Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Geezer on January 31, 2009, 06:13:46 pm
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I have a new project that I need help with (LOTS of help) :-*
I have lots of "clean" amps, but not any that really do the overdriven thing....at least not like the following linked video!! :-[
After watching THIS video of Peter Stroud (http://www.youtube.com/watch?v=zc3-CiPe09I&feature=channel_page) playing one of his 65Amps "Soho" 18w'ers, I really want to build one!
Of course, I (we ;) ) must figure out what in the heck it is!
From the "clues" that Peter gave in the demo, I have drawn my "best guess" at what the topology of the amp could be.......see the drawing below. I have it in .gif (for easy viewing) and in .sch form (for others to make corrections/add their input).
I'm just doing a basic "block diagram" right now. I'll work out a preliminary schematic later on......
#1 - EF86 Pentode is the input stage (a no-brainer)
#2 - Blackface Tonestack(??) ....I get this idea from Peter's comment, "we wanted to get that sort-of dipped midrange that allows for a cleaner tone". I don't think a 5F6-A/JTM45 "Tweed" TMB would give that mid-dip(?)
#3 - He states that the Tone circuit is "12AX7 driven"....I supect the 12AX7 is AFTER the tonestack (as shown). Wouldn't a pentode, into a triode, then into the tonestack be too much gain before the stack?? I envision it like the AB763 BF circuit....pentode, then tonestack, then a triode "recovery" stage before the PI.
#4 - The "BUMP" switch/circuit....I'm thinking it bypasses the TMB tonestack, and inserts a 5E3 style Volume/tone circuit....what do you think?
So my main questions (to start) are....
What tonestack?
Where should the tonestack be placed?
Where do the 12AX7 triode stage(s) fit into the picture?
Does the "Bump" circuit seem resonable as I've envisioned it?
This is just a start.....I'll worry about figuring out that really great sounding Master Volume later!
All comments & help encouraged & appreciated!!!
Geez'r
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I think you nailed it pretty well. But why the 6 step volume control on the bump channel. Why not just a pot? Something special going on?
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Does Dan Boul still post over to 18watt.com? He'd surely know (although maybe he won't give it all up since he does make a living making and selling those things)..... I haven't been to 18watt since the forum "cleansing" but he was a regular contributor, he seemed like a nice guy on the internet.
j.
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why the 6 step volume control on the bump channel. Why not just a pot? Something special going on?
I wondered the same thing. Maybe they just wanted to make it a "boost" ("Bump" up) and not have the possiblility of actually reducing the volume, so they limited the scope of the control so that the range is only the same amount or more volume....less volume is not possible. A selector switch would make that fairly easy to accomplish....Just a guess(?)
Does Dan Boul still post over to 18watt.com? He'd surely know (although maybe he won't give it all up since he does make a living making and selling those things)..... I haven't been to 18watt since the forum "cleansing" but he was a regular contributor, he seemed like a nice guy on the internet.
I've already gone over to 18watt & looked around. Dan readily acknowledges that he learned pretty much everything he knows from thoses guys. I even found the post where he announces the official incorporation of 65Amps! But it seems any posts he made about real innovations or ideas are gone with the "cleansing" :-X .
I'll look around some more (@18w) & maybe even pose some of the questions over there.
Thanks for the responses! G
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DaGeezer,
If you're looking for an idea with a boost accenting mids ................... you might consider this?
I used a heavy duty full sized DPDT when switching tone stacks. However this accomplishes a similar feature with a PAB boost that adds a cap when engaged to allow more mids thru. Plus the mid pot on this can get either mid scooped or a pretty full mid tone.
I added a PAB to the Little Wing Reverb and another mini-toggle SPDT (center off) to switch a 820p or .002 cap (paralleling the treble cap). With either of those caps paralleled and the PAB on ......... it gets some excellent overdrive!
The thing with this is that you can use a mini-toggle DPDT.
With respect, Tubenit
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Can you really drive a "standard" tonestack after EF86? I believe there was some "pentode high impedance" thing involved in this.
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It's funny how quickly we can all come up with what might be the circuit. We are all such a talented bunch of folks. :)
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Can you really drive a "standard" tonestack after EF86? I believe there was some "pentode high impedance" thing involved in this.
Thats just what I was thinking....you got an extra triode why not use it for a cathode follower before the TS. Or use a 6U8A (pentode/triode) for the gain/cathode section on the front and parellel the 12AX7 after the stack.
my 2 cents
BL
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Blind Lemon had teh same thought I did and posted as soon as I open the reply box!
I think the EF86 would have enough gain to drive the tone stack, but it might benefit
from a CF between it and the TS.
Maybe that is what he meant by "the TS is driven by a 12AX7".
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A lot of their ads (e.g. http://radicaltone.com/65_amps_soho.php) say: "65 - SoHo 20 WATT (2xEL84), Tube rectified, EF86-12AX7 Cathode Follower with BumpTM circuit and Defeatable Master Volume"
That's where the unemployed triode works ...
The tube line-up is 1xEF86, 2x12AX7, 2xEL84 (http://www.proguitarshop.com/index.php?CategoryID=316) so it must be close to tubenit's sketch. The bump control obviously switches volume/level and tone at the same time, so there is probably some cap switching involves, which explains why they didn't use a pot.
There's another demo of the SOHO on youtube (http://de.youtube.com/watch?v=4vqk9AOOjdg) that might be of interest. Maybe someone with a trained ear can figure out more details by listening.
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"Exactly" 6G6. I went over and listened/watched the clip (very nice BTW) and for some reason I get the impression the "Bump Circuit" is not a parallel TS. :-\
BL
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The Dr Z Route 66 has a ef86 with treble and bass controls, ltpi.
(http://www.prowessamplifiers.com/schematics/images/Route66.jpg)
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I may be wrong, but I would guess that a triode gain stage after the EF86 would make it very difficult to produce a clean sound, due to the high output level of the pentode. You would either need a voltage divider or couple the cathode follower directly to the EF86 and use the second triode after the tone stack, so the topology would be:
EF86 - 12ax7/CF - TS - 12ax7/gain - PI - power section
Could the bump circuit then be a switchable cathode resistor/cap-combination for that post-TS triode gain stage (apart from disengaging the TS)?
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I have been thinking about building something like this in a VJ chassis. I was either going to use da Geezer's schemo for converting a VJ to 5E7 with 2 EL84's or Tubenit's Nightclub Guy schemo as a starting point. This looks like it has elements of both. The EF86 frontend and the CT. This looks like a very cool little amp with lots of tonal possibilities. Thanks guys, as always. Jmac
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Here's they're PR on the amp on they're site, perhaps give a few more clues:
65 – SoHo (20 watts) – Designed to be a super-functional amp for working players and enthusiasts who desire widely varied tones in one amp. The SoHo can transition seamlessly from the best of American tones to the sweetest British tones via the “Bump” function and NOT switching channels. The Bump will radically shift the tone-stack functionality achieving astonishing renditions of dozens of excellent, natural sounds. In addition to the “Bump” function, there is a defeatable Master Volume that is revolutionary in that is slopes the tone stack as you turn the Master down, giving the Master a very natural sound and ultra-low volumes. The Master can be turned on for “Home” mode and completely defeated for “Gig” mode. This amp is a must for studio players, small club gigs and enthusiasts who need to play at reduced volumes.
Regards,
Dyna
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Could the bump circuit then be a switchable cathode resistor/cap-combination for that post-TS triode gain stage (apart from disengaging the TS)?
This is what I was thinking... Something like tubenits fat/full/bright switch, but with more options/different values.
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Could the bump circuit then be a switchable cathode resistor/cap-combination for that post-TS triode gain stage (apart from disengaging the TS)?
This is what I was thinking... Something like tubenits fat/full/bright switch, but with more options/different values
Except.....Peter states in the video that the Bump feature disables the TB tonestack...... :-\
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I may be wrong, but I would guess that a triode gain stage after the EF86 would make it very difficult to produce a clean sound, due to the high output level of the pentode. You would either need a voltage divider or couple the cathode follower directly to the EF86 and use the second triode after the tone stack, so the topology would be:
EF86 - 12ax7/CF - TS - 12ax7/gain - PI - power section
I'm thinking Heinz has hit pretty close to it here....(?)
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couple the cathode follower directly to the EF86 and use the second triode after the tone stack
What would that look like? How would you couple a 12Ax7 triode to a EF-86???
What would you change in this schematic to make that work??
With respect, Tubenit
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What would you change in this schematic to make that work??
Nothing.....that sure looks "direct coupled" to me....
May someone else will say otherwise(?)
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There's a lot of dc on the cathode of that cathode follower. Need to block that dc.
EDIT: I changed the GIF/SCH schematic (2 posts prior) to match Sluckey's critique. Thanks, Sluckey!
With respect, Tubenit
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I just watched the proguitarshop.com video again and they say that as you back off the bump level you have less bass and less gain. That sounds like a switchable cathode bypass cap to me.
The bump tone seems to be a simple treble filter, but I just can't figure out how that could be connected without needing another switch. Anyone have an idea?
EDIT: the volume pot is in the wrong place. It remains effective even with the bump active, so it must be right before the triode gain stage's grid (after the left side of the switch)
EDIT#2: also forgot the coupling cap sluckey mentioned
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Ok, here's the correction of the previous mistakes.
Peter Stroud also talk about a special master volume that preserves the tone as the volume is turned down. As the tone tends to get fizzy at lower volumes, I guess that this feature is a variable treble filter that rolls off treble in proportion to the volume. This could easily be done with a dual pot as drawn. Will this work as expected?
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Heinz,
GREAT ideas (as usual) ........... keep em coming! Thanks, Tubenit
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Heinz,
GREAT ideas (as usual) ........... keep em coming!
Yea, Baby!!! Now we're talkin'! ;D
I feel another build comin' on! :P
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EF86 > CF > tone stack is a VERY good idea, and works very well giving good gain and good tone. Of course the devil is in the details. The key is the screen voltage and supply voltage of the EF86.
Greg
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Here comes another design. I think I found a way to connect the 'bump tone'. It's a parallel mini-tonestack that comes into action when the regular ts is switched out. Both ts stay in the signal path but only one becomes effective while it's grounded.
The bump level selector is the same as in the last design. Maybe it's not only switching bypass caps but also parallel cathode resistors. I don't know how that would change the tone and if it would be a step in the right direction.
I feel another build comin' on!
That solves the question who's gonna test all these ideas...
EDIT: the master volume is now switchable
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EF86 > CF > tone stack is a VERY good idea, and works very well giving good gain and good tone. Of course the devil is in the details. The key is the screen voltage and supply voltage of the EF86.
Yes, what I've read is that the "target" ballpark is keeping the screens @ 2/3 voltage of the plates or less. I expect alot of experimentation will be needed (as I'm I total novice when it comes it input pentodes).
I think I found a way to connect the 'bump tone'. It's a parallel mini-tonestack that comes into action when the regular ts is switched out. Both ts stay in the signal path but only one becomes effective while it's grounded.
Heinz, that seems to stay in line with what Peter said in the video.
I think we're getting close to having the "basic" topology in order!
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I expect alot of experimentation will be needed (as I'm I total novice when it comes it input pentodes).
Copying the input stage of an existing amp will probably be a good start. The pentode stage as drawn by tubenit is from a Matchless DC30, for an AC15 the plate resistor is 220k and the screen-to-ground resistor 1M. I have made tests with both configurations (with signal generator and scope) and they work equally well, but I cannot say anything about their tonal qualities. I don't expect much difference, but I'd start with the AC15 values because I like its tone.
The gain of that stage is around 200, so even with the tonestack loss the signal level may be too high for the following triode stage (V2B). I have inserted a 220k resistor before the volume pot to bring down the signal level, but that is just a guess.
Now someone's got to start experimenting and find out if this design actually points into the right direction. I have a power stage module and a brand new pentode preamp module (see http://dhost.info/jschem/t7 if you don't know what I'm talking about) that would make a great start and leave only the tonestack/bump section for prototyping. However, I won't be able to do any experiments this week due to my job workload. I'll keep you informed as soon as I start tinkering, but you'll have to be patient.
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I won't be able to do any experiments this week due to my job workload. I'll keep you informed as soon as I start tinkering, but you'll have to be patient.
Then you'll be well ahead of me...I wouldn't start for several weeks, so patience won't be a problem on my end! ;)
Sounds like you have the right setup to prototype. Mine will just be a "build it & tweak" type thing, using parts I have around. I have some good quality 50's/60's PT's & OT's (7k to 10k primaries) that came out of hi-fi & PA stuff running 2x6V6's that should work well.
Tubenit has agreed to do a layout, as soon as we have nailed down the topology.
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There you go! Looks good so far and not too complicated. But then...who knows if it actually works...
EDIT: Heinz was kind enough to edit a layout I posted to correct it. I deleted mine to avoid confusion.
Tubenit
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T,
look @ Heinz post right above yours....he has attached "#9" schem that (I think) fixes your drawing of the MV/Tone...
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Wow! I missed seeing it. Thanks Heinz!
I'll review it and edit anything that needs it and repost it or affirm that it's done and looks right to me. Maybe I should delete all the other schematics that aren't being used? What do you think ........ delete or leave em?
With respect, Tubenit
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DaGeezer & Heinz ,
Take a look at the schematic and layout and let me know if this is a good workable 1st DRAFT for the build? Feel free to edit and change anything you wish. I would think Hoffman's 18watt PT & OT could work well with this.
With respect, Tubenit
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Just stumbled across this drawing on an idea to limit EF-86 microphonics. Haven't used it but thought I'd share the idea. Also saw where some individuals were using shrink tubing on half of the tube to cut down microphonics.
With respect, Tubenit
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I have seen a large hi temp O ring used as well. John at Pacific T.V.sells isolation tube mounts: http://members.shaw.ca/pacifictv/socket.htm#specialp
(http://members.shaw.ca/elissag/b9ashk.jpg)
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DaGeezer & Heinz ,
Take a look at the schematic and layout and let me know if this is a good workable 1st DRAFT for the build? Feel free to edit and change anything you wish. I would think Hoffman's 18watt PT & OT could work well with this.
With respect, Tubenit
Nice layout! So a 21 parallel turret board should work? Cool stuff, I just happen to have a head chassis with three noval sockets... Hmm, love the sound of the ef86, wonder if it sounds similar to Gab's sound garage Voxer?...
You guys are amazing, from description to design, I'm impressed...
Regards,
Dyna
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weber has some bases that are "floating" and are suspended by strings.
i love that o-ring trick. it works so well. =] does anyone have the link for that online? i forgot which ones are for this. =/
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Ted Weber has the o-rings>>>>>>
https://taweber.powweb.com/store/tubesktord.htm (https://taweber.powweb.com/store/tubesktord.htm)
Tubenit, I've been pressed for time today & will comment on your layout as soon as I can....Thx again, Geez'r
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I've been ordering my o-rings from McMaster-Carr. Can't remember the specific size at the moment, but they were hi-temp (400-500 degree F) orange silicon, and about $10 for a bag of 100.
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I am really looking forward to trying this out. I don't have much experience with EF86's. What are the differences between running the EF86 into a 12AX7 and running a 12Ax7 into an EF86?
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I don't have much experience with EF86's
Apparently none of us (here) do! So "join the club"....but we're gonna' have some experience when the dust settles! ;)
What are the differences between running the EF86 into a 12AX7 and running a 12Ax7 into an EF86?
Once again, not sure, but to me the diff is getting that fat pentode tone 1st, rather than "fatting up" a 12AX7 tone(?)
The Matchless Clubman does the latter, a 12AX7 paralleled, into a pentode....not EF86, but a 6SH7 (lower gain than an EF86, I think)
http://www.schematicheaven.com/newamps/matchless_clubman.pdf (http://www.schematicheaven.com/newamps/matchless_clubman.pdf)
FWIW, I'm going to use a 6CA4/EZ81 rectifier in mine, just because I have a nice NOS RCA in the bin. It's a bit "stiffer" than a 5Y3, & I won't need to have a 5v supply.
To help me speed things up a bit, I'm toying with the idea of modding one of my existing amps to this design (one that I never play anymore). The one I have in mind already has pretty much everything in place, except for the recto socket. But, it's a Epi VJr chassis, so I'll have to see if there's room across the front for all the controls (?) :-\
I think I'd have to "squish'em" in, Trainwreck style!
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Oh....and I have some isolated/shock mount sockets from old 50's/60's military equipment. I'll have to see if I can get them out without destroying them!
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Tubenit,
the sch & layout look good to me.
As I said above, I'm going to be using an EZ81/6CA4 recto, so I'll look that pinout up & change it on one copy (as an option to the octal recto).
I think the closest 5v octal to the EZ81 (which is what they use in the Soho) would be the 5V4.
A 5Y3 is going to be much softer/saggy-er & drop more voltage.
Thx again, G
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Thanks for that info Mr. Geezeer. I'll be building mine in a VJ chassis as well. I was going to build something with 6V6's but now i'll put EL84's in it. I have already installed a couple of 9 pin sockets for pre tubes so I guess I'll add a couple more. And an octal socket. I also have some concentric pots I bought a few months ago that should work nicely with the limited front panel space. This should be a very interesting build. Thanks again, JMac.
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I just went & measured the front exposed faceplate area of the VJ....~11" across (exposed). If I leave the on/off switch in place (I already have a standby switch beside it on the right side) that leaves ~8" to place the pots. If you cozy them side by side (almost touching) you can get 1 pot/inch, so there's room for 7, but we only need 6 (if we put the switches above or below the function/pot they control)....the input jack could be done that way, too, above the volume pot.
If I butt the pots right up against each other, I think I could even squeeeeze in another pot for a mid control, if I want (which I just might!
Below is a scale drawing of the faceplate. I'll have to use smaller knobs, but I really like the Davies style that are found on Trainwrecks anyway, so that's cool (no chicken heads need apply!)
(http://www.pedalpartsplus.com/mm5/graphics/00000001/Black%20Knob%20crop.jpg)(http://www.pedalpartsplus.com/mm5/graphics/00000001/Green%20Knob%20crop.jpg)(http://www.pedalpartsplus.com/mm5/graphics/00000001/Red%20Knob%20crop.jpg)
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Here's the face with a Mid pot added....still plenty of room, even with 1/8" between each pot case.
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I couldn't resist...
This is the first prototype of tubenit's schematic. It has a 6BM8 power section (similar to the Little Wing) instead of the EL84/12AX7 combination.
I have not built a tonestack for the preamp (which is equal to 'bump' enabled) and V2B has a 10k cathode resistor without a cap. Even without the bypass cap the gain is way too high to be usable. The signal is either off or distorted, so a lot more attenuation before the volume pot is required (or may a lower pot value). I'll play with the values when I have more time & post the results.
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V2B has a 10k cathode resistor without a cap. Even without the bypass cap the gain is way too high to be usable.
I'm thinking that V2B is not needed at all.........many/most EF86 designs just go straight from the EF86 > to the tonestack > to the PI.
However, Peter clearly stated (in the video) that the tonestack is "12AX7 driven", so maybe we should try:
EF86 (V1) > to the 12AX7 cathode follower (V2A) > to the tonestack > to the PI (leaving off V2B altogether).
All you would need to do to try this is bypass the last triode on your prototype & take the TS directly into the PI.
If the TS seemed to need "a little more" drive, maybe we could parallel the V2 triodes for the CF(?)
