Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on July 11, 2010, 04:12:24 pm
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DaGeezer and I have been trading ideas off forum trying to come up with an amp that will do both clean and OD well. With his trail blazing and help ........ this is what I have come up with. It sounded great with the band and cut thru the mix very nicely.
Basic idea is sort of a cathode biased D_mble but with a 5879 in the second gain stage of the OD. It doesn't sound like a Dumble to me but it does the blooming thing pretty good, IMO. I'd been calling it a Wrecked D_mble because it has more crunch than a D_mble does, from my perspective.
Thanks to DaGeezer for his help and inspiration! I wouldn't have come up with this on my own. I wasn't even thinking in this category til he got me started with his Maxim OD.
:wink:
SCH file is here: http://www.el34world.com/Forum/index.php?topic=9636.0
Sound Clip is here: http://www.el34world.com/Forum/index.php?topic=9637.0
With respect, Tubenit
EL34 EDIT:
To avoid confusion, all current schematics, layouts and materials list for this project can be found here
http://www.el34world.com/Forum/index.php?topic=9636.0
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Uh......don't let Tubenit mislead you! This is HIS design! :grin:
Yes, I have been working on some pentode stuff, but this is in another league!
Looking @ the schematic, what you've basically done is build a sort-of mini SE trainwreck (3x gainstages into a pentode), then feed that into a bigger PP poweramp. Maybe someone has done that before, but I've never seen it.
It's kind'a along the same lines as the Guytron amps, but BETTER, as those are PP into PP, and yours is (sort of) SE into PP, yielding much better harmonics (I suspect).
The soundclip is fantastic.....I think this design is a real breakthrough for getting great tube overdrive at ANY volume. I suspect that you could feed the preamp into pretty much any clean poweramp & get similar results, since the tone is "happening" in the preamp tubes & pentode. I bet a simple pre-PI or post-PI MV would do the trick to control the output volume while allowing the preamp to do it's work.
Good job, my man! :wink:
Geez'r
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Attached is my take on the COS....simplified with a single tone control in the "clean" section.
NOTE: given DaGeezer used a "tweed" tone stack ............ we're calling his design the Tweed Overdrive Special
To put it mildly, this amp ROCKS! :headbang:
The cleans are truely clean, or mildly overdriven if you crank up the gain a bit....goes from country clean to bluesy drive with a few simple adjustments of the gain & tone control.
The OD section is unbelievable....everything I have "heard" in the great amp clips I've heard on utube & such, but easily obtained without a bunch of tweaking of gain/drive & tonestacks....no "unobtainium" here!
I love the single OD tone control! Basically just get the distortion amount right with the gain & drive, then add or take away hi-end crispness with the tone control.
I played the amp with the band tonite (got lots of compliments) & then played for ~~ an hour after everyone else left.....I didn't want to stop. I went from chicken-pickin to blues to classic rock tones with a stomp on the channel select footswitch & by simply tweaking the guitar volume/tone controls.
The only thing I'm thinking is that I may want to reduce the mids "slightly" on the clean channel, as it seems a tad mid heavy when playing by yourself, but it sits very well into the band mix, so I'm not sure I want to do that. Maybe a cap selector switch for more or less mids(?). I'll have to experiment with that a bit......
Anyway, THANKS to Tubenit for the great design!
Geezer
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Tubenit, guys--the COS OD section is a great innovation! Wired it up last night (though I ended up using a 6AV6 to drive the 5879 and a few different values). I can get this one to bloom into feedback now at a lower overall volume but it is still pretty articulate. Thanks for sharing your R&D. Regards.
dennis
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I'd be interested in hearing about your findings. I finally got my mosfets from Mouser which were on backorder for while but I haven't had the time or willingness yet to redesign and make the changes yet.
Jeff, you've seen my schem but don't know if you still have it? If I put the VVR into it, would it be best to replace my vol control right before the pi? Or is it better to leave that and install it somewhere else?
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I don't know what is "best" but in both amps I did with VVR on the poweramp and LTPI, I did still keep the master volume prior to the LTPI.
With respect, Tubenit
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The Tweed Overdrive Special got a workout "live" today W/ the band & it performed WAY above my expectations .... so "natural" sounding & so easy to dial in the desired clean & OD tones with the simplified tonestacks.
The added mids of the clean section (which I thought I wouldn't like) turned out perfect to help the guitar parts "sit" so perfectly in the mix between the piano, bass & acoustic guitar parts!
The amp was such a pleasure to play, it was truely "inspiring"......it's now officially my favorite amp.
Thanks again, T!
Geezer
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To put it mildly, this amp ROCKS!
The cleans are truely clean, or mildly overdriven if you crank up the gain a bit....goes from country clean to bluesy drive with a few simple adjustments of the gain & tone control.
The OD section is unbelievable....everything I have "heard" in the great amp clips I've heard on utube & such, but easily obtained without a bunch