Just thinkin' out loud....Thanks for the report Heinz!! ;)
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You're probably right. The EF86 input stage does not need another gain stage. However, I doubt that the guys at 65amps would leave a triode unused. A double CF does not make much sense, either.
Thinking about it...a different idea comes to my mind. We could use two separate CFs after the EF86, one driving the tonestack (normal path), the other going directly to the volume pot (bump path). These can be enabled/disabled by a switch that disconnects the load resistor from ground (as with the tonestacks in the current design). An additional benefit of this design would be a full separation of the two signal paths.
But then we lose the bypass cap switching at V2B. That means that we now need a new idea how to implement the bump level...maybe switch bypass caps for the EF86?
I can't draw a new schematic right now (being at work) but I'll post one when I get home.
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Could this schematic be edited to work????
Tubenit
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Late to the game, but this text keeps me thinking: "it slopes the tone stack as you turn the Master down." Coupled with something Peter said in the first video, I wonder if the "Master" is mis-labelled. Maybe the slope resistor is being changed? That would affect frequency response and insertion loss of the tone stack assuming TMB. Combine a variable "Raw" control like Dave Allen's (mid boost/tone stack defeat) with a variable slope resistor and you might get a similar affect. Also, given Heinz's observations about excessive gain, is it possible that the "Master" is actually dumping gain (or not) before the tone stack?
It's great to watch you all working this out. Thanks!
BTW Jeff there was a lengthy discussion on the 18Watt forum within the past year or so about one of the London amps and how they got an EF86 preamp stage to sound so sweet. Might be worth chasing.
Chip
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link for that 18 watt discussion please lol! their search function just doesnt work lol. =]
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My bad - I haven't been visiting the 18Watt forum for longer than I thought, plus it was the 65 Amps "London" model I was thinking about. It looks like every useful post has either been deleted or the attachments (i.e. schematics) have been removed. There was a lively and interesting discussion on the subject.
Guess we can't blame Dan Boul too much, although someone will (or already has)reverse engineer(ed) his circuit.
Chip
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I've been meditating over the circuit again and spent some time with duncan's tone stack calculator. Here's a new idea. The 'normal' signal path is a standard Fender tone stack, the 'bump' path is a simple CR-filter with switchable caps and a tiny low-pass filter for the tone. I have investigated many possibilities around the slope resistor, but this seems to be the most likely solution. All other tone stack modifications either do not 'bump' or leave the tone controls effective when 'bumped'.
I'm no longer convinced that my previous master volume design is right. It doesn't make sense anywhere before the PI. I'm not sure if Peter's "it slopes the tone stack" is correct. He also says that the tone stack is disabled when the bump section is on. So how can it be part of the tonestack (which is disabled) and still be working? I did some internet reading on master volumes and decided to try a simple post-PI cross-pot, which many say does sound very natural. We'll see.
Excessive gain was another problem of the first design. The AC15 runs the EF86 output directly into the PI and sounds fine, so that signal level seems to be about right. I have now tried to maintain that level at the PI input by attenuating the signal at the volume pot. The ballpark numbers can be found in the schematic.
I'll make some experiments (hopefully this evening) and post the results.
@tubenit: Jeff, I apologize for not using your schematic. I did not want to re-invent the wheel, but I had trouble with the snap grid and could not draw any straight lines, so I started from scratch.
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Heinz,
Great job on the schematic! As usual!
Da Geezer,
Can you give a thumbs up or edit Heinz's schematic and then I'll edit a layout to match it.
Thanks, Tubenit
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Jeff, I'd hate to see you waste your time...I'd wait with the layout until the design becomes clearer.
I'll post some results tonight. We'll see if this design is on the right track, or if we'll have to come up with a complete new idea (again)...
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Here are some test results:
- the cross-pot master volume is NOT what we want. It sounds very fizzy when turned down. I'll try a dual pot post-PI master volume next.
- the second triode stage adds too much gain. It should either be omitted or we have to lower the signal level even further. I don't really understand this. The 'bumped' preamp is almost identical to an AC15, which feeds directly into the PI. How can this be controllable on the AC15? It's either off or distorted on my amp. Does anyone have experience with AC15s? If the gain stage is dropped, what could we do with the unused triode?
- I have only prototyped the bump channel so far, it's late and I can't turn the volume too far up, but it sounds pretty good. The bump tone pot works and switching the caps changes gain and bass response just as intended. The cap values are not conclusive, they're probably too large. However, the overall 'bump' design does not seem to be too far off.
It's almost midnight now, so I'll continue my experiments on wednesday. Good night everyone!
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Thanks for the work & update, Heinz!
what could we do with the unused triode?
I know it's hard to do ( ;D ), but it's really OK to leave a triode unused! It's entirely possible that's what 65 did.....
I think EF86 > 12AX7 CF > TS > PI is the way to go....is that the arrangement/circuit you have now?
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How about using a 12DW7 instead of a 12AX7 in the preamp to lower the gain?
Amps like the D'Lite have four gain stages prior to the LTPI. So there has got to be a way to attentuate some gain with a 12AX7,IMO. I am also wondering if the 6BM8 tubes are easier to overdrive than the EL84?
With respect, Tubenit
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"So there has got to be a way to attentuate some gain with a 12AX7,IMO"
No cathode cap and a large cathode resistor; maybe 10k to start. That would definitely lower the gain. If that's not enough, a voltage divider after the tubes output would also lower the gain. It's all a crap shoot, just fiddling around till it sounds right. It's a pain but the only way I can ever figure how something will sound. Getting it from inside my head to coming out of the speakers is not an easy task for me... but it's what I really love about building these things.
Dave
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I know it's hard to do ( ;D ), but it's really OK to leave a triode unused! It's entirely possible that's what 65 did.....
:'( What a waste.
Of course, we could use the triode for something completely different, e.g. a tremolo...
I think EF86 > 12AX7 CF > TS > PI is the way to go....is that the arrangement/circuit you have now?
Yes, that's the topology. Plus the TS can be switched out of the circuit.
How about using a 12DW7 instead of a 12AX7 in the preamp to lower the gain?
Yes, I'm sure that would work, ANY triode will serve as a CF. I just don't have any...
Amps like the D'Lite have four gain stages prior to the LTPI. So there has got to be a way to attentuate some gain with a 12AX7,IMO. I am also wondering if the 6BM8 tubes are easier to overdrive than the EL84?
The D'lite and other high gain amps attenuate the signal after each gain stage with a voltage divider. We only have ONE gain stage...maybe we could attenuate the signal before the CF, as Chip suggested some posts ago. Some more tinkering is required here.
The 6BM8 needs a higher drive voltage than the EL84. I'm sure they are not part of the problem.
No cathode cap and a large cathode resistor; maybe 10k to start
I tried that. It definitely reduces voltage gain, but it also reduces voltage swing. Large signals distort easily, which is why e.g. Marshall and Soldano add these stages. You can only keep these clean at low signal levels, which brings us back to our problem. Leaving the triode unused seems the most likely option at the moment.
If we would change the PI to a split-load PI we would free another triode and could remove one 12AX7. This would essentially leave a one-stage pentode-preamp amp (not much overdrive), which might be good for blues, but would be a different amp.
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It is not drawn neatly since it was done in a hurry. However, for illustration purposes ......... does this represent the
latest thinking on this amp? Feel free to edit it.
With respect, Tubenit
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I've not got the $ to buy an EF86, but I found a couple of NOS RCA 5879's in my stash, so I think I'm going to try that tube 1st in my build.
From the info I've found on the web & the data sheets, it seems to have slightly less gain than the EF86....possibly a good thing(?)
For the price of (1) EF86, I can buy dozens of the 5879's.........so the chances of finding a lower microphonic specimen will be better.
What'a ya think?
http://www.mif.pg.gda.pl/homepages/frank/short/001/5/5879.gif (http://www.mif.pg.gda.pl/homepages/frank/short/001/5/5879.gif)
http://www.mif.pg.gda.pl/homepages/frank/sheets/079/5/5879.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/079/5/5879.pdf)
http://www.pmillett.com/tubedata/HB-3/Receiving_Tubes_Part_2/5879.PDF (http://www.pmillett.com/tubedata/HB-3/Receiving_Tubes_Part_2/5879.PDF)
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5879's sound great and are cheap, so yeah its a great choice. Look at lots of Gibson circuits for how to use those, especially the '59 GA-40.
Greg
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EF85 and EF184 work well, too, but need slightly different part values. I have tested some and haven't had any problems with microphony.
I have finally found some time to continue my experiments with the prototype.
These are the results:
- the dual pot post-PI master volume is the way to go. It sound very natural at low volumes.
- I have reduced the gain of the EF86 by using a smaller cathode bypass cap and by splitting the plate resistor. The preamp is now controllable, even with the second triode stage re-inserted.
- I'm no longer sure about the cap switching. It does change tone, but not quite as you can hear in the demo videos. Either the cap values are off, or the whole circuit is wrong. I have drawn an alternative circuit that could do the job, but haven't tested it yet.
- The amp has that wonderful VOXy shimmer and sounds best when overdriven. It would be a nice amp even without tone stack and cap switching.
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In the interest to continue generating ideas to consider. This would be like the D'Lite PAB but called "Bump" and the cathode caps on the second gain stage would be the "Bump Tone".
I have used both of these ideas in amps and like the tone/gain changes & the simplicity of it.
With respect, Tubenit
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Just to keep this active & near the top....I'm starting the conversion of one of my "November" amps into the 5879 (EF86/pentode input) project circuit.
The topology is pretty much the same in the power section, so it shouldn't be too hard to convert.
I'm going to start with just the pentode > CF > Tonestack > PI configuration 1st, then try different things/ideas for the "Bump" feature (after I've got the "basic" circuit right).
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OK, got it wired with the topology I stated above (pentode > CF > Tonestack > PI) and it sounds good, but needs more volume & gain to the PI, so I'll add the 2nd triode driver before the PI & see what happens.
As for the tone, I am getting a "taste" of the pentode tone.....slightly compressed & very smooth & full, just needs more "push" into the poweramp. ;D
I'm very excited & hopeful for good results!
Also, of the (2) 5879 tubes I have, (1) is slightly microphonic, but not so much as to be a problem (slight "ring" when tapped), but the other one is very quiet, indeed! The quiet one also seems to have a bit more gain to it....
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You want more gain? I'd want less... :o
Anyways, I love the sound of the amp!
What is your B+? And how did you wire up the pentode (bypass cap, plate resistor)? Could you please draw a quick-n-dirty schematic of the pentode stage?
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When I had the pentode connected as shown in your last drawing (split 100k's w/ the CF input from the junction of the 2x 100k's), the gain was WAY too low....in fact, very clean! & the overall volume was low, even with the vol pot all the way up.
I now have it wired as originally drawn/"traditional" (200k Rp w/ the CF input tapped from the plate of the pentode)....NOW we're talkin'! It still needs some tweaking of values, but it is very close to the tone in the video!
Current config = 200k Rp, 1Meg Rs w/0.1uf to ground, 2.2k Rk & 2.2uf bypass cap, 33k grid stopper.
Voltages = B+ node 255v, plate 105v, screen 65v
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I get the picture. You tried the 'normal' setting with the tone stack. I tried the 'bumped' circuit without the tone stack.
The tone stack reduces the signal roughly by a factor of 4. I reduced the signal by 2 with the split plate resistor and by 2 before the volume pot. That makes a total reduction by 2*2=4, so the circuits are similar gain-wise. Seems I tried to get clean tones from the 'bumped' circuit that it wasn't designed for. I'll change the pentode plate resistor back to the original configuration and attenuate the signal in the bump branch right before the volume pot.
I have tested my latest idea with the second triode stage and bypass cap switching, but it does NOT have the same effect as the level switch in the video. The coupling cap switching seems to be more likely.
It still needs some tweaking of values, but it is very close to the tone in the video!
Very interesting! My experiments indicate (to my ears) that the tone of the 'bumped' amp (tone stack bypassed) is very close to the video, as well. Obviously, we're not too far off..
Have you added the second gain stage, or does it seem unnecessary now?
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Have you added the second gain stage, or does it seem unnecessary now?
it seems unnecessary now, but we'll see once I get everything hooked up.......reworking the tonestack a bit now....
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HI, the March 08 GP magazine sports a picture of the SoHo chassis. It's too small to glean everything, but I did notice that the MV is definitely a dual pot. If there's anything else you'd like me to look for in the picture, I can try looking with a magnifier, although it sounds like you're getting close. CraigB
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Thanks for the info! Could you scan it?
EDIT: found the article online http://www.guitarplayer.com/article/65-amps-soho/Feb-08/33519
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Yeah the article doesn't show the picture. I will scan it and try to blow it up some. If the file is too big, can I e-mail it to you? CraigB
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There's a tiny pic in the article on the right, but it doesn't reveal much. You can't even tell resistors from caps...
I will scan it and try to blow it up some. If the file is too big, can I e-mail it to you?
That would be very nice! You can send it to the email address from my forum profile (just click on the envelope below my name in this post). I'll upload it to my website and post a link so everybody can participate in the quest.
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Go here - http://www.tonecentral.net/gearpics/ (http://www.tonecentral.net/gearpics/) down the list are several good Soho gut shots.
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Wonderful! Thank you very much!
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Been following this - sorry I didn't find it sooner!
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No need to scan the little postage stamp size picture I've got! Those are great shots. I wonder what the cap and resistor are there on the MV dual pot??? CraigB
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I'm currently trying to trace out the circuit. Unfortunately, the most complicated parts are in the most crowded areas. From what I've found so far I can say that that the MV is unlike the usual circuits.
Our ideas were close in some spots but completely wrong in others. I'll post what I have when I'm done.
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Here's as much as I could find out. Please check for errors.
Assuming I got it right...the circuit is certainly interesting. The MV switch adds gain by boosting a triode stage and the attenuator part of the MV strangles the signal before the PI. There's also some high frequency filtering going on, but unfortunately the component values are invisible.
The bump works by lifting the tone stack from ground, so we were pretty close with our guesses.
Why did they use a switch for the bump selector? Could it be related to power dissipation (especially with the bump enabled)?
And why did they use a triple switch for the MV instead of just lifting the MV circuit from ground?
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WOW....Thanks for that piece of work, Heinz!
It will take a while to digest all that.
In glancing at some of the missing (?) values, some of them, at least, can be guessed at by the surrounding component values...others will just be a "try & see" approach.
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Minor Update: I corrected some drawing errors.
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Well, I'll be re-wiring my test bed for the pentode>triode>CF>TS>PI configuration.
The pentode>CF>TS>PI sounds "good" in it's own right, but not like the SoHo.
BTW, doesn't that newly discovered configuration look a bit like a 5F6-A/JTM45 circuit w/ a pentode?
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I changed my prototype to reflect the new knowledge - and it sounds just great! 8)
The bump circuit works as expected (boost factor, level and tone control) and the normal tone controls provide the usual range. I'm not sure if I got the master volume circuit right. I've been meditating over the pictures for some time, but I can't see for sure whats behind that resistor on top of the pot. I'll go about this later.
I have compiled a new schematic with the component values I expect to work. The .sch file has a second sheet with my prototype (call it the 'SoHeinz' ;)) which includes some simplifications to the bump circuit. A pair of 6BM8s saves one ECC83 and makes a nice 4-tube amp with good sound and decent volume (still enough to make my wife complain).
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I rewired mine to match your last posted schem (just the basic circuit....no bump or MV).
Sounds pretty good with the volume low, but it either goes into cutout OR starts oscillating @ ~ 1/4 way up.
If I severely drain some of the signal after the volume pot (39k resistor to ground) then it calms down & sounds really good.
I'll be trying more tweaks today....may be lead dress, since this is patched together in an older build(?)
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Wanted to say that there really is a difference in tone of the pentode compared to a triode (even a paralleled dual triode, which I heard some guys compare pentode inputs to).
It's kind'a "jang-ly" sounding & responds really well to pick attack....cleans up with a light pick, then distorts when dug into! Also responds well to the guitar volume, and it's a "smooth" type of compressed overdrive, but still retains definition....I like it alot!
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Heinz, I am supposing that the screen resistor on V1/EF86 is mis-labeled? You have it as 470R, but that doesn't work well with my 5879....I'm thinking it should be 470K(?)
Even that value doesn't sound good on mine, so I have it back to the 1M.
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Geez, Mine has a 2.2M
The Pentode preamp tube is fat and jangled. I have an amp with a parallel triode and a EF86. I switch between them. The Pentode is the lead channel.
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Oops! Of course you're right, it should be 470k. The 470k/1M resistors make a voltage divider that keeps the screen at about 2/3 of B+. The 100n cap provides some stabilization to the screen voltage. This circuit has a gain of about 100-120 and a voltage swing of nearly 180Vpp (B+ 250V) before it distorts visibly on the scope. You can see the compression set in at about 100Vpp.
I haven't seen this arrangement before, but it works about the same as the 1M/100nF combination. In that case the internal screen resistance is the second half of the voltage divider. A 2.2M resistor will set a lower screen voltage. If the plate and cathode resistor remain unchanged, this will result in a reduced output signal voltage swing which means it will need less input signal before it starts to distort. This may be what you want...or not. Maybe a guitar pickup signal can't even drive it to this point.
The AC15 uses a 220k plate resistor, 2.2k cathode resistor and 1M/100nF for the screen screen.
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I'm still trying to figure the master volume circuit out. Here's an enlarged piece of the gut shot with some remarks.
Unfortunately, there's no way to look behind that brown resistor that sits on top of the pot. There are two wires coming from behind that resistor, but I can't see what type of component it could be. The black blob is a cap that is probably connected to the left lug of the pot but where exactly does the right leg go to?
Could my schematic (see above) be correct?
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Heinz, the Rk on V2A......is 47k correct? That's really cold biased & takes all the life out of the tone on mine.
On the master, your guess looks pretty good, but there does seem to be some other component under that brown resistor....almost looks rectangular, like a wima type cap(?), but that doesn't really make sense, since they used Sozo caps everywhere else..... :-\
I'm REALLY getting frustrated with my attempts to get this "basic" circuit right (no bump or MV on mine yet).
It sounded better to me without the extra triode (although we now know that the Soho uses it).
Mine has way too much gain (ocsillates like crazy if turned up), is too "chimey" (too much highs), is too boomy without the bass turned all the way down....I need to step back a while & try to get my perspective straight again.... :(
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DaGeezer,
I am wondering if some of the oscillation is from the complexity of the build & layout? I would be tempted to try something like this which has the mstr vol prior to the LTPI. I would guess (maybe incorrectly) that it would be less prone to oscillation?
With respect, Tubenit
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Thanks T, I am thinking I will like that arrangement & will try it.
I found this article ( http://www.freewebs.com/valvewizard/pentode.html (http://www.freewebs.com/valvewizard/pentode.html) ) that describes that the "standard" values given for pentode operation (Vox AC15 & such) is for absolute max gain, & gives some suggestions & guidelines for values more suited for guitar use......
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The 47k cathode cap for V2A is just a guess. The component is not visible in the pictures, just its connections. I get too much gain if I use a substantially lower value. With the 47k the range goes from clean to very distorted, just as it should be. Choose an appropriate value if this value doesn't work for you.
One important aspect my prototype seems to be the B+ voltage. It needs 240V or more for the preamp, otherwise it does not clean up, even when the guitar is turned down.
Strangely enough, mine is very bright and sparkling. Could the different pentode type you are using make such a difference?
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One thing to think about is that it's possible for an individual to get used to a mental picture of the way things should "BE"! And I continue to realize within myself how much of how I approach music is a mental thing.
My Carolina Blues Rocket will oscillate if I crank everything to 10. That really bothered me at first but I "got over it". It's a high gain amp. I will NEVER play everything cranked to 10. I discovered the most that I like (and will tolerate) is vol at 10, gain at 7 and mstr vol at 5. That's it! It's unbearable to me beyond that. AND .......... I have to crank it beyond what I would ever use it at to get any oscillation. And I play it waaaaaayyyy less cranked than that with the band I play in.
So I adjusted my thinking ........... my amp does everything I need that amp to do & I enjoy playing it.
I realize if I were trying to sell one and stated "you can't crank everything to 10 or it will oscillate" then I might not have any buyers. But the amp is for me.
Just my two cents worth! With respect, Tubenit
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I have played with my prototype for some time now (my ears are ringing :o) and tried both my guitars (a yamaha pacifica and an ibanez LP-type with active pickups). It seems that the amp likes humbuckers better than single coils. You'll also notice that Peter Stroud always demonstrates the amp with a LP.
That (nice) bright jangly tone appears if the signal level exceeds a certain value. If either the guitar or the volume is turned down too much it sounds somewhat dull. Maybe the reason why humbuckers sound better is simply the higher signal level.
If you want to reduce the bass you can use a smaller cap value for the 470k/10nF pair right after the CF. I've been wondering about this cap value (it's more of a 'broad' cap than a 'bright' cap) but it sounds fine in my amp. Reducing this cap may also take away some of the extra gain.
I have also installed two 47pF caps in parallel to the 220k grid resistors of the power tubes. You might want to try this to reduce the oscillation.
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da Geezer quote:
"On the master, your guess looks pretty good, but there does seem to be some other component under that brown resistor....almost looks rectangular, like a wima type cap(?), but that doesn't really make sense, since they used Sozo caps everywhere else....."
Like to say upfront that I don't know diddly (but given have a chance would love to meet Bo), so I searched on the physical appearance and found RFI suppression caps which have a similar footprint to the appearance of the component back of the pot, some are available with a metal sheath over the plastic case.
http://www.evox-rifa.com/technote_pdf/rfi_fact.pdf
http://www.lcr-inc.com/pdf/MEY-Y2.pdf
Whether it makes sense to place one in this section of the circuit is beyond me.
Just a stab in the dark....
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Well, I'm getting really close to the tone, but have had to make some major changes to get there (as follows):
*Completely changed the tonestack to 5F6-A/JTM-45 specs. The BF stack was just too glassey & bright & lacked midrange. The new TS is smooth & full. Used a 500p treble cap, .02/.02 mid & bass.
*Changed the pentode setup to: 100k Rp / 470k Rs / 820 Rk / 2.2uf Ck / 0.1uf Cs
*V2b Rk changed to 3.3k
I am currently experimenting with adding just a touch of NFB....seems to help calm the pentode hisssss down a bit & smooth out the overdrive a little more.
Still working.....once I get the basic circuit right, then it's on to the MV (any progress, Heinz??)
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OK, here's what the 5879 Soho preamp looks like now, & it sounds EXCELLENT!!! (see .sch file below)
Once again, it is just the basic circuit (sans bump & "tweaked" MV) but it nails the tone!
It took a long time to get the pentode component values correct, but I settled on ~ 1/2 way between the AC15 values & the lower gain values that the "ValveWizard" suggested (see previous link to his article).
Once I got the amp sounding "pretty good", I remembered that the OT I had used was way oversized (and cheap, a 40-50w Weber China unit).
I subbed in an old 1963 Schumacher 18w unit I had & the amp CAME ALIVE! :o
Once you get the amp dialed in, then all you have to do is adjust your guitar volume to get jangly clean or fat overdrive....I can even replicate the "bump" function using my Barber DirectDrive set to clean boost. That kicks it up to heavy overdrive that is just like what's heard on the video.
I'm going to use the amp tonite @ a gig for the "true test", but have really high hopes. If it sounds anything close to what it does in the shop, then this circuit is a keeper!
I'll update after the gig tonite........
To recap, the attached preamp is run into an 5F6-A/JTM-45 type tonestack (500p/.02/.02/33k slope), then a 1M pot (MV), then to the PI & poweramp as shown earlier.
Oops, forgot to say that I reduced the PI plate resistors (x2) from 100k to 68k....that gave it more clarity & definition when overdriven. Also, the 470k/.01uf R/C after before the tonestack is very important. It shifts the frequency in a way that really sweetens the tone.
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After watching THIS video of Peter Stoud (http://www.youtube.com/watch?v=zc3-CiPe09I&feature=channel_page) playing one of his 65Amps "Soho" 18w'ers, I really want to build one!
*************************************
Another variation on the theme...
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Got it! IN SPADES!!!!!! ;D
After almost giving up on this project (pentode circuit values were really frustrating me!) I gave it another shot this morning & got it right!
Attached is the "final" schematic, & here are the basics (once again, this does NOT have the "bump" feature):
*** Used the "ValveWizard" values that are mentioned in an article I linked a few posts up. REALLY cleaned up the pentode & it drives the next stages into sweet overdrive in a way that is VERY pleasing to the ear.
*** Used "smoothing" caps parallel with the plate resistors of the 5879 & 12AX7 stages. That removed the harsh "hash" & gives a super sweet tone! (did I mention the tone is really sweet! 8) )
*** Have a switch on the V2A cathode bypass cap to switch a 25uf cap in/out. That gives a great range of tones with only a slight re-adjustment of the tone controls.....very high gain with it switched "in".
NOTE: I think this could be used as a "bump-like" feature if another volume & tone control were switched in at the same time....(I'll work up a drawing & see what you guys think)
***Marshall type tonestack with a Master/trim pot after....this really helps to control/adjust the amount of drive to the PI. Very useful.
This is a GREAT sounding amp. In fact, it is the best overdrive amp I've built so far! Simply NAILS the tone(s) heard on the videos of the pentode amps we've been seeing (Soho, AC15 Heritage, etc).
It's not a "one trick pony", either......wide range of tones are available, from fairly clean to blues, to heavy R&R overdrive. Really does a nice "bloom", too! Get in front of the cab & hold a chord....she just sustains into ringing overtones.....I love it!
Have I mentioned how much I like this amp (& how sweeeet the tone is)? ::)
Thanks to all who contributed, especially Heinz & Tubenit, but really everyone who chimed in with ideas!
Geez'r
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DaGeezer, I'll try to draw up a (paralleled) layout for it tomorrow and post it. Let's put this in the SCH library for sure!
Thanks for the hardwork and perseverance!! When you can, try to post some sound clips. I'm interested in what your specific amp sounds like.
Thanks, Tubenit
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Yes, Thanks again for doing all the hard work for us ;)
I have been wanting to do a pentode front, I have a few old EF86s and MANY 6AU6's around...but that'll probably be down the road. Now I've got something to copy.
3 Cheers!
j.
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DaGeezer
Can you help out please? Can you edit the B+ rail ? And I don't know if you used a tube rectifier and if so what kind?
Maybe post voltages for the major B+ rail nodes too?
Thanks, Tubenit
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Here's the updated schem (w/ B+ rail).....
I used SS recto & separate nodes for each section (so there's an additional node over Heinz's)
This used to be a 5E7, then a November, then who knows what, now it's this, so maybe not optimal setup, but it works! G
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can you guys take a look at the layout board and confirm it looks right
Looks right to me....
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hey DaGeezer,
I know you left out a "boost" but if you wanted one this would be a really easy one to try. You can experiment by simply removing the connecting wire from bass to treble and use a wire with alligator clips to hear the difference. The paralleled cap with the DPDT version really gives it a huge boost.
I use a PAB on all three of my amps cause it gives a great tone for me and it's an easy mod to do.
With respect, Tubenit
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might i suggest seeing what it sounds like when you send the ef86 output (post volume) to the pi, bypassing the 12ax7 stages (like an ac15)? if it sounds good, then it's a simple dpdt switch to have both configs available to you.
the pentode/triode switch ala vox is also a tried and true option.
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I've been playing this thing for hours! :o can't....stop.....
I've done another "tweak"....
I now have the V2A bypass cap wired to an "on-off-on" switch....
* center is "off"/no bypass cap/low(er) gain
* "up" is the 25uf bypass cap, best with the Strat/Tele/P90 Single Coils
* "down" is a 0.1uf cap, best with Les Paul/Humbuckers
Honeymoon, I know, but so far this is absolutely the best sounding amp I've ever built! There are so many tones in there by turning the gain up & the trim (into the PI) down, or vise-versa.
Oh....forgot to mention that I have installed a Cross-line MV (ala Matchless, but 500kA) and it works fantastic! Really increases the versatility...I LOVE this amp!
Sluckey is sending me an "off the wall" pentode to try in another build.......I'm building another one of these to run "Stereo" in my stereo 2x12 cab!
I'll update the schemo with the new additions..........
might i suggest seeing what it sounds like when you send the ef86 output (post volume) to the pi, bypassing the 12ax7 stages (like an ac15)? if it sounds good, then it's a simple dpdt switch to have both configs available to you.
the pentode/triode switch ala vox is also a tried and true option
I tried it that way before I found the "magic" pentode circuit values....maybe I should try it again, but I really don't think it could sound any better than it does right now (but it could be "different" in a good way...)
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> responds really well to pick attack....cleans up with a light pick, then distorts when dug into! Also responds well to the guitar volume, and it's a "smooth" type of compressed overdrive, but still retains definition....I like it alot!
Assuming normal picking and pickup, and roughly 250V supply: a triode will not overload (much), a pentode's higher gain probably can overload. And for most biases, the pentode's overload is more complex than a triode.
> I believe there was some "pentode high impedance" thing involved in this.
"Pentode" covers a wide spectrum. For maximum low-frequency voltage gain you work very high impedance and low current. For driving loads, or maximum high-frequency bandwidth, you can work a large pentode with a low impedance albeit at high current.
So "it depends".
> what I've read is that the "target" ballpark is keeping the screens @ 2/3 voltage of the plates or less.
Well, the get-it-working easy path is:
Pick a plate DC load resistor based on your audio load impedance.
Note the Typical ratio of plate/screen current, usually 3 to 5.
Find a screen resistor 3 to 5 times bigger than your plate resistor. (You see why 220K+1Meg is a common setup.)
Start with 1K cathode resistor and adjust so that plate sits "about half" of B+.
That works, is stable and consistent with supply or tube changes, will give good gain and output (after you bypass screen and cathode).
In gitar amps, "good" may not be "best" so you will be, as bluesbear wisely says, "just fiddling around till it sounds right.".
Tubes:
6J7/6SJ7 is the classical audio pentode, and IMHO 6J7 is mighty hard to beat. True, it has no glow (metal).
6AU6 is the classical post-war all-purpose sharp pentode. It is however a bit bigger than we need for most audio chores (300mA heater).
5879 is intended for audio gain applications. 150mA heater. It claims low microphonics. I've worked with a lot of 5879 gear, neither great nor crap.
> an "off the wall" pentode
Bah. 5654 is just a premium 6AK5, which happens to be (IIRC) the very first of the wretched "miniature" tubes. The "mini" idea was partly about size, partly cost, and was essential for getting gain at very high frequencies (dozens of MHz)... trying to work an Octal at 20MHz is like tapping a drum-roll with a 20-foot fishing pole. Huge quantities made for OEM and service, still available cheap ($5), and really a fine little bottle.
"Pentode microphonics" is partly about complicated internal structure, partly because they are hard to make well never mind "solid", but do not forget that pentodes can give HIGH gain per stage, and if you stacked good triodes to the same gain you might have the same microphonics.
Pentodes will always give higher hiss than a similar triode; this may be a non-issue with modern pickups.
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>And for most biases, the pentode's overload is more complex than a triode.
Yes, it's very complex....never have been able to get this tone out of a triode.
>what I've read is that the "target" ballpark is keeping the screens @ 2/3 voltage of the plates or less.
I'll try to post the voltages on the schematic today so that some analysis can be done.....
>In gitar amps, "good" may not be "best" so you will be, as bluesbear wisely says, "just fiddling around till it sounds right."
Yes, I "fidded" a bunch....almost gave up. I had read from several sources though, that "the effort was worth the time" (when it came to pentodes) and they were correct!
>5654 is just a premium 6AK5
Any suggestions on a starting point w/ the 5654? Should I just use the circuit values I have now & go from there??
http://tubedata.itchurch.org/sheets/084/5/5654.pdf (http://tubedata.itchurch.org/sheets/084/5/5654.pdf)
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I'm working on adding the voltages to the schem now.....
BTW, since most of the tone seems to be coming from the preamp (still has the cranked poweramp tone, even with the MV turned way down...has to be the pentode), then I think this amp would translate into a larger poweramp very nicely (4xEL84's/30w, 2xEL34/50w....heck, even a 4xEL34/100watter would probably be good.
Just food for thought..... ;)
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DaGeezer,
How much hiss is there at idle? And any problems with oscillation or microphonics that you hear?
I know they are different critters ...... but do you like this amp better than the Little Wing?
After you post voltages, let's place this in the SCH files? OK?
With respect, Tubenit
(I corrected the layout a few posts earlier to reflect the crossline master volume. I usually use Hoffman's 1MA pot with a spst to turn the mstr vol off/on when I use a crossline).
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Schematic with voltages is attached.
OK....now, as I was taking the voltages I found an error (a possible "Happy Mistake"?)
Look at the attached jpeg...the left diagram shows how the 470k/0.01uf r/c is supposed to be wired.
The right side diagram is how it actually IS wired.
Looks like the CF is biased really cold...is that correct? It's going thru 570k resistance to get to ground, instead of the normal 100k.
I'm almost afraid to correct it, but I will, in order to see what the difference might be in tone. It may be even better (?), but if it's not (or worse) then I'm putting it back to the "wrong" way. ;)
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How much hiss is there at idle?
Slightly more than a single triode....possibly ~ the same as paralleled triodes (not too much at all) at normal operating volumes. You can hear the hiss pretty good, though, if you completely dime all controls.
And any problems with oscillation or microphonics that you hear?
None whatsoever....I can tap on the 5879 w/ no ringing at all.
I know they are different critters ...... but do you like this amp better than the Little Wing?
Completely different, and yes, I like it much better, but this was the tone I was looking for....pretty much overdiven to some degree at all times....
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OK, I fixed the "mistake", as follows:
** Changed the 100k Rk on V2B (cathode follower) to the correct placement, before the 470k/.01u r/c network, directly from the cathode, instead of after the r/c network (see drawing above).
This changed the character of the amp in a bad way.....lost much of the sweet overdrive tone... :'(
** Remembered that I had raised the V2A Rk to 3.9k to get the gain under control. So, I put that Rk back to the standard 1.5k, hoping to put some of the overdrive back in....It worked!
The amp now sounds even better! There's more clarity to notes & especially chords. I guess that's the result of everything being biased "correctly".
WooHoo!!
I'll change the schem to reflect the different components & resulting voltage changes.
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Updated / corrected schematic>>>
EDIT: schematic and layout is now in the SCH library
http://www.el34world.com/Forum/index.php?topic=6176.new#new
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> This changed the character of the amp in a bad way.....
The output of the overloaded pentode is like a hundred volts.
The signal needed at the power-amp driver to overload the output tubes is like one volt.
So if you ever want to hear input stage overload, you NEED a BIG loss in between.
The tone-stack loss is roughly 20:1. Not enough.
Your "mistake" is a ~~5:1 divider. 20:1 * 5:1 = 100:1. Now you have the pentode overload near-balanced with output section overload. Turn it down a trice, you have whacked-up pentode and near-clean power stage. You can shift it down to whacked-pentode and wispering power stage, or up to where both pentode and power stages are suffering for your art.
Yes, the GE data for 6AK5/5456 Amplifier is a known-good starting point. Fender-fans will start at RL=100K Rgf=240K, which are the ballpark conditions Fender used, and fairly valid for high-gain audio amps generally.
This table gives conditions for 200r and 100K drive. A pickup is 5K, a tube or tone-stack is more like 100K. The differences are not huge, and probably less than the differences you'll wind-up "fiddling in" while tweaking for tone. So RK of 200 or 300 is a pile of brown-stripe resistors, 220 or 270 to start.
The higher RL values give higher gain but less bandwidth. The 510K loading for nominal gain of 390, with typical stray capacitance, will give bandwidth of 10KHz or even 3KHz. And of course a gain of 390 is way too much for any single audio stage unLESS you are looking for gross distortion. Which you are, so go ahead and whip it.
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Fair Radio will sell you pull-out 6AK5 for $8.... for ten!. $50 for a hundred. I remember 33-cent tubes when the vacuum-bubble burst in the late 1960s, happy days are here again.
http://www.fairradio.com/catalog.php?mode=view&categoryid=1417
Very early data-sheet for 403A/6AK5/5654 tube:
http://www.westernelectric.com/spec_sheets/403A.pdf
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Fair Radio will sell you pull-out 6AK7 for $8.... for ten!. $50 for a hundred. I remember 33-cent tubes when the vacuum-bubble burst in the late 1960s, happy days are here again.
http://www.fairradio.com/catalog.php?mode=view&categoryid=1417
Very early data-sheet for 403A/6AK5/5654 tube:
http://www.westernelectric.com/spec_sheets/403A.pdf
nice! :)
do you think the pentode in the link below would do well as a low watt SE for reverb or 1-2W practice amp? maybe use 6ak5 or 6au6 for driver? ???
http://tubedata.milbert.com/sheets/084/4/418A.pdf
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DaGeezer, Hope you won't mind a bunch of questions ....... I am planning to convert one of my amps into your
HoSo 56. Some of the changes in my schematic are for convenience to work with what I already have in the donor amp.
1) I was gonna use one input. Do you actually use both the high and low input?
2) I have some different tone stack pot values (like a 500k bass) Any concern .... do I need a 1M?
3) I am gonna try a PAB since it's already there.
4) I am gonna use the passive effects loop since the amp won't have reverb (gasp!) & I can use digital delay
5) I already have a NFB switch installed. With it on, it should clean up the tone some. With it off, it's like your amp.
6) Do you have both a Trim pot after the tone stack and a crossline Mstr Vol ??? Should I have both? If I
had only one ........ which would you use the Trim pot or crossline Mstr Vol?
7) You had one of the phase invertor plates showing to be hooked up to node B ......... is that correct or simply a
mistake in how it's drawn? ( Are they both hooked up to node C?)
8) I really like 2.2k screen resistors on power tubes because it adds some compressed tone. Think those will work
OK?
9) I'm gonna use a GZ34 rectifier instead of the solid state rectifier. Any concerns?
10) I am gonna use a 32uf at node A & 10uf at node E. Think that's OK?
11) I typically use Orange Drop with tone stacks and anything prior to the LTPI and Mallory150 in phase invertor.
That sound reasonable? BTW, my tone stack caps are mounted on a terminal strip soldered to the mid pot.
At your convenience, can you answer the questions? Anything else that I should be mindful of? I'm gonna order some stuff from Hoffman and get started rebuilding this amp in the immediate future.
With respect, Tubenit
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I've been following DaGeezer's progress with great interest. Unfortunately, I haven't been able to experiment for more than a week, but I finally had some time yesterday to evaluate different MV concepts. I'm still trying to make the original SoHo design work, but to no avail.
The MV before the PI has an audible influence on the distortion at higher gain settings (which is why you build this amp). Turning down the MV in this spot also cleans up the tone to some extent. Obviously, the PI contributes to the overdriven sound. The crossline MV sounds very harsh and thin when turned down, at least in my amp. I don't know if this is inherent in this design or if it is a flaw in my prototype (DaGeezer will hopefully shed some light on this). I liked a dual pot after the PI best. It sounds very natural, even at bedroom levels. Maybe this helps with question 6.
if you move the PAB-switch down below the mid pot and use a rotary switch to switch in resistors from 10k to infinity, you will have the 'original' bump feature with some in-between settings. It will probably give you more low end as well.
I have also played with large screen resistors (my pair of 6BM8s need 4k7 at 300V) and I like the compression for playing clean or slightly overdriven. Adding a cap to stabilize the screen voltage helps to define the tone at high gain settings (my personal taste). You could tie the screens together, use a single 1k (double wattage) screen resistor and add a switchable cap (a few uFs will do). It's a simple mod and you may like the effect (and revert to the original design if you don't).
Good luck with your conversion! I'd love to hear some samples when you're finished.
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1= I used 1 input/33k
2= prob OK...either more or less bass control, don't have time to calc it right now....
3= OK
4= OK
5= Good move
6= yes/both....trim pot is very important to get the tone right. MV only to get the overall volume down.
7= Both PI plates to "C"....schem wrong
8= 2.2k OK
9= GZ34 great!
10= cap values OK
11= Suspect OD's will make it brighter (?)...but should be OK.
>Anything else that I should be mindful of?
I also put some comments on the schematic thread....take a look there.
Gotta' run.....G
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DaGeezer,
Thanks man! I already removed the chassis and ordered some stuff from Doug. Looking forward to this build and will keep you posted. You sure have come up with some fantastic ideas and amps!
Thanks for all you do contributing to the forum!! I did read the SCH notes & they were helpful.
Best regards, Tubenit
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I'm a bit choke-ignorant... what would be appropriate choke specs to look for, for this power supply?
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Choke = 5H/120Ω DCR, but only because that's what I had on hand...anything close (like Doug's "small" Fender type choke) will work just fine.
Found on this page>>> http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&thispage=parts1.htm&ORDER_ID=!ORDERID! (http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&thispage=parts1.htm&ORDER_ID=!ORDERID!)
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DaGeezer,
Just getting some clarity here
Try different values for the "smoothie" caps across the plate resistors (probably lower, if you want more of the Voxy "chime")
When you say lower values ............ I am presuming you mean smaller values. A 250p would give more high frequencies than a .001 right? And .001 would allow more "lower" frequencies to pass thru?
With respect, Tubenit
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By lower I mean, as in: 250p is lower value than 500p
Don't know about the lows, but yes, a 250p would cut out less high frequencies than a 500p.
I would just start with no smoothing caps at all & add as/if needed, to your own taste & to your guitars high freq response....
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OK...."new twist" idea here, for critique & opinions.
The "Boost" cap (bypassing V2A Rk) works wonderfully, but needs some tweaking of the tonestack to sound it's best. And when it's tweaked right, it sounds incredible!
So, I thought of making it footswitchable (w/ a relay) AND.....at the same time (with another DPDT relay) switch in another full tonestack dedicated solely to the "Boost".
I have attached a schematic of the proposed mod.
In "Normal" mode, the amp would be just that....."Normal".
Then, when the "boost" is switched in, the following would occur:
#1 - The V2A bypass cap would the switched into play, and...
#2 - The "Normal" tonestack would be totally bypassed (out of the circuit) & the "Boost" tonestack would be switched "on-line".
The "Boost" stack would even have it's own "Trim" pot to preset the volume boost.
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I have attached a schematic of the proposed mod.
??? Where?
I ordered my parts from Hoffman yesterday and they came in today!!! How's that for fast service!
Thanks for continuing to share! I am looking forward to this build.
With respect, Tubenit
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Where?
Sorry, saw a mistake in the drawing, so I had to quickly delete them, make corrections & re-post....they should be there now..... :-[
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OK, you asked for some thoughts ............ If I am understanding your schematic both tone stacks are the same with one connected to a boost w/a trim pot? Is that correct? If that's incorrect, then disregard the following.
Two tone stacks which are the same seem redundant to me? Anyway to simply switch in the cathode cap and a trim pot? I'd personally try to work with one tone stack.
Have you tried the PAB idea? It should just take a couple of minutes to experiment with by simply lifting the bass to treble pot connection. I use it on all 3 of my amps and it works great for me. On a couple of amps I switched in a bigger cap with the PAB. Thing I like about it is that the PAB gives just the right boost I am looking for. Not too much or too little.
I will start my rebuild this wkend & will keep ya posted on the progress.
With respect, Tubenit
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The switchable, two tonestack thing is basically what mesa is doing with the Lonestar amps (albeit they have an extra tube stage available too).
Schem: http://www.schematicheaven.com/boogieamps/boogie_lonestar.pdf (http://www.schematicheaven.com/boogieamps/boogie_lonestar.pdf)
It is a bit redundant, but it is also kind of cool for shaping two separate tones.
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Here is another idea that may be better than the previous two ideas I shared? It switches out the ground on each pot so they are not interactive. And you could always add a cap on the trim pot for the boost.
With respect, Tubenit
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both tone stacks are the same with one connected to a boost w/a trim pot?
Well, sort of.....except both are exactly the same AND both have trim pots.
When the boost cap is switched in, it boosts the high frequencies into ear piercing levels (depending on how the treble & mids are set) and the bass can be overbearing for distortion w/ humbuckers.
However, if I adjust the treble, mid & bass controls, it has an amazingly wonderful tone!
So, the idea is to have "presets" for the T/M/B when the boost is activated, and go back to the old T/M/B settings when the amp goes back to "normal".
EDIT: the extra trim pot allows volume of the boost to be adjusted to louder or softer as needed.
Two tone stacks which are the same seem redundant to me
Yes, redundant, but necessary to achieve the tone I'm getting with the boost & re-adjustment of the tonestack, without having to actually re-adjust, & to make it instantly footswitchable.
Have you tried the PAB idea?
Yes, I tried it....unfortunately it doesn't sound anything like what I'm getting with the "boost".
The switchable, two tonestack thing is basically what mesa is doing with the Lonestar amps
uh-oh! Should I be expecting a "Cease & Desist" order from a certain "Mr. Smith"?? ;)
I guess what I was really wondering is...is there a better/more "elegant" way to do the switching?
Your last suggestion (diagram) got me to thinking that maybe just a ground lift would achieve what I'm looking for(?) & I would be able to eliminate at least one relay...I'll see if I can work something like that out.
Be back later w/ a new drawing (maybe)....
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How about this.....lifts the grounds of the unused tonestack & isolates it, so as to have no effect on the "used" stack. At the same time, grounds/ungrounds the boost cap.
At least I think that's what it does(?)
Someone check me.....TIA
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Hey, I think that looks good and would work just fine. I am taking it that you're wanting to be able to switch to boost "on the fly" and have the tone preset. If that's the goal, then the dual tone stacks and trim pots make sense to me. Unfortunately I won't have room on the chassis I have since I can only get 6 pots on there.
With respect, Tubenit
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I am taking it that you're wanting to be able to switch to boost "on the fly" and have the tone preset.
Yes, that's what I'm shooting for.
Thanks for the look over....and this is not something that's necessary at all for this to be be a great amp, just "gravy". 8)
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> do you think the pentode in the link below would do well as a low watt SE for reverb or 1-2W practice amp?
418A? That's a $50 tube, when you can find it.
If you have a secret stash under the bed, sell it.
The abuse ratings are not much different from 50L6, 6Y6, etc... the low-voltage radio power tubes. Since these are 10% the price of 418A, it seems a waste.
Where the 418A shines is sensitivity. Where the other tubes need 8V-12V peak drive, 418A can be rigged to swing wildly with 1V-2V drive. In high-frequency work where gain is difficult and costly, that can mean a big advantage. In audio amps, you always need more than one stage and with 2 or 3 stages you usually have ample gain in hand.
418A as a minimal gitar amp needs an additional gain of 30 or so. So you could do it with a half an AX7. And sometimes when you need heavy reverb, your level after the first preamp is marginal for 12AT7 or 6F6 tank-driver and may lead to another stage; 418A will whack the tank.
Except: the maximum grid resistances are quite low. We are used to pushing this spec on the big fat tubes. You gotta be careful on this one: the high gain goes with very small grid-cathode spacing. It could have quite high grid leakage. If the grid starts to drift positive, it could go into runaway and melt. A low grid resistance is acceptable in MHz and RF circuits which inevitably have low impedance (capacitive strays). But we like high impedance in audio circuits.
Unless you can get 418A for a lot less than $50, a 6F6/6V6 plus maybe half a twin-triode is a lot more economic.
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> do you think the pentode in the link below would do well as a low watt SE for reverb or 1-2W practice amp?
418A? That's a $50 tube, when you can find it.
If you have a secret stash under the bed, sell it.
The abuse ratings are not much different from 50L6, 6Y6, etc... the low-voltage radio power tubes. Since these are 10% the price of 418A, it seems a waste.
Where the 418A shines is sensitivity. Where the other tubes need 8V-12V peak drive, 418A can be rigged to swing wildly with 1V-2V drive. In high-frequency work where gain is difficult and costly, that can mean a big advantage. In audio amps, you always need more than one stage and with 2 or 3 stages you usually have ample gain in hand.
418A as a minimal gitar amp needs an additional gain of 30 or so. So you could do it with a half an AX7. And sometimes when you need heavy reverb, your level after the first preamp is marginal for 12AT7 or 6F6 tank-driver and may lead to another stage; 418A will whack the tank.
Except: the maximum grid resistances are quite low. We are used to pushing this spec on the big fat tubes. You gotta be careful on this one: the high gain goes with very small grid-cathode spacing. It could have quite high grid leakage. If the grid starts to drift positive, it could go into runaway and melt. A low grid resistance is acceptable in MHz and RF circuits which inevitably have low impedance (capacitive strays). But we like high impedance in audio circuits.
Unless you can get 418A for a lot less than $50, a 6F6/6V6 plus maybe half a twin-triode is a lot more economic.
PRR, much thanks for taking the time to answer my question(s), i'd still like to play with a pair, if for anything, just out of curiosity and doing something just a bit different. sorry to say, no stash of them.. although i wish i did. lol!
at some point in the past, you suggested using an EL84 for a reverb driver and i did just that, actually, i used a 6gk6... it took some tweaking, but i did get it to work quite well and built it into an amp that i currently use.
to me, the 418 is intriguing enough to fiddle with. if i can find any, i'll keep the forum updated on the results.
$50.00 is not a problem... i've spent much more renting a bottle of scotch :D
geezer, i should have started a new thread - my apologies to all for the thread hijack... i'll be :-X now...
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DaGeezer,
These are the two boost ideas I am considering. I probably will try the PAB with the paralled treble cap first since I have used that and really like it. Paralleling the cap can shift the frequencies to the mids (which I like more). My guess is when you tried the PAB you did not parallel a cap also?
The other is following in your footsteps but taking in account that my chassis just flat does not have room for more than 6 pots. So I will have to use a switch to adjust for boost volume and tone. I will try this one second.
Any thoughts on these? I will start on the amp this wkend.
With respect, Tubenit
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My guess is when you tried the PAB you did not parallel a cap also?
Yes, that's correct. :-[
Both of those look like good options to me.
Hope all goes well with the build....have fun!!
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I know that pins 4 & 5 are the heaters for the 5879 V1 tube.
Normally, I keep the same "color" wire on the 12AX7 tubes. Like in this example, the white wire goes to the 4&5 pins
hook up and the black wire to pin 9.
My question is should I hook up the white wire to pin 4 or pin 5 of the 5879 to minimize hum? or does it matter?
This layout is going to be my first try prior to tweaks. I'm going to try the PAB with a paralleled cap 1st since that has worked so well for me previously.
Tubenit
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doesn't matter. :)
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Stripped the amp of what wasn't needed for the conversion ..........
Then got the board built and installed, the power tube sockets rewired for EL84's, the pots stripped & remounted with buss wire, and the preamp sockets stripped ready for rewiring. Mostly just need to run wires for the preamp sockets to the board, from the board to the pots and switches and check for errors ..............
..........then the smoke test! The tubes came in today so I have the parts to fire it up. Gonna install a standby switch since I'm playing out more. If all goes well should have it working by tomorrow evening.
Tubenit
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If all goes well should have it working by tomorrow evening.
Wow, you're makin' some serious progress!
I decided to forgo the dual tonestacks on this amp (but will add it to the next one, where the chassis will be designed especially for the "HoSo56").
I found that if I adjust the tonestack "down" just slightly (especially the treble) then when the boost cap is engaged, I can get the tone back where I want for the overdrive by simply rolling back the guitar tone control a bit to cut down some of the high end!
I'm installing the relay board now (all it will do is lift or ground the boost cap)
Can't wait to hear how you like yours (or not ??? ) ;D
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DaGeezer,
There has got to be a simple solution to toning down the treble when you throw your boost switch!!???
Can you try tacking in a paralleled 500p or .001 with the boost on and see if that tone is closer to what you want??
The reason I use a paralleled cap on the PAB's (most of the time) is to knock of the treble and add more mids and warmth.
I am confidant I will like the amp. And I anticipate like all of them that I'll tweak some stuff. I really like the mid tones alot and a smooth overdrive. I think I can dial that into this amp pretty easily.
With respect, Tubenit
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There has got to be a simple solution to toning down the treble when you throw your boost switch!!???
Yes, and I think I've found it! (Thanks for the encouragement to keep looking for a solution.)
What I ended up doing was "draining" some of the highs off at the treble cap with a .001 cap to ground.
I haven't been able to crank it up to performance levels (kids sleeping) but I think I'm at least close.
See the attached schem.....
I tried the last idea you posted (bypass the treble cap with another cap) but it just gave too much mids....this amp is already "mid heavy". The cap to ground seems to drop a bit of highs without shifting the mids too much....we'll see, once I get to turn it up....
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WOW !!!!
DaGeezer, you have come up with another VERY cool sounding amp! It does sound similar to those video/audio clips. It is a pretty diverse amp. Engage the NFB and leave the other two boost switches off and it has a nice clean tone. Disengage the NFB and engage one of the boost switch, it sustains and overdrives very nicely.
Super touch sensitve and really really nice harmonics with a pretty smooth sounding overdrive. The amp does not have any more hiss than the Little Wing or the Carolina Blues Rocket at the same volumes. I like a quiet amp and the hiss is not a problem for me at the levels that I will play it at with a band.
I view the NFB switch a must on this amp for me. It offers another very very nice tone and the NFB completely eliminated the hiss making it super quiet even at reasonably loud volumes. It still left the amp very touch sensitive with nice harmonics.
I like the PAB switch with a paralleled .001 on the treble pot cap. That works perfect for me! Without the .001 it was waaayyy too trebly. The .001 leaves it with about the same balanced tone (as with the tone controls) but boosted.
I choose a 2.2uf instead of a 25uf to be switched in on V2-3 cathode. Sounds smoother/better to me with less grit for my taste.
I actually like it better without the crossline master volume engaged. But the mstr volume does give it another sound which makes it more compressed sounding and you can control the volume. I'm using one of Hoffman's 1M pots with the spst. I suspect that I will probably play it not engaged more than with mstr volume.
DaGeezer's Little Wing is a great amp. I am more wow'd by this HoSo 56T than even the Little Wing. I prefer the tone of this amp more than the other great EL84 amps that I've had a chance to really play on for a while.
More later ........... I took some voltages with a 5Y3 which I probably will continue to use. It put about 300v on the plates. My preamp volts are pretty similar to DaGeezers but I used different power rail values.
Thanks Geezer !!!!! With respect, Tubenit
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See....told 'ya!
I view the NFB switch a must on this amp for me.
I'll have to try that, although I just don't see how this amp could sound any better! I've been playin' it all day!
I choose a 2.2uf instead of a 25uf to be switched in on V2-3 cathode
Oops....I just went & checked mine....it's 1uf! I had it labeled wrong all the time, sorry... :-[
The 25u is def too much....
I actually like it better without the crossline master volume engaged.
Me too....I'm going to put a switch on mine. Like Peter says, it definately "opens it up".
On the "cap to ground" thing mentioned in an earlier post, I settled on a 390p. It gave the best balance of hi-cut to boost. If the cap is too large in value, it actually negates the boost, cutting volume & tone. I have updated the schem of my version to reflect what I have now.
IMO, everyone should build one of these (a pentode front-end). Every slight twist of either the gain or trim pots gives a new tone that is very useable......I feel that I don't even need OD pedals anymore....this amp is better than ANY OD I've ever tried.
My next project is to try one of the 5654 pentodes (that Sluckey graciously sent me) in a 2xEL34 Plexi-type amp, and also in one my Super SE amps using the same basic preamp as the HoSo.......
Havin' more fun than a human being should be allowed to have! ;D
Geez'r
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Do you think the preamp will work driving a single EL-34?
I have some EF-86 tubes and even a spring mounted socket.
Would a 12AT7/5751/12AY7 tube work better than a 12AX7? Less gain to have to deal with only driving the one power tube.
Any ideas on how to make this work for a 10 watt SE amp.
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I have been thinking of trying an EF86 amp for a while now as well and have been very intrigued by this thread. The last few days I have been puzzling over the bump channel and Master questions discussed.
Tonight I went over to the 65amps site and read everything I could about the SOHO.
Says "20 WATT (2xEL84), Tube rectified, EF86-12AX7 Cathode Follower with BumpTM circuit and Defeatable Master VoltageTM"
I was puzzled by the term "Master Voltage"... Turns out they scale the B+ with that knob to get lower volume without destroying tone. It is not a master VOLUME in the traditional sense.
As for the bump control, I wonder if it has more in common with the Hammond Organ swell circuit than a Fender tone control. It is a variable capacitor control that allows more mids to pass through as the level is increased. As it reaches max level, the bass and highs roll off a bit giving a tight compressed mid-heavy scream. I have not looked at the pics of the guts of the SOHO yet but I will soon and see if I can learn anything.
I am even more interested in pursuing this project than before and am so impressed at the skill of all of you in coming up with schematics and prototypes in such a short time.
Keep it up!
-Matt
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This project is looking pretty good. I am trying to round up the parts I need to build mine. You guys rock, as always. Jmac
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In another video on YouTube, Peter Stroud mentions a 6-position rotary tone control. I have a rough schematic put together that I need some help with. I have no idea how to switch between the two "channels" if you will. and I am not sure of the values everywere but it is a start.
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PRS_Matt,
Can you post your schematic in a GIF or Bitmap format? Also, did you read all of the thread on this amp? We may have possibly already covered some of the ground that you're referring to??
Macula56, If you don't have the trannies yet, I think Hoffman's 18w trannies with a 5Y3 would be excellent. You can change the B+ rail closer to what I used to get the same preamp volts range.
With respect, Tubenit
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thanks Tubenit. I want to build something with 4 EL84's. I started another thread about that as I was looking for info on OT's for that project.
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Tubenit, I put up an image of my draft.
I did/have been reading through this thread for several days now. Not so say I have not overlooked something or forgotten some details. I have spent the last several minutes re-reading some of the early posts and realize that a lot of this has been covered. Sorry :-[
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PRS-Matt,
Looke at where you have the B+ hooked up on the EF86. It needs to be at the juncture of the 330k & 2.2M resistor.
You're asking the right questions!
And it looks like you're retracing some of the ground we covered. I can't remember for sure but I posted several schematics early in the thread that I later deleted & I think one of them was pretty similar to what you've drawn? I think Heinz ruled it out because it created way too much gain? Not sure about that though.
With respect, Tubenit
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Thanks tubenit. I just stumbled on one of your schematics from somewhere early on in this discussion and found nearly the same design.
Too much gain from the CF or from the gain stage after the TS?
I will adjust the V1 B+ connection. Thanks!
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Well, looks like I'll probably be building another HoSo56 real soon.
I took the prototype up to my friend who owns one of my "Little Wing" amps, the same guy who did the sound clips for me. He played the HoSo about 30 seconds and a big grin came across his face! ;D
We only had a few minutes...he tried it with a Strat. I told him it really screams with Humbuckers & P-90's....and after we unplugged it, I remembered that he didn't even turn on the "boost", so he has something more to look forward to!
I'm going to finish all the "final touches" on the HoSo Proto & take it back to him to use for a week or so. If he likes it as much after that as he did today, he's going to order one. I think he will ;)
Also, I'm getting ready to try one of the 5654 pentodes tonite....hope it goes as well as I expect.....G
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Various "other" pentodes tested tonite & here are the results:
Note: all tubes were tested with the exact same circuit values as the "original" 5879 pentode.
**5654(AKA 6AK5 / EF95) - http://tubedata.itchurch.org/sheets/084/5/5654.pdf (http://tubedata.itchurch.org/sheets/084/5/5654.pdf)
More hi-end & seemingly more gain than the 5879. Also seemed to be more sensitive to variations of input signal gain. I think some tweaks of the circuit values will be needed to get the absolute best tone out this little tube. That said, it sounded excellent! Just rolled off the treble a bit & backed down the gain & it sweetened right up. Also, ZERO microphonics, which would be a big plus if used in a combo. Tapping on the tube yielded complete silence! Another "plus"...they are ~$5 each just about anywhere on the net!
A Big "THANKS" to Sluckey for providing the test units of the 5654's!
**6AU6(A) - http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AU6A.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AU6A.pdf)
Note that this tube has the same basic pinout as the 5654 (see above)....just connect pins 2 & 7 on the socket & you can pop in either one!
This was the best sounding of the (3) tubes I tested. I had the same basic tone as the 5879, but in the smaller 7-pin format. No circuit changes are needed, it has great tone with the same values. Only darwback seems to be microphonics.....several of the 6AU6 examples I have were noisey if tapped, but I found a 6AU6-A that was almost silent, so good examples are possible. ~$10 each.
**6SJ7 - http://www.pmillett.com/tubedata/HB-3/Receiving_Tubes_Part_2/6SJ7_6SJ7-GT.PDF (http://www.pmillett.com/tubedata/HB-3/Receiving_Tubes_Part_2/6SJ7_6SJ7-GT.PDF)
Octal pentode with metal jacket....high on the "cool factor".
Tone was the "weakest" of all (3) tested, but still an excellent sounding tube with seemingly less gain than the others tested. I think with some circuit changes it would sound excellent....seemed to have more "chime" when the treble was boosted. Microphonics were again kind-of high on the several examples I tried, but some were better than others. There is a MilSpec version that is "Red" in color out there (5693) that has better results on the microphonics, but are $35 each! Ouch! http://thetubestore.com/nos-5693.html (http://thetubestore.com/nos-5693.html)
The 5879 is still the hands-down winner in this particular amp, but as I said, I think all of these can be tweaked to give super tone.....as has been said, the effort is well worth the time!
Geez'r
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DaGeezer,
Had a chance to play my 56T again tonight. Cool amp! I really like it alot. Playing it, I'm not sure what I could tweak that would make it sound any "better"? It is remarkably expressive. Very nice tone for playing slide too.
If the 250p cap across V2-1 were removed ...... would that remove some treble? I won't have a chance to experiment for a few days with it, so I thought I'd go ahead and ask. That's the only thing I can think of that I might try at this point?
Thanks for the additional info you provided about your friend's response & also about the other tubes. . I'd love to hear your friend record "Spanky Blues" (or something comparable) with the HoSo 56 for a comparison to the Little Wing.
I am dialing my amp with volume on 7, treble 3, mid 7-8, bass 6-7, trim about 5-7 and mstr vol around 4-5 to get my favorite tone/s so far. Switching the 2.2uf cathode cap on & no NFB. Guitar about 8. Very little hiss at all. Less than the Princeton Reverb I used to own with those settings. I also like the NFB engaged with the guitar on 9-10 quite a bit.
What are you using mostly?
With respect, Tubenit
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If the 250p cap across V2-1 were removed ...... would that remove some treble?
No, that would add treble. Try a 470/500p.
What [settings] are you using mostly?
I really vary it alot (there's so many cool tones in there!), but I suspect fav would approx be:
Gain-5, Treb-3, Mid-3, Bass-4, Trim-8, MV-6
On the MV, I had a 2Meg pot that I put in there & it works great without having to switch it out circuit. I can turn it up all the way, then disconnect it from the circuit & there is no difference. It still has a good, smooth "sweep" also, so that's what I'm going to try for a while.
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On the schematic, do you have a "B" where you should have a "C" on the lower part of the 12AX7?
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Geeze! I got very interested in this amp you have designed. I have never met a EF86 amp which I liked. This could be the one. Today I went to my component supplier and by chance he had four 5879 NOS tubes in his stash. I bought all of them with to my mind reasonable price (some 12 USD/tube). Now my question is: Can you put that tube to V1 instead of EF86 without modifying anything in your schem? Harry
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Harry,
Now my question is: Can you put that tube to V1 instead of EF86 without modifying anything in your schem?
If you're asking if DaGeezer's schematic can be used for a 5879 in V1. That is what he is using in V1. He is not using an EF86. Hope that helps.
http://www.el34world.com/Forum/index.php?topic=6176.0
With respect, Tubenit
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hows about a 6SJ7? does that require and extra's or is it interchangeable?
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:)Thanks Tubenit! I am very interested in to hear also your opinions of different tubes in V1 position like DaGeezer described earlier. I do not have any deep experience of using this kind of tubes in preamps. A friend of mine just built a Matchless Clubman and he was suprised how good sounding it turned. It has also EF86 as the first preamp tube. Harry
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harryk,
This is my 1st experience with any kind of tube like this and it's a favorable one. I have used a paralleled 12A_7 on all my scratch amp builds prior to this.
My impression is the Matchless uses a EF86 in the V2 position with a paralleled 12AX7 in V1.
http://www.el34world.com/Forum/index.php?topic=597.0
http://www.schematicheaven.com/newamps/matchless_clubman.pdf
With respect, Tubenit
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::)Either I have to go to eyedoctor or I have a worse and worse dementia because I do not see anything and do not rememer anything anymore :-[. Luckily we have more fresh people here to guide us loser like me ;). Anyhow I have to build this amp this spring in any case. I also wait some sound clips of your amps. Harry
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hows about a 6SJ7? does that require and extra's or is it interchangeable?
I think that (6SJ7) was covered a little ways back....10 posts up, under "pentodes tested"
Harry, this is also my 1st experience with pentodes @ the input.....so glad I "went for it"!! This amp has the most "fun factor" of any amp I've ever built, or even played....
G
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On the schematic, do you have a "B" where you should have a "C" on the lower part of the 12AX7?
I thought we fixed that! :-[
It should be "C" on both plates of the PI.....
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I think that (6SJ7) was covered a little ways back....10 posts up, under "pentodes tested"
yeah, maybe if i tried reading instead of asking, lol.
thanks Geez, Jmac.
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> all tubes were tested with the exact same circuit values as the "original" 5879 pentode.
That's unfair. Like testing my ThunderBird with Honda-size tires. Or trying a fine Gibson acoustic with head-banger rock strings and titanium pick.
With "same" values, one tube may be happy and another may be jammed in a corner (plate voltage way high or low).
That may cover some of your observation about 6J7. It is a good tube but "smaller" than those RF pentodes. It may be starved, barely working right. It sure "can" have as much gain as the others, although you may have to raise all resistances to find it.
Throw the DVM on the plate and adjust cathode resistor so all plate voltages are "similar" within 20%.
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That's unfair. Like testing my ThunderBird with Honda-size tires. Or trying a fine Gibson acoustic with head-banger rock strings and titanium pick.
;D Exactly .....that's why I noted that no circuit values were changed.
Throw the DVM on the plate and adjust cathode resistor so all plate voltages are "similar" within 20%.
I will hopefully have time this weekend to try it again with that method....
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I love pentodes on the front end but the cap across the Rp makes me wonder if this might be a good place for a cascode- lotsa gain but with less pronounced high-end. Should I volunteer?
jamie
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Ordered an RCA 5879 today....
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Ordered an RCA 5879 today....
Cool!
I'm making the faceplate for my amp (former Epi VJr head) right now..... ;D
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Somebody else's take on a 5879-based amp, with youtube demo videos:
http://ampgarage.com/forum/viewtopic.php?t=8138
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HotBluePlates gave me a capacitor substitute box that sure helps when tweaking amps!
I changed two caps on my 56T. The paralleled treble cap is now a .002 instead of .001. And the cap going into the phase invertor is now a .01 instead of a .0047. This allowed more mids and more overdrive and a smoother sound to my ears. It will get a pretty decent Billy G type sound with the PAB switch on.
DaGeezer mentioned that he couldn't quit playing his amp. I'm having the same thing happen. It's a great amp and is definitely super touch sensitive and expressive. Probably the most "fun" amp I have played. I'm using a delay in the passive effects loop and the amp sounds huge for a small amp.
With respect, Tubenit
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Hi guys,
My eyes are all blurry from trying to read all of this thread over last night and today as I have great interest since I have made three EF86 amps over the last 6 months.
This has sounded very close to my second amp which I ended up using the EF86 switchable into a 12A_7 (X,T,U) cathode follower stage or input directly into a single EL84.
The EF86 has a pentode/triode switch option going into TB stack which is also bypassable. This yields more control of the EF86 as well when you don't want so much gain for jazz/country playing. The difference in my amp is that I made the tone stack and switching ability between the cathode follower so that I had control of the pentode tone when not going through the 12A_7 (which does give instant levels of more gain) for versatility and my own experimentation. After the 12A_7 CF stage it has a volume and treble cut similar to the Vox AC Heritage amp (this is what inspired this design in the first place) and also a bright switch on the volume pot which helps when using a Les Paul / warmer guitars. Overall I think it's pretty similar to what's going on here w/ great designing at work by DeGeezer and Tubenit. Great job you two (and others)!
Great thread everyone!!! I'm really stoked to see this. BTW for those worried or thinking EF86's have too many issues, in my last amp I double up on using two EF86's - one driving the switchable tone stack and another used as a driver stage both w/ their own volume pots - a truely fun and totally simple but effective little practice amp. Depending which tube you drive more it's really got a great vibe. It could be a great higher powered amp IMHO but that's another thread/story.
Cheers, Joe G :)
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in my last amp I double up on using two EF86's - one driving the switchable tone stack and another used as a driver stage both w/ their own volume pots - a truely fun and totally simple but effective little practice amp. Depending which tube you drive more it's really got a great vibe. It could be a great higher powered amp IMHO but that's another thread/story.
Welcome Joe!
Do have a schematic for that amp?(or any of the other amps you referred to?)
Thx, Geez'r
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Sure can but I don't want to mess up this thread since I'm new and I don't want to step on anyone's "guitar chord" err toes. Let me know what's appropriate and also what file format should I scan with to be able to post?
Thanks for the welcome :)
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I would love to see any SE amps you have made especially with an EF86.
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Joe,
We're interested! Post in a GIF, JPEG, etc.....
When you try to post it, it will show the type files and size allowed.
Thanks for sharing and welcome aboard the forum!
With respect, Tubenit
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Sorry for the delay as I hadn't drawn up a corrected version from when I finished this. It's as close as I remember and w/ a little picture help I think it's pretty accurate? I wanted to show the amp I was telling Doug H about when I was ordering parts and never got around to doing it and visiting the forum until now (and also 2 more amps later :-[). I've learned so much in the last couple of days from you guys, it's been eye opening. But, when I was involved w/ this there wasn't any info that I could find really except some version I remember that had a "squishy" control for the EF86. I tried it originally but it was like a compression effect and I didn't end up using it. I started out having that, a tremolo circuit, and a pentode/triode switch for the EL84 w/ this but aborted that all as well and ended up w/ this instead. As I'm sure you all know, funny how things don't end up where your intentions usually begin?!? Anyways, I hope this can add a little bit to the forum amp fun?
Quick description: EF86 w/ pentode/triode switch going to TB or bypass (w/ a little low end filtering here for this at higher levels) going directly to EL84 output OR switch this output into CF stage (this sounds great w/ 12AX7 or 12AT7) to kick things into higher gear then this output has a treble bleed and volume control going into EL84. Any constructive criticism is welcome. Thanks.
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Thanks that's exactly what I have been looking for as a start to an SE project using the EF86.
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I hope it will work out well for you. The name is a bit lame but it was basically built w/ the AC-4 circuit in mind originally. I didn't want to call it a JC-4 because it might make someone think of the Roland JC-120 or whatever and I didn't want to conjure up any thoughts of a solid state amp :o
Please email me when you're done, I'd like to see another and what you came up with. Thanks.
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Well, that's it!.....this thread is "officially" the largest thread ever on the Hoffman Forum (more posts & views than any other!) THANKS to all who have contributed!
I am currently working on a version using 6BM8's for the PI & output tubes (ala Little Wing), so it will be the "HoSo" preamp into the LW poweramp. I'm hoping for good poweramp overdrive without the need for a Master Volume.
It will also have the ability (via a "parallel" wired 7-pin V1 socket) to run the 5654 & 6AU6 pentode tubes.....
I'll post results in several days.
Once again, just to make it clear (for those of you considering building this amp) that this is absolutely the best amp I've ever played....either current production/vintage/modern/boutique/younameit!
I simply can't stop playin' it...it's THAT good!
G
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The EF86 tubes in the amps that I've built are little tone bottles of pleasure. My recent posted amp and current newest double EF86 amp has me feeling the same way as yours does for you. In fact while I was reading this thread originally, I was doing so plugged in at the same time, ;D To see what you and the others were doing and conjuring up was like a shot of adrenaline as I've spent a lot of the last year messing around w/ this tube.
What was your latest/last final schematic? I think I DL'd appx. 20 of them between you and tubenit getting neat ideas along the way. It sure is a great design idea and would be a fun build & nice amp to own.
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I actually amp going to use an 18 watt chassis I got on ebay cheap and see if a champ type PT fits.
From right to left will be input-multi position switch-tonestack. Some of the holes for input jacks could be used for mini switches.
I was think of having 2 preamps to choose from the EF86 and a 12AX7 with different inputs.
I'm going with an EL34 and all handmade SOZO caps.
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Tubenit and DG, I have a couple quick questions as I put together a parts order to get started on a pentode preamp project of my own...
Where are you sourcing 2.2uF caps for the cathode bypass caps on V1 and V2? The only ones I can find so far are $20+ Hovland MusicCaps and $30+ Audience Auricaps. There has to be an affordable option somewhere.
Tubenit, are you using a SPST switch and a DPDT switch right next to each other in the PAB part of your circuit?
I am excited to get going on this!
Thanks,
-Matt
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Bigdaddy - all sounds good!
Matt - these caps are non-critical. You can go to Radio Shack or any local electronics supply if you want and get polarized or non-polarized for a dollar - maybe get two in a pkg?
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Ahem, Doug has em for .75 cents a piece. I heard he sells parts.. ;)
Regards,
Dyna
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My bad, thought that would be looked at first. :-[
Hey Mo! I can't see, I can't see...
Why not!?!
I got my eye's closed - nyuk nyuk ;)
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Where are you sourcing 2.2uF caps for the cathode bypass caps on V1 and V2?
Look near the bottom this (linked) page, under "More caps not listed above" >> http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&thispage=parts4.htm&ORDER_ID=!ORDERID! (http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&thispage=parts4.htm&ORDER_ID=!ORDERID!)
If you don't already have some, go ahead & order a few 1uf & 4.7uf also.....your "taste" may want a little more or less gain in those positions & it's easy to just "clip" different caps in to see the differences. Don't be afraid to experiment w/ different values. Even a 25uf is not out of the question to try.....
My bad, thought that would be looked at first. :-[
No Prob! We just like to keep reminding folks that "our sponsor" has a great selection of parts & the absolute best service anywhere around! ;)
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The latest (& I anticipate final) version of my amp is right here:
http://www.el34world.com/Forum/index.php?topic=6176.0
I got my 2.2uf caps from Hoffman.
PRS-Matt, I am not sure exactly what you mean by the spst and dpdt right next to each other? If you mean literally the answer is no they're not but having them next to each other would be fine. I use the PAB playing quite a bit. I tend to leave the 2.2uf (spst) on most of the time.
As a side note, because my amps all have the passive effects loop ........ I can experiment with the preamp of one going into the power amp of another. After experimenting, I plan to change the preamp of my Carolina Blues Rocket into the 56T preamp. The 56T (5879) had a fatter sound & was more touch sensitive to playing, but not as much overdrive as the CBR preamp.
With respect, Tubenit
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Once I got the amp sounding "pretty good", I remembered that the OT I had used was way oversized (and cheap, a 40-50w Weber China unit).
I subbed in an old 1963 Schumacher 18w unit I had & the amp CAME ALIVE! :o
Can you elaborate on this, and explain the change made it come alive?
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The smaller OT (about 1/4 the size) just gave the amp more "feel", which I suspect is simply more output distortion(?) The BIG Weber OT (intended for a Fender Bassman/Bandmaster type amp/2x6L6's) just didn't let the little EL84's breathe, I guess......
G
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Doug has em for .75 cents a piece.
I missed them all the way down at the bottom. I just looked in the section with all the other small value e-lytics. OOPS!
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Sounds like the littler output transformer is probably like you said distorting (saturating?) compared to the big iron, but hey in the amp world the right kinda distortion is a good thing no matter where it comes from. I like the new 5e3 head I did but even though its only a 20watt transformer (ot) the 1600 series of Hammons are probably way overkill, my other one sounds good with a universal 125e but doesn't have as much clean as the new head, then again I'm running 6l6's but I think thats less of a factor, the 6v6's still sound cleaner in the new head than with the littler Hammond iron. With el84's I've had good luck with a fender deluxe reverb replacement tranny which was cheap and works great for those. Happy accidents are what this amp stuff is about sometimes I think (what little I know of it). :D
Regards,
Dyna
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PRS-Matt, I am not sure exactly what you mean by the spst and dpdt right next to each other? If you mean literally the answer is no they're not but having them next to each other would be fine. I use the PAB playing quite a bit. I tend to leave the 2.2uf (spst) on most of the time.
I was completely confused by your answer so I went back and looked at the schematic, then compared it to the layout. It seems there is a switch in the schematic that is not in the layout. I have attached examples. Please feel free to call me a moron if I am still missing something.
Thanks,
-Matt
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PRS_Matt,
Yikes an error in the schematic drawing. There is no spst below the dpdt.
Sorry 'bout that, Tubenit
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No worries :) Just making sure I was not missing something.
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Ordered an RCA 5879 today....
Cool!
I'm making the faceplate for my amp (former Epi VJr head) right now..... ;D
What transformer setup are you using on this thing Geezer?
Greg
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I don't know what DaGeezer used but I'd think Hoffman's 18w PT and OT that he sells would be a great match for the build!
With respect, Tubenit
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18w OT for sure would be great, Doug's "Mojo" 18w is very similar to old 60's Schumacher unit I'm using.
The PT on mine came out of a 60's Hi-Fi amp, & I think it's actually 250-0-250, because the Little wing uses a 250-0 PT with a SS bridge recto, & I'm getting the exact same B+ rail voltages on both amps.
Tubenit is using a 270-0-270 PT & getting great result, also...so I think your OK with anything from 250v to 275v 2ndaries.
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A note to those building this amp (or any amp w/ 5879 pentode):
In adapting this preamp to the Little Wing, I copied the B+ rail exactly. All the voltages were correct (nodes A, B, C & D) with the exception of node "E", which feeds the 5879. It was approx 163v, just slightly higher than the HoSo voltage. I thought "it won't make much difference"....oh yes, it did!
The amp had much more gain (in a bad way). I could only turn the gain pot up 1/4 way before it was totally distorted.
So, I changed the 5k dropping resistor between nodes D & E to a 22k. This brought the plate voltage on the 5879 down to HoSo levels (~138-140v).
Totaly changed the character of the amp! Still have a few "tweaks" to do, but the amp is currently VERY close to the HOSO tone.
Moral of story = watch the 5879 plate voltage! Keep it below 150v, preferably in the 130-140v range for the best tone.
Geezer
ps...I will draw up a "Little Wing Five" (pentode) schematic as soon as I get a change (and have it "finalized...yeah, right ::) ) & post it.
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Interesting info, DaGeezer! Thanks for sharing it. I've got 158v on my 5879 plate and have the volume at 7 with it. It doesn't seem too distorted to me. However, I only have about 300v on the EL84 plates. I'm guessing all of those interact somewhat. Maybe it's a difference between the 6BM8 triode/pentode vs. 12AX7 & EL84?
I won't have time this wkend, but I'll experiment with lower voltages on the 5879 and post what I find sometime next wk. Maybe it will improve an already excellent tone!
With respect, Tubenit
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Please give me feedback on this: when dealing w/ preamp pentodes, if you increase the Vp, don't you get more "gain" but in a "cleaner" less distorted way? However, you're saying that you "lowered" it and it cleaned up the distortion?
For my only pentode experiences (EF86), when the Rp was 100k I got cleaner sound than when I used a 470k it became more distorted.
When I left the Vp alone and changed the screen grid resistor's values - going down to 470k gave much more distortion and then going higher the tube cleaned up a lot more - I think I experimented up to 5.6M?
Another thing I found that was interesting was that when I lowered it to appx. 1k, the amp got a spongie/sag kind of feel and using the standard 2.2k to 3.3k gave much firmer/normal response.
Is the 5879 tube different than the EF86 in any of these ways? Also, aren't most if not all pentode's basic behaviors similar from one to another?
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I think one of the factors in talking about this issue to consider is that gain and distortion are not the same thing, if I am understanding that correctly? And DaGeezer was commenting that the higher voltage had more gain and then distorted "too early".
I think I might know what he is referring to? At times when I've experimented ....... I would find the voltages changed the distortion from a harsh to a sweet type of distortion. And I found it wasn't so much the amount of gain that I wanted to dial in but the type of gain meaning did it have the best tone to my ears.
Hot Blue Plates gave me a resistor box where I can hook it up and simply dial a variety of plate resistors in while using a 25k pot to dial in the cathode resistor value. I've done that on a few amps and found I could find what was a tonal sweet spot for me. I think I'll try that approach on the 56T 5879 tube. However, it sounds fabulous as it is already.
With respect, Tubenit
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Please give me feedback on this: when dealing w/ preamp pentodes, if you increase the Vp, don't you get more "gain" but in a "cleaner" less distorted way? However, you're saying that you "lowered" it and it cleaned up the distortion?
jojo, thanks for you help here.
I'm a real newbie when it comes to pentodes in the preamp, so anything I'm doing is just trial & error. When I adjusted the Vp (which was the only thing that was different than the "good sounding" HoSo amp) it "sweetened" the tone/distortion to a more pleasant type.
For my only pentode experiences (EF86), when the Rp was 100k I got cleaner sound than when I used a 470k it became more distorted.
When I left the Vp alone and changed the screen grid resistor's values - going down to 470k gave much more distortion and then going higher the tube cleaned up a lot more - I think I experimented up to 5.6M?
Yes, I suspect that (screen grid resistor's value) is where I should do some adjustment to find the sweet spot....it's very close right now, just needs a little more adjusting. It still breaks up much earlier on the gain pot dial than the HoSo, and it's not the power tubes (6BM8's), it's definately the preamp distorting.
Another thing I found that was interesting was that when I lowered it to appx. 1k, the amp got a spongie/sag kind of feel and using the standard 2.2k to 3.3k gave much firmer/normal response.
Did you mean to say "1Meg", 2.2 & 3.3M?? I don't see where 1k, 2.2k, or 3.3k fit in here(?).
If that's what was intended (which I suspect, typo) then that makes some sense to me. I'll try raising the Rs value & see what the results are.
Thanks again.......G
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Another thing I found that was interesting was that when I lowered it to appx. 1k, the amp got a spongie/sag kind of feel and using the standard 2.2k to 3.3k gave much firmer/normal response.
I had alot of "drive time" today w/ work & was thinking about this....are you possibly referring to the Rk/bias resistor??
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While comparing this amp's schematic to the layout, I noticed that one of the caps is different from one file to the other;
in the layout, the cap on lug #3 of the board is a 1uF, whereas its corresponding cap in the schematic is .1uF
Which would be the correct value? or would it matter? Another also, is on lug 14: schematic, .047uF; layout, .0047uF. which is the correct value?
Also, the trim pot; I'm assuming it's a hidden trimpot, and not on the front panel, as the layout would seem to suggest?
One more thing; the bright switch -- DPDT -- the way it's drawn suggests that in one position, neither of the caps are in the circuit, and in the other position, they're both in parallel to ground. Is this the case?
S^G
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While comparing this amp's schematic to the layout, I noticed that one of the caps is different from one file to the other
The last schematic on the schem thread ( http://www.el34world.com/Forum/index.php?topic=6176.0 (http://www.el34world.com/Forum/index.php?topic=6176.0) ) has the correct values for my version. I've not been updating the layout...just sub in the values from the schem.
Also, the trim pot; I'm assuming it's a hidden trimpot, and not on the front panel, as the layout would seem to suggest?
No, it's on the front panel....it's very important in being able to adjust the signal strength/gain into the phase inverter, to control PI distortion.
The "trim" designation was just something I called it....it could be called "volume" (so then you'd have "Gain/Volume/Master Volume"), or "Drive", or anything you like.
One more thing; the bright switch
Mine doesn't have that feature....Tubenit will have to answer that one.
Geez'r
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Another thing I found that was interesting was that when I lowered it to appx. 1k, the amp got a spongie/sag kind of feel and using the standard 2.2k to 3.3k gave much firmer/normal response.
I had alot of "drive time" today w/ work & was thinking about this....are you possibly referring to the Rk/bias resistor??
Yes the cathode's bias resistor! Sorry if it wasn't clear before. I originally thought that I could lower it for changing the gain like a 12AX7 but pentode's gain doesn't work the same way.
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One more thing; the bright switch
Mine doesn't have that feature....Tubenit will have to answer that one.
Geez'r
Sorry.. after looking at the schematic again, I realize it's the boost relay, relay 1a, 1b, not a bright switch... I think Tim Horton's slipped me some decaf this morning... yeah, that's what I'll blame it on... ;)
Aimed at both Geez'r and Tubenit, as well as anyone else who has built this amp: --> Do you have any pics detailing the inside of the amp, that might help in supplementing the layout? (as well as show the newcomers "how it's done")
S^G
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Thx jojo.
I have been trying everything (different Rk/Rp/Rs for the 5879) on the 6BM8 version & keep coming back to the original values as the best > Rk=470R/Rp=56k/Rs=470k.
I think I've finally got it right (tone-wise) by going back thru the B+ rail & getting all the voltages correct at each node by adjusting the dropping resistor values to get the correct voltages all down the line. I started by measuring node "C" (as there is not much I can do about nodes "A" & "B"....the plates & screens). The "C" voltage was too high, so I increased the 10k up to 15k. Then I checked nodes "D" & "E"....they were now at correct voltage (per the HoSo voltages I had measured & noted on the schematic). That greatly helped to bring the tone more in-line with the original EL84 amp.
As with most higher gain amps, small variations seem to have large effects on tone. Alot of tone tweaking may be needed with each indivdual build of this amp to get the best sound results.
Another change I've made (for the 6BM8's) is a different primary imp on the OT. I originally had an 8k OT in there, but was getting some harshness, so I tried higher & lower imps. I had an old OT from a Fender Musicmaster Bass amp (1978) that is supposed to be 8k, but I had measured & marked as actually being 11k primary......put that in & it's the best sounding of all the 6 or 7 units I tried.
The 6BM8 version is definately lower output than the EL84's. I did put a MV on it (had the old "Presence" control hole to fill) but find I have it "off" or fully dimed most of the time. Of course, I will probably still want it for use in really small/quieter venues.
I'll post a "6BM8" version of the schematic soon. I'm in the process of tweaking the "boost" circuit values now & am getting "pretty happy" with the overall results, although the original EL84 amp still has the better "tone edge" ..... as of right now......still working ;)
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Do you have any pics detailing the inside of the amp, that might help in supplementing the layout? (as well as show the newcomers "how it's done")
S^G
I don't (although Tubenit may)...I'm not posting any pics of mine :-[ .
They wouldn't help you much anyway. It's a "prototype" unit & very messy inside (especially compared to my normal standards...see below ::) )
Right now, I wouldn't recommend this build to anyone not experienced in full scratch builds....it's still in the evolutionary development stages & so far needs lots of "tweaking" to sound it's best.
I'll see if I can do a layout that acurately depicts what the amp layout "should" look like, although Tubenit's drawings are usually pretty good to go by. If I do it, it will probably be later this week.
Here's what my amps usually look like (& the prototype doesn't!)
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All I need is a schematic that has the latest correct information on it, maybe also w/ voltages would be helpful. I'm not sure that I know which of your guys latest drawing is being used? However, I do have a brand new 5879 tube on it's way :), not sure if I'll use the 6BM8? Not sure why this and not EL84/6V6 (not that it's a big deal at the moment)? The only issue is that the tube site NJ7P doesn't show much in the way of specs for this tube, especially for class AB1 use...
I'll check back in a couple days (funeral out of town). Thanks guys.
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My final schematic and layout (and DaGeezer's final schematic of the HoSo 56) are here:
http://www.el34world.com/Forum/index.php?topic=6176.0
This layout board follows the layout I drew with the except of the B+ cap to the 5879 being on the board.
With respect, Tubenit
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Quick question... Schematic shows 5Y3 rectifier and 32uF first cap. I thought the 5Y3 should only handle a 20uF as a first filter cap.
Am I missing something?
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Fair question about the 32uf and 5Y3. It has worked for me in a couple of amps with no problems so I will continue using it. I've seen amps that were build that exceeded the rating of a particular rectifier tube significantly. I had a Bogen CHA-20 that was a good example of that.
You can always use a 20uf in that place & it will still sound good.
With respect, Tubenit
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Thanks Tubenit. I just soldered the 32uF in the chassis yesterday afternoon--just figured I should ask before firing it up.
Another question. I am using a 6AK5 for V1 (cute little tube) and in order to make the most of its own design, I grabbed the data sheet for it. http://tubedata.milbert.com/sheets/137/6/6AK5.pdf One column reads 120V for both plate and screen and the next column reads 180V plate and 120V screen. Which way should I go there? Shouldn't the screens be a fraction of the plates by like a ratio of 5:1 or is that plate/screen current?
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The Hoffman Stout uses a 40uf filter cap on the first node off the 5Y3 so it must be ok.
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You guys using newer 5y3's or NOS? I just wondered, urban legend was that nos stuff didn't like bigger caps much, true or no? Just wondering... I know like the sovtek can take it (not as much voltage drop and I've had a 50% failure rate, but the 2 good ones worked fine).
Regards,
Dyna
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You guys using newer 5y3's or NOS?
I am using NOS GE and Sylvania. Guess we'll see how they handle 32uF... :)
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You guys using newer 5y3's or NOS?
I am using NOS GE and Sylvania. Guess we'll see how they handle 32uF... :)
Leo's 5F2-A had two 16uf filter caps in parallel for the first node. He sure wasn't using Sovtek tubes! ;) Effective capacitance is 32uf, but I don't know whether or not having two caps in parallel reduces stress on the rectifier tube.
Chip
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Tubenit
Hot Blue Plates gave me a resistor box where I can hook it up and simply dial a variety of plate resistors in while using a 25k pot to dial in the cathode resistor value.
While you were dialing the pot to smack the sweetspot did it ever made scratch noises?
I´m considering something along this lines but as a permanent feature. I also considered the possibility of using a rotary switch - make before break - but a pot would be the ticket...
Thanks in advance
With Respect
Best Regards
Rzenc
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I don't remember if it made scratchy noises dialing that pot? The Soma 84 out of TUT 5 uses an interesting treble boost that uses a pot that is dialed. I had something similar on the original Carolina Blues Special.
I'd probably use a 25k pot paralleling a resistor if you were going to try that.
With respect, Tubenit
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I have always without fail used a 40-20-20-20 can with any P-P amp using a 5Y3. That's probably 25 amps. There's never been a problem with the 40mf. The only rectifier tube I've ever had fail was a 5AR4. It started arcing after 4 years of 3 to 4 nights a week of jobs. Sadly, it was a Mullard. I'm willing to bet the 40mf didn't have anything to do with it. It just got moved around and shaken up too many times. Few of those gigs were more than 1 night per venue (that'd be too easy).
Dave
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Would this preamp design work well with 6V6's? Or is the 18watt PI and power important to the sound?
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Tubenit is using it w/ 5881's & seems like it even better!
http://www.el34world.com/Forum/index.php?topic=6316.0 (http://www.el34world.com/Forum/index.php?topic=6316.0)
I have just about got it tweaked into the Little Wing/6BM8 format. I think you could sub in 6V6's with very little changes in circuit values, if any were needed at all.
I'm thinking of a quad of tubes......a pair of 6V6s (or 5881's) & a pair of EL84's, cathode biased in pairs w/ the ability to switch off each pair or combine both pairs.
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Yes, it works beautifully with 5881's & I do like them better than the 6V6's which also sound good. The CBR-5879 can change out between those two with no rebiasing. I've tried them both but keep coming back to the 5881's which have better harmonics. The 6V6's do break up more quickly.
I love this amp & really feel it's the best 5881 amp I've built to date. Certainly the most touch responsive and expressive.
Having said that, ........ IF I could only build one amp with the 5879 ....... it would definitely be DaGeezer's HoSo 56.
;)
With respect, Tubenit
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Anyone contemplating a 4xEL84 version? I've got a good, working layout that can be adapted to a 2xEL84 version easily that is also a bit different from Jeff's. It's not drawn, but I would take the time to do one up, if anyone is interested. (I need to practice making my express drawings look as good as these other guys'.) Craig
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Sure, why don't you post a layout if you time and are willing to. Not very many guys seem to use the paralleled terminal approach that I use & your layout may be more to their liking and/or a better layout.
When you're done, just put it in the SCH file for DaGeezer's HoSo 56
With respect, Tubenit
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Tubenit, your layout looks great as usual, perfect for those universal-type terminal boards like you used, or terminal strips, and best for tweaking values until you get it right.
I'll post mine in the .sch area when I get it drawn up.
Craig
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My 6BM8s came in. Any word on the 6BM8 version? Just paste the hoso preamp onto the Little Wing power supply and power amp?
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Please post your layout CraigB. I am building a 4 EL84 version and would like to see what you have come up with. Thanks, Jmac.
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My final schematic and layout (and DaGeezer's final schematic of the HoSo 56) are here:
http://www.el34world.com/Forum/index.php?topic=6176.0
This layout board follows the layout I drew with the except of the B+ cap to the 5879 being on the board.
With respect, Tubenit
(http://www.el34world.com/Forum/index.php?action=dlattach;topic=5901.0;attach=7904;image)
This answers the question I was gonna ask on this, it appears those cheap tagboards can be used quite well for this amp. Think I've seen em for like 6-7 bucks, I still would like to build one of these sometime.
Regards,
Dyna
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Those tag boards are "OK" if you don't experiment and remove and replace parts very much. If you do alot of tweaking and experimenting the flimsy tags rip off. One of my boards is pretty beat up looking from all the experimenting & soldering.
But if you know exactly the layout you're gonna use, they are reasonably easy to work with if all you need are paralleled terminals in your layout.
Something like Hoffman's turret boards would be sturdier and less prone to problems, I think. I think if I were building for someone else who played out frequently, I would NOT use the tagboards.
With respect, Tubenit
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Thanks for commenting on them, I wondered since they were pretty cheap. Theres other options, I know were to get the same using just eyelets with another row down the middle I think.
Regards,
Dyna
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Quick question on the Root 666 designed posted earlier in the thread. What power transformer would work for that amp? Are they using something like a JCM900 100 watt PT to get that 370-0-370 vac?
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My 6BM8s came in. Any word on the 6BM8 version? Just paste the hoso preamp onto the Little Wing power supply and power amp?
Sorry for the delay....very busy lately (Thankfully!)
Yes, to start, just take the HoSo pre & paste it onto the LW power section, which would include the 6BM8's triode section as the PI.
In tweaking this amp, it was a bit of a challenge & took a long time to get right. It seems the 6BM8's take less signal to drive them into overdrive, as there was too much distortion at 1st. I tried different PI values & such, but ended up using a 12DW7 (ECC832 JJ) in V2 to get the gain down to acceptable levels. That tube (if you're not familiar) is 1/2 12AU7 & 1/2 12AX7, so that dropped the gain a good bit & got the amp to sounding more like the EL84 version. I also raised the Rk on V2A to 3.3k to further clean it up.
There are probably other ways to go to reduce the drive to the power section, but being pressed for time, & it worked very well, I stuck with the tube substitution instead of stressing over it too much. I realize that most folks aren't going to have a 12DW7 around, so you may want to try other methods of signal reduction(?)
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Theres other options, I know were to get the same using just eyelets with another row down the middle I think.
I usually put another row of Hoffman's eyelets down the middle, but only in the holes where I need them.
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Yeah the one in question has turrets with a row down the middle, its a universal board and a bit pricey, if I were going to do it I'd probably just get some eyelets and board from Doug and do my own. I know some don't like eyelets, but I don't mind them and they're easier to set with the modest tools I have. I find a wide pointed centerpunch flares the backside just fine, I also do the board ghetto style with a hand drill and no drill press, I'm in a APT so it makes it hard to have a drill press around like I did in my Pre-divorce days. Provided you get a clean bead of solder I haven't had problems with them. (If you tweak things all the time, then yeah Turrets are the way to go, I tend to get it working, tweak a little and then button it up so I can play it out)
Regards,
Dyna
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[/quote]
I realize that most folks aren't going to have a 12DW7 around, so you may want to try other methods of signal reduction(?)
[/quote]
Thanks DG. I really appreciate your help.
When I started the amp hobby I hacked up an Eico Stereo 40, and have a couple 12dw7s left over from that. Finally--something to use them for. The transformers are doing very well in the world's only dumble clone with a cathode biased 7591 power amp.
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What manufacturer 5879's do you guys have? I've got a few Philips ECG and a couple RCAs, which look like they were all made in the same factory, or at least with the same materials. Interesting to me is that each one of them sound a bit different from the other, more so than the difference between different 12AX7s, for example.
Anyone achieving success with other pentodes, like 6AK5 and 6AU6, etc.?
I'm hooked playing into a pentode. Theres so much complex ring and zing and makes even the most difficult parts to play seem easier to do.
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Well, ....... I had one of my 5879 tubes go very very microphonic. Never heard anything like that before with a tube. I view it pretty much unuseable at this point. It was in my 56T combo.
So I am interested also in knowing what brands might be best. Mine was a Sylvania.
I'm also curious what "makes" a tube go microphonic. The tube started out just perfect. I didn't play the amp for about a wk & then when I fired it back up it had the microphonics. It seems like it was not a gradual change but a sudden shift to being microphonic.
BTW, my son was in town & he had a chance to play all three of my amps ........ and his favorite was clearly the 56T! He thought it was the most expressive. Second favorite amp to him was the "Blues amp" (schematic in SCH library).
With respect, Tubenit
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Found this explanation:
"Loose electrodes, the metal pieces inside a tube which conduct electricity, are affected by vibrations generated by the loudspeaker. This causes the space between the electrodes to vary, and as a result the electrical fields to vary within the tube. This changes sound into current vibrations, which is positively fed back through the tube, and an annoying howl results. These vibrations can affect the electrodes through the socket or the base, or by sound waves striking the glass envelope of the tube.
"To help control microphonics Fender surrounds some tubes with a spongy, sound-absorbing material. In other amps and old radios mounting the sockets on rubber may also help. If both of these precautions fail the tube is too microphonic to be used, and should be replaced. You don't necessarily have to throw it away, as it may work fine in an amplifier with a separate head and cabinet."
REFERENCE: Marcus, Abraham. "The Electron Tube: Service Notes" Radio Servicing: Theory and Practice, Third Edition, pg. 165, par. 2, Prentice Hall Inc, 1960.
One of my Philips 5879's was destroyed when I cranked the GA40 for the very first time :'( I've had the same thing happen with EF86. I now make a shockmount subchassis for my preamp tubes, and actually on this build made a subchassis for the 4xEL84 too. Works like a charm. I will post a photo so you can see what I mean.
Glad to hear your son got to dig in and try the 56T and others. Your combos are really cool with that nice woodworking. I can see from the picture of the 56T how the soundwaves from the speaker would tend to pummel the 5879.
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OK, it's obviously a limited "test" but I compared the 5879 tubes I have. 2 Sylvanias, 2 RCA and a Raytheon.
Both RCA's sounded darker and perhaps smoother but had the least character/expression to them. The Sylvanias had the best chime, expression and harmonics, IMO ..........but one of them has gone microphonic in a relatively short period of time. The Raytheon sounded reasonably close to the Sylvanias and also had nice harmonics but perhaps not as chimey sounding. I thought there was a pretty noticeable difference between the RCA's vs. Sylvania & Raytheon.
Not much of a review but I thought we could start comparing some observations about the 5879 since it's kind of a new tube to many of us. Anybody else have a comparison or description they could share?
Tubenit
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> I thought the 5Y3 should only handle a 20uF as a first filter cap.
The capacitance hardly matters. If you have enough uFd to hold ripple to usable level, you have BIG current surges.
What matters most is resistance. Some bottle-rectifier datasheets spell this out. A complete calculation is mind-bending. However a "right size" power transformer intended for tube rectifiers -probably- already has a reasonable amount of resistance. If it is a copy of a PT from a tube-rectifier amp, size suitable to the rectifier you want to use, you can probably use 16uFd or 60uFd. If it is like the lump I just found in my office, 2 Ampere rated output, even 2uFd will probably weld a 5Y3's guts together.
> One column reads 120V for both plate and screen and the next column reads 180V plate and 120V screen. Which way should I go there?
Neither. Look at the currents: 7mA! You almost never want that much current in an audio voltage amplifier. Reverb Drivers rarely suck that much. (However if you did need a gain of 100 with a 20K output impedance, and did not mind the brutal drain on your B+, it works.)
I lost track of what plate resistors we are using. But since audio preamp loads are in the general range 100K-500K, reasonable plate resistors are 50K-500K. Typical preamp B+ voltages are 200V-400V. Plates and resistors usually split the B+. So typical currents are 200V/50K= 8mA to 100V/500K= 0.2mA. 1mA-2mA is most common.
The 7mA suggestions are for choke loaded amplifiers. Chokes are very much better loads BUT at audio frequencies a "good" choke is not very good and very expensive. 6AK5 and kin are aimed at radio frequency stages, where a great choke is a few pennies.
So the Typical Operation conditions on the 6AK5 sheet are NOT directly useful for you.
And the difference shown for 120V versus 180V is very-very small. Most parameters change less than 5%, when real tubes vary 20% one to the next at the same conditions. It isn't clear why they show the two different voltages. Mostly, pentode plate voltage does not matter for small-signal work. But maybe the forms looked bare with just one set of conditions.
You will probably want MUCH lower G2 voltage. How much lower is hard to say.
> Shouldn't the screens be a fraction of the plates by like a ratio of 5:1 or is that plate/screen current?
The plate/screen current ratio WILL be around 4:1 for 6AK5 at any reasonable plate voltage. The screen is a picket fence with 80% open space. Throw BBs at it, how many go through and how many hit pickets? A high wind in your favor (high plate voltage) may funnel a few extra BBs between pickets; wind in your face (low plate voltage) and more BBs may veer into the eddies behind the pickets.... but as page 2 (http://tubedata.milbert.com/sheets/137/6/6AK5.pdf) shows, the plate/screen current ratio is very consistent right down to where plate current collapses.
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I read through the whole topic, and I think none of you guys tried a vox top boost tone stack, any particular reason why that is so? I was actually pretty much convinced that the 65 amps were using a variation of a top boost but with a ef86 as the first gain stage rather than a single triode.
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Hmmmmmm........sounds logical, since 65 pretty much says straight out that they have used Vox amps as a model for their amps!
Maybe I'll try it in the 7W version.
Or, you could build one & let us know how it works out! :wink:
Gz'r
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Hi Geezer, well everytime you see Dan Boul in his youtube videos talking about a new amp release he seems to have his book of vox by his side, and I've seen from other videos he is quite keen on vox amps... besides their first amps were pretty voxy inspired (soho included). I have a ac15 twin from music ground JMI, a sort of clone of a vintage ac15 twin, and I can tell for a fact that the london's ef86 channel does get similar tones, however that's a totally diferent front end from the soho.
As a matter of fact I'm just about to build a prototype with this type of front end just for kicks, however I will be using a brown vibrolux output/power transformer replacements, and make it usable for 4x6v6s or 2x6l6s (mostly 6L6s I guess), but if I can share what conclusions I come to I'll be happy to. I'll only start building after the 18th of september, as I'm on vacation right now. I'm guessing this will turn out, if everything goes right, much closer to their stone pony or monterey models, which is fine with me, since the stone pony has become a favourite of mine among their videos.
thanks
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the stone pony has become a favourite of mine among their videos.
I'll have to check that out......
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I read through the whole topic, and I think none of you guys tried a vox top boost tone stack
Isn't it the only difference between the Vox tone stack and the marshall (HoSo) stack the treble capacitor (50pF Vs. 500pF)???
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Hey Bruno,
I know you tend to like to design your own amps ........ however, it sounds like you're considering something similar to my
Carolina Blues Rocket (at some level).
I have a SCH schematic and layout drawn right here:
http://www.el34world.com/Forum/index.php?topic=6368.0
I have not added the VVR yet, but plan to in the next few wks. I did add it to the 56T and it works fantastic, IMO!
Let us know how you like your build when it's done.
With respect, Tubenit
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hey tubenit,
I do love trying to come up with my own things, but I also love trying out others designs, like your carolina blues special, with the dano centurian tone stack, I've tried it in a prototype chassis. Wonderful tweedy sounds, although the tonestack is a little different than what I was used to.
For this build I'm not considering anything special, I just want to try the front end of the 65 amps, and since the only set of transformers I have laying around the house is a brown vibrolux set, I guess they'll have to do. So yeah, the schematic you posted is pretty much what I'm after, although I'm going to install Dana Hall's VVR just like you did and like 65 amps uses. I'm going to scale only the inverter and the power section too I guess. I'm leaning towards trying the Vox tonestack though.
The differences between the marshall and the vox stack are not only the 47pf, if you check the schematic. However a cool ay to see how both react is the tonestack calculator.
thanks for the schematic tubenit, I have most of the components here which were spares from other builds. And I have a bag full of 6PS series caps, so I guess I'll use those. The only thing I need to buy are octal sockets. I even have a aluminum chassis ready.
PS: I'm not too sure why you guys use 33k to simulate the 68k resistors, in a single jack wiring, the 68ks purpose is to have the low input, if you don't use that input the 33k while emulating the traditional wiring, simply degrades sound, why not just leave it out? I've done so in several builds, one less part right?
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the 33k while emulating the traditional wiring, simply degrades sound
Not sure what you're saying here??? Paralleled 68k resistors are 34k which is close to 33k.
How does that degrade the sound? It's what you hear plugging into many Fender amps.
With respect, Tubenit
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well when fender used the two 68k it was in order to obtain the low input, if there is no low input, then no resistor is necessary at all, the only reason you guys are keeping it is to simulate the effect of having the 68k. A resistor in the signal path there simply degrades signal (which was a necessary bad thing in order to get that second input, but no in this case), and since the main function of the 68k resistors is to obtain the low input and there is no low input in a single input amp, well it just seems pointless to me to use any resistor at all.
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it just seems pointless to me to use any resistor at all.
Bruno,
At my home if I use a 10k input resistor ....... I sometimes pick up radio waves thru the amp. A 22k or larger eliminates that.
With respect, Tubenit
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Hi tubenit,
well a shielded wire between the jack and the first stage grid will acomplish good isolation too... however in good truth I never had problems even without using the shielded wire and no resistor. But I guess it depends on the zone you live in.
I have even built an amp from a trainwreck express schematic, which has tons of gain, due to the way the amp was designed. No resistor between input and first gain stage's grid. No problem with radio interference. I even have a youtube clip of the amp being played really clean. But I've played it under all kinds of circunstances with no problems.
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Bruno, a point you may be missing is that series resistor (33K, 47K, 68K, whatever) and the interelectrode tube capacitance (grid to cathode) form a high frequency shunt filter. Hi freq parasitic oscillations or radio freq interference (RFI) that may be picked up by your guitar or amp input is shunted to ground through that hi-freq filter and the low impedance cathode circuit. This resistor is sometimes called a grid stopper or suppression rersistor. This could be important to you if you are playing in the vicinity of strong RFI. And including that resistor even on a single input amp will not degrade the guitar signal, unless there's another resistance involved that forms a voltage divider. Even then, I wouldn't call it 'degrading'. I'd just call it 'dividing'. :grin:
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Hi sluckey,
I'm well familiarized with grid stoppers... I just find them a bit pointless in a single input design unless you really are having problems with external interference. And then shielded wire helps there. A voltage divider is a different thing alltogether as your whole signal is dropped before hitting that first stage.
Just another idea, obviously, in testing we're basically free to do as we please... any way thanks guys!
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Well, you wanted to know why we use them. That's why. Plus, my brother-in-law works for Ohmite in the resistor painting section. He's in charge of the orange paint gun. :grin:
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Lots of external interference where you live then, hey? :P
So that must mean free resistors, lucky you, care to send me a few?
I'll tell you how the build went later on, might even try 7591 tubes much like the stone pony or 60s ampegs.
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Hey Bruno,
Enjoyed your video clip. Amp sounded good to me and very expressive. Liked your blues rifts and rhythms also. Good stuff. Thanks for sharing it.
Best regards, Tubenit
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thanks tubenit, I'm actually a lousy player...
I built it, but the amp isn't mine, it's too loud for my purposes... besides it can do high gain stuff, this video showed the other side of the spectrum as I felt not enough videos had been done on trainwreck style amps clean tones. I usually stick with amps that have less gain, although I really like the trainwreck.
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Lots of external interference where you live then, hey? :P
Yep. This is the noisiest planet in the Sol system.
So that must mean free resistors, lucky you, care to send me a few?
All I can get are orange. Sure you want some? :wink:
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Lots of external interference where you live then, hey? :P
Yep. This is the noisiest planet in the Sol system.
well you never know, now do you?
So that must mean free resistors, lucky you, care to send me a few?
All I can get are orange. Sure you want some? :wink:
I'm all stocked up to get me through this build, thanks!
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hey dyna, I don't want to drift this topic away from the subject, so I can open up a new topic regarding the trainwreck copy I built with a couple of photos if you'd like.
However... I was playing at home, at very low low low volumes, and with a stratocaster with JM rolph 64 pickups, which are like a vintage strat and very low output. So yeah this thing makes chimy tone, it's a very dynamic and versatile amp, the most dynamic I've ever heard to be honest. It does get into very high gain territory.
please procede with the hoso topic...
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No problem, I'll remove it, sorry for the hi-jack!
Regards,
Dyna
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I've been scrutinizing the pictures linked earlier in the thread. I really want to try the circuit out with the ef86, as I already have that tube.
I found a few things that seem different to me, than the posted schematics.
To me it seems that the cathode resistor on the CF is not 100k. Comparing it to the 100k resistors to the right of it (the plate resistor for v2A and the "slope" (?) resistor for the tonestack), it seems to me that the color rings are: brown, grey, yellow, gold = 180k.
Now, I'm not too good at the theory here... what will the difference between 100k and 180k on the cathode do to the sound?
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Hi ,
I get myself registered here to thank you all for that great thread (and forum ) I've been reading for a while now . I've just finished building an amp based on that topology (pentode/triode/CF/tonestack) with a SE 6V6 . Sounds huge ! the pentode has a great unsual sound . I started with a cap selector (as the "FAC" in old orange amps , and VOC ac15 I think) but I just took it off this evening , didn't bring something new to the sound.
I'm still working on it , devil is in the detail :laugh: , but I'll post a schematic once it's done !
I'm also very interested in 65 amps designs , seem to be "original" designs (which is pretty rare in "boutique" market , they're all clones of something "vintage" ) . I"m trying to figure out what it's the "ventura" , the Tupelo/lil elvis could be also a great stuff to know :icon_biggrin:
fuzz
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I'm still working on it , devil is in the detail :laugh: , but I'll post a schematic once it's done !
Welcome fuzz and why wait?
Why not post it now and what your thinking and what your looking for? The guys love the process too as much as the end product.
You don't wanna steal away their fun, do ya???? :undecided:
Start a new thread on it.
Brad :l2:
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I'm just a new kid here but I stumbled upon this one by accident and I'm really happy with the results...I shoved this Bogner Ecstasy overdrive circuit into a Sovtek Mig 50 in place of the pc board preamp and just left the power stage in tact and WOW....not too far off of that heavy overdrive sound near the end of this clip. Build it on it's own separate board and plug it in...If you don't love it,,,sell it to me (for parts cost only, of course :icon_biggrin:)
Use this preamp, and you will not be sorry:
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This can be fun but nothing really new there SG. Typcial cascading gain stages to CF design. I find it interestingly somewhat alarming, poorly designed, and excessive use of parts. Why put in high-end dumping techniques and then use high pass filters all at the same time? And why use voltage dividers which also affect tone and high end loss when split-load plate resistor set-ups could be used w/out these detrimental affects? But then who am I to criticize? :w2:
*BTW, how did this thread suddenly turn from EF86 amp project...design input sought to HoSo56 amp project...design input sought???
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You know Keo, I was just trusting that Reinhold knew what he was doing...
I should've also checked the date and the direction of this thread before posting....chalk it up to a rookie mistake.
This is my first forum, and you guys have my full attention.....I can't get off the site because I'm learning something new every minute.
I got lucky with one build and I'm tryin to shove this preamp down everyone's throat :l2:
I'll get back to my shopping cart and try not to hijack any more threads :icon_biggrin:
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I'm not saying he doesn't know what he's doing, just that his circuit has contradictions going on and that I doubt Leo or Jim would've done the same if not for parts count alone. They were ingenious yet very frugal at the same time.
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I'm still working on it , devil is in the detail :laugh: , but I'll post a schematic once it's done !
Welcome fuzz and why wait?
Why not post it now and what your thinking and what your looking for? The guys love the process too as much as the end product.
You don't wanna steal away their fun, do ya???? :undecided:
Start a new thread on it.
Brad :l2:
yes why not after all !
I make it clear and post it !
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Hi guys,
I didn't forget about the schematic but I can't draw a clean scheamtic about this amps. I'll try to get a clean hand hand drawn one and post it (plus my camera is really annyoing with batteries) . Unless there's a website allowing drawing schematic ?
I'm also thinking about putting a switch to get the pentode alone into the cathodefollower + TS for a "clean" and sending it into the 12AX7 for a kind of dirt side (or gain boost).
The EF86 (6N32P in my case) + 12ax7 is really crunchy even at low gain settings.
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https://www.circuitlab.com/ (https://www.circuitlab.com/) Hi Fuzz, that is a web-based schematic program, free to use, you just have to register. Then there is also Express PCB which is a free download, easy to learn (even for me) and most everyone on here uses. Hope this helps! :smiley:
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https://www.circuitlab.com/ (https://www.circuitlab.com/) Hi Fuzz, that is a web-based schematic program, free to use, you just have to register. Then there is also Express PCB which is a free download, easy to learn (even for me) and most everyone on here uses. Hope this helps! :smiley:
The best part about ExpressSCH besides being free is that tubenit already has SCH schematics for many amps. Go check out the Schematics section here...
Chip
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https://www.circuitlab.com/ (https://www.circuitlab.com/) Hi Fuzz, that is a web-based schematic program, free to use, you just have to register. Then there is also Express PCB which is a free download, easy to learn (even for me) and most everyone on here uses. Hope this helps! :smiley:
really cool webiste ! i tried to use "webschematic" but didn't get something clear as I wanted. But there're no tubes or my eyes are tired ?
I remeber that I used to use expressSCH on an old machine , really easy software . I must have it on the hard drive .
I'll try to be quicker with that schematic !
I browsed few days ago and I was happy to see that there're a lot of good original projects in here !
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I know this is old, but I'm curious if you ever got to this project and finished it. Also, It seems to me that the circuit would go more like this...
EF86-12ax7CF-tonestack-12ax7-PI-el84's. The way I heard the amp, and peter stroud explain it, this is the way I would assume the bump circuit is done, again my assumption, Bump circuit is they put a resistor underneath the mid resistor to make slight boost, and more mids and warmth, also disabling the rest of the tonestack from there. Then using a relay to switch that resistor in, they use another pole of the relay or a second relay to connect in the tone control. Same with the level, relay that in, and it goes from there. I don't think they are actually switching between two different circuits in the tonestack section.
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I'm interested in this topic, too, since I need a new project after I finish my first build (modded 5E3)!
This is an awesome work you guys have done. I wish I could find sound samples and/or videos on YT.
Apparently, the most complete schematic (leaving HoSo closer to the original) would be the HD VVR Draft #2, with the Mosfet based VVR (Dana / Hall style) and the bump selector and level knob.
Rectifier tube should / could be EZ81 as the original (any other changes needed?).
I searched around but could not find additional comments on this version, nor a layout file that would be definitely handy.
In case we get a final version of this and layout, I can add a BOM file.
Thanks again to all contributors.
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A recent video with Dan speaking about this amp.
http://www.ustream.tv/recorded/37928597 (http://www.ustream.tv/recorded/37928597)
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Apparently, the most complete schematic (leaving HoSo closer to the original) would be the HD VVR Draft #2, with the Mosfet based VVR (Dana / Hall style) and the bump selector and level knob.
I personally would pass on that one and build Geezer's version. I'd use a 6AQ5 or 6V6 for my personal preference but the EL84 version is great also.
And I have built amps with both VVR and PPIMV that I liked. However, on this amp, I'd use the PPIMV instead of the VVR.
With respect, Tubenit
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Thanks, tubenit!
I'm not really familiar with PPIMV. Is it that 2.2M pot between the 12AX7 and EL84s? It's a dual gang pot?
Regarding the Bump circuit, do you think the one in the schem I sent is more complete?
I'm still noob in this, but it appears to me that it has Bump level and Bump tone dials (as in the original) and this schem you sent has two foot switches, one Boost (? middle?) and a Treble boost, as well as a Trim dial (volume?).
Thanks, again.
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It's a dual gang 250k pot with 2.2M resistor from wiper (middle terminal) to ground.
I'd use Geezer's bump/boost design instead. You can always leave him a PM IF he doesn't chime in on this one.
It's Geezer's design and he has done ALOT of experimenting with it.
With respect, Tubenit
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Sorry if this has already been addressed, but Geezer's 3-18-09 "Final" schem shows a .1uF cap on the 5879 screen, but his layout shows 1.0uF. I'm assuming .1uF is correct as it's more or less in line with the RCA datasheet.
Just checking!
About to begin a HoSo56 variant build with a post-TS second 5879 driving a SE KT66 in Ghetto UL. Naturally I don't expect it to sound anything like Geezer's. Just need a break from building Dumbles, and this is using all major parts I already have, so no big $$ layout.
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.1 is correct.
About to begin a HoSo56 variant build with a post-TS second 5879 driving a SE KT66 in Ghetto UL. Naturally I don't expect it to sound anything like Geezer's. Just need a break from building Dumbles, and this is using all major parts I already have, so no big $$ layout.
Well that sounds very cool! Can you please share the results when you get it done and let us know how you like it.
As an FYI, I have found 5879's in an OD position can benefit in smoothness considerably by adding a cathode follower. You could even use a mosfet CF in that manner. (take a look at the D'Mars ODS as an example)
with respect, Tubenit
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Thanx tubenit! I thought so.
I'll be happy to post it when it's done. Here's the current schem, I figure there's enough voltage out from the second 5879 to overcome impedance mismatch losses and still give the KT66 enough oomph to get 10 or 12W out of it.
If not, I bought some 6U8A to replace it and do the CF into the KT66.
The Ghetto UL I ran into on Ampgarage, it's a PP OT feeding the screen from the primary CT, then the plate. Not shown but I have a small 20H choke to put behind the KT66 screen gridstopper, to smooth out the screen supply and cut the voltage down to about 300V.
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Thank you so much! I'm pretty much computer illiterate I'm afraid. I can use PSUD 2 and TubeCad and SE Cad. I will try to learn .sch though, it will take me some time I think.
V2 I changed to a 12DW7 because the 12AU7 makes a better CF. V2B cathode 39K or 47K. V3 screen cap is .05uF, to cathode not direct to ground. Cathode bias resistor is 1.1K (two 2.2K in parallel), bypass cap 4.5uF (nominal 3uF). This is set up to maximize voltage out, not gain. V1 is set up for max. gain. KT66 cathode resistor will probably start as 300r, as I'm told 250r is pushing the tube too hard (Shuguang KT66). 10uF nominal bypass caps I have measure 12uF. Voltage D in PS about 250V.
I've also been advised to try .033 TS caps in place of .022 because the SE output can use the extra lower end. Makes sense but I'll try it with .022 first.
Choke is Triad C7X 10H 90mA. Dropping resistors 8K2, 12K, 15K.
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FogHornLegHorn,
Thanks for the response. I've edited it. Any other edits needed at this point?
The ExpressSCH is super super easy to use. Very intuitive. It would take you all of 5 minutes max to use it given your skill sets with those other programs. :icon_biggrin:
Just click on a component and change the value when/where needed.
NOTE:
Not shown but I have a small 20H choke to put behind the KT66 screen gridstopper, to smooth out the screen supply and cut the voltage down to about 300V.
With respect, Tubenit
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FogHornLegHorn,
Are you on TAG? And if so, what's your handle on that forum?
Just curious.
with respect, Tubenit
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Hi tubenit,
would you have by any chance the layout for the DG 56 HoSo? (the schem you sent recently)
Thanks!
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You're new here, so ........................
:icon_biggrin:
PLEASE download ExpressSCH which is a FREE download (if you have not already done so).
http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm (http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm)
I drew up Geezer's version with a PPIMV in the layout for you.
With respect, Tubenit
IF you're interested in other versions of the original HoSo56 (such as with 6BM8 tubes) look here:
http://www.el34world.com/Forum/index.php?topic=6176.0 (http://www.el34world.com/Forum/index.php?topic=6176.0)
IF you want a version with "pseudo channel switching" and a layout look here:
http://www.el34world.com/Forum/index.php?topic=10999.0 (http://www.el34world.com/Forum/index.php?topic=10999.0)
IF you want a version with 6V6's and an extra "gain stage boost" (schematic and layout) then look here:
http://www.el34world.com/Forum/index.php?topic=15863.0 (http://www.el34world.com/Forum/index.php?topic=15863.0)
IF you want a version that will support 6V6's and 5881's and has reverb then look here:
http://www.el34world.com/Forum/index.php?topic=9049.0 (http://www.el34world.com/Forum/index.php?topic=9049.0)
LOTS of options for you!
With the ExpressSCH program, you will have access to 100's of editable schematics and layouts here:
http://www.el34world.com/Forum/index.php?board=12.50 (http://www.el34world.com/Forum/index.php?board=12.50)
With respect, Tubenit
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Yes, my handle on TAG is my real name, David Root. Thanx again for your invaluable help with this project.
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Yes, my handle on TAG is my real name, David Root. Thanx again for your invaluable help with this project.
David, welcome to EL34 World!
I've appreciated your posts on TAG. You've generously offered great information & I am impressed with what I have seen about your builds & insights into building. Thanks for joining us!
With respect, Tubenit (10thtx on TAG)
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You're too kind! One thing I try to do which I find really helps is read the tube data sheet closely (no s**t Sherlock).
But all datasheets are not equal, for example the GE 6L6GC sheet has far more info than any other.
Closer to what I'm doing now, I had been using the Tung-Sol 5879 datasheet, but I recalled that the RCA sheet had recommended to ground pins 2 & 6, because that will reduce hum level from the AC heater supply by shielding grid #1 and plate from the heater.
Pins 2 & 6 are marked NC so I can't see how this would work if they're not connected to anything, but, hey, these guys knew what they were doing.
This morning I re-read the RCA sheet and the very helpful bullets on the maximum voltage chart that explain how the screen, cathode bypass and plate coupling caps' values were derived. Very useful in picking the right values for a driver application, which is the second 5879 in my current build. I had read these before but it didn't sink in until now.
As it happens it looks like I have used lower cap values that will be good for overdriven condition ie cut bass below 250Hz or so. How they will sound in clean mode remains to be seen, but I cut only the bypass caps, the coupling cap is as per the chart.
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Nice... I'm going to start buying parts.
The schematic has SS rectifiers, which I think will be great to save a little on the tube side (and possibly PT).
What diodes are those?
What are good specs for the PT and OT? (current, taken the voltages in the schem are ok)
After I build it, is it "easy" to include a reverb if I already consider the space for tube and tank?
thanks!
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Which version and power tubes?
6AQ5, 6BM8, 6V6, 6L6 or EL84?
We need more information from you.
I found it reasonably easy to add a one tube reverb, so if you have space and would be happy with a subtle reverb, then I think it's doable.
With respect, Tubenit
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I'll build the 56 & PPMIV you posted recently, with EL84s.
I hope the PPIMV has the crunch that the VVR gives to the original 65s.
If this gets good, it will prob be my main amp, so I'll leave some space for reverb, that I like, as well as send / return, footswitch, etc.
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The HoSo schematic (HoSo & PPIMV) shows the filter capacitors of 40uF, 20uF, 16uF and 8uF. I know that even though not common here in Brazil, 16uF is common in amps.
I know I can use 22uF caps in the place of 20uF, but not sure if using 47uF won't be a too big difference in the place of 40uF.
I could not find 40uF or 20uF caps even on eBay, except some engine / machine starter capacitors like this one
http://www.ebay.com/itm/AC-Motor-Capacitor-Washing-Machine-Start-Capacitor-CBB60-450VAC-40uF-/110906187167?pt=LH_DefaultDomain_0&hash=item19d2861d9f (http://www.ebay.com/itm/AC-Motor-Capacitor-Washing-Machine-Start-Capacitor-CBB60-450VAC-40uF-/110906187167?pt=LH_DefaultDomain_0&hash=item19d2861d9f).
Is that right? Should I use one of those??? Seems quite weird.
If so, how to place them on the chassis?
thanks a lot!
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IF your rectifier can handle 47uf use that.
22uf instead of 20uf is fine.
10uf is OK if you can't find 8uf.
Their not that critically important, IMO.
With respect, Tubenit
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Thanks, Tubenit!
I'm also trying to figure out which diodes to use as SS rectifier. I understand that with SS rectifier I might need a PT with less voltage than would with a tube rectifier, right?
Will a 500v PT provide me with de 350v B+ using SS rectifier? Will these diodes handle the 47uF cap you mentioned?
This is my second amp project and I'm sorry if I'm asking basic things. I really want to build the HoSo56 and there aren't many pages on this amp.
I know I'll post many questions as I move forward, but I already know I'll get stuck when defining the Choke (which to use?), Relay (is that even a component or a circuit I need to build)? and some components that are in the schematic that are not in the layout (maybe taken for granted...)
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Hi Marco, if by 500v PT you mean a 250-0-250, then yes that'll give you around 350 unloaded; actually a bit more because our wall voltages are often more than 120. The diodes won't have a problem with any size reservoir cap you want to use. Use the diodes that Doug sells, the 3A/1000 volt jobbies. Probably more than you need but at .50, pfft.
Use the choke that Doug sells, it'll be fine. I believe there is a BOM somewhere on here for the HoSo.
As far as the relay goes, *if it were me* I'd leave a little room for the components, but first get the amp built and working the way you want, and then add the relay. I put the relay in from the start, and spent mucho time trouble shooting my mistakes with it. Eventually I disconnected it and voila, the amp was quiet as a mouse and sounded great.
One thing to note is that humbuckers will sound much darker through this amp (built to schem) than single coils, so you may want to play with the cathode bypass caps. The 5879 gives it a nice smooooth sound that I'm sure you'll love!
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... The way I heard the amp, and peter stroud explain it, this is the way I would assume the bump circuit is done, again my assumption, Bump circuit is they put a resistor underneath the mid resistor to make slight boost, and more mids and warmth, also disabling the rest of the tonestack from there. Then using a relay to switch that resistor in, they use another pole of the relay or a second relay to connect in the tone control. ...
Sounds about right to me. What I *think* I heard in the video is a large resistor (100k-220kΩ) between the "mid pot" location of the tone stack and ground to defeat the tone circuit, giving a midrange boost and the impression of more gain (cause it's not being tossed in the tone circuit). At that point the added tone control comes into play, and being a single-knob is probably something like a tweed Deluxe tone control. That is, it seems to make the sound more-trebly or more-bassy; naturally there could be some other 1-knob variant that does the same thing but in a slightly different way.
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First of all, I'd like to thank Tubenit for the work on the layout for this.
I spent a few hours on this thread this weekend and trying to understand more of this project before moving on. I'm still using the "HoSo 56 & PPIMV.sch"
I already got a 20 x 2 turret board, EL84s, 12AX7s and a 5978, as well as tube sockets. I did not look into chassis yet because I still don't know if I'll make it a big amp with room for future expansion (reverb, foot switch, tube rectifier, etc) or as small as possible to take to band rehearsals without regret.
I hope I now have more focused questions for my build. I have added an extra sheet in the sch layout file, it would be great if someone could help validate that.
So, some questions:
- We just use 2 diodes and not a 4-diode bridge? I know diodes will drop less voltage than most tubes, so, will 270-0-270 HV secondary take me to 350v DC output? Or should I get a 250-0-250 transformer? (I'll order PT custom here in Brazil, with 127v primary that I get at home - up to 132v sometimes)
- I've tried to create an output impedance selector (see layout sheet 2). Is that possible / does that exist? What's the easiest way of doing this?
- What kind of switch is used for the boost? (I'll include in a BOM I've built and post here)
- 20x2 turret board is enough for the existing layout, but I realized it does not include any of the big capacitors. Guess I should look into buying another small board for these and the rectifier diodes, right? (I tried drwaing this on sheet 2)
- I saw that some of you built this amp in a very small chassis. Maybe too small. What chassis size would you recommend (I'll also have to order custom, probably).
I see that no one has yet posted any sound sample or video.
More pictures and gut shots would also be of great help.
Thanks a lot!
- marcoamf
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I'd use a chassis that is a minimum of 17 or 18" X 2" X 6.5". OR you can use a tweed style 18watt type chassis.
IF you are using solid state rectification, I'd use the 250-0-250 PT which will give you around 350 v on the EL84's. Frankly, I'd prefer to use a 250-0-250 PT with tube rectification so you have more options.
You can use a relay switch for the boost. Doug sells all the parts on his website. And there is an abundance of info in ARCHIVE under relay switching.
With respect, Tubenit
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Ok, I'll try to go with tube rectification.
Any difference between EZ-81 and 5AR4 that should be considered? Cost, maybe...
So still keep 250-0-250, right?
Any comments on the other questions?
Thanks a lot, again!
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I'd personally go with 250-0-250, but that's just my personal preference because I think it would allow me the most options.
solid state 250 x 1.4 = 350
GZ34 1.3 = 325
5V4 1.2 = 300
5Y3GT 1.1 = 275
With respect, Tubenit
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Hey.
I've been looking at the gutshots that someone posted a link to, back on the second page of this thread.
There's something that puzzles me a bit.
If I'm not mistaken, there are some components in shrinkwrap under the two resistors before one of the big caps.
I'm not entirely sure what to call those resistors, but I think they are the ones that you usually swap out for the LarMar ppimv.
What puzzles me are those two thingies under the resistors... What could they be? I haven't seen other amp schematics that had any components parallel to these two resistors.
Anyone got a good gues?
Regards
Anders
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I've been looking at the gutshots that someone posted a link to, back on the second page of this thread.
There's something that puzzles me a bit.
I don't see that picture on page two. What is the reply # you are referring to???
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Hi,
I was told................?? there are series connected diode/zener diodes connected in parallel with each of the grid resistors. This would help reduce any blocking distotion.
Havn't seen the amp in the flesh to comfirm this.
Cheers,
John
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Now that you mention diodes + zener it occurs to me that I have seen something similar before on 18watt.com
People seemed to often have trouble with fizz/buzz and there is talk of a "Paul Ruby fix" to help reduce this.
I''m curious though, 'cause in that circuit those two resistors to ground (are they called grid-leak resistors?) are 470k's.
In the circuit in the pics, they could look like they are 100k's, and I assume that would mean the signal is hitting the el84 less hard, so I wonder why they could be needed.
If I understand it correctly, this fix only reduces the fizz originating in the el84's, or am I wrong?
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Hi,
Maybe this is the site you are refering to.
John
http://www.paulamps.com/18watterbuzz.html (http://www.paulamps.com/18watterbuzz.html)
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That seems to be the original work for the info I found on 18watt.com yes.
But I'm still curious - I haven't met other amp-designs with lower value for those mentioned resistors, that needed such a fix.
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Hey Tubenit, I saw your pictures of the HoSo but none were from front panel.
What are the switches, knobs, lights and plugs and order? I'm looking for reference for the "real state" of the front panel.
thanks!
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Publishing again on this forum since I finally finished my build.
Here's a simple demo of the sound:
https://youtu.be/cKPd9zB1R-0 (https://www.youtube.com/embed/cKPd9zB1R-0)
Thick bass (I can't say it's as thumpy as my 5E3, though), chimey with great harmonics.
Nice clean sounds with the volume knob down and low gain knob.
Awesome crunch with gain up, bump on and bridge pickup.
Definitely a must build.
Only one voltage that doesn't match the schematics. 70v on V3 cathodes is at 45v on mine. The rest is on the 10% range.
Any clues why this happens?
I want to build an effect loop for this. Also would like some tips on how to do it. Anyone?
thanks to all of those involved in this project!
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I want to build an effect loop for this. Also would like some tips on how to do it. Anyone?
Sure, this works well and I use it on the D'Mars ODS build.
Very nice job on your build! Thank you for sharing the video. Sounds good.
With respect, Tubenit
AND there are LOTS of examples here: http://el34world.com/Forum/index.php?topic=10208.0 (http://el34world.com/Forum/index.php?topic=10208.0)
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I still haven't been able to figure out the issue with the voltage at the v3 (second 12AX7) cathodes.
These are the voltages I'm getting:
https://dl.dropboxusercontent.com/u/7505544/voltagens_Soho.png (https://dl.dropboxusercontent.com/u/7505544/voltagens_Soho.png)
I just feel the smell of something burning when I play too much (BTW one of the best amps I've ever heard).
I smell it around the 2 12AX7's and the relay board.
Do you think it could be a defective tube? I don't think so, so I didn't have the courage to put another one there.
I've checked the voltages on the filaments (a bit high, around 6.4v) and remade some soldering wherever suspicious.
As you can see from the images, even though in general my voltages are lower than specs, they get actually higher at C, D, and E.
Apparently no issues with the 10K/5W resistor.
:dontknow:
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I think I found my mistake, I'm using 250k dual gang pot for the ppimv.
For some reason I believe it was the only one available at the store when I bought it (and still now, there are no 2M dual gang pots in here).
I don't believe I could use it, right?
Might take some time until I get get my hands on 2M dual gang pots...
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I think I found my mistake, I'm using 250k dual gang pot for the ppimv.
That dual pot IS supposed to be a 250K, not 2M. That MV will not affect the voltages you have on the PI tube. I would not be concerned about the voltage on your PI cathodes if you like the sound.
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Thanks, sluckey, but isnt the plate dissipation much higher than acceptable?
Could be unrelated, but the smell of smoke was a bit suspicious (near the 12ax7s)
If the pot is 250k, should the resistors on them be 2M?
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but isnt the plate dissipation much higher than acceptable?
No. It's actually kinda low. Your low cathode voltage and high plate voltage are indicators that the tube is drawing a small current (about 1.2ma) so plate dissipation will be low. If plate dissipation was higher than acceptable, the tube current would be higher, resulting in a higher cathode voltage and a lower plate voltage.
If the pot is 250k, should the resistors on them be 2M?
Correct. But, please confirm which type of MV you have. Your pic with the voltages and red arrows shows a simple 2M crossline MV but your schematic from November, 2014 shows a dual gang 250K MV with 2.2M resistors. Which do you actually have in the amp? In either case, the MV pot(s) will not be the source of your burn smell, nor will they be causing something else to burn. That's providing the MV is wired IAW the schematic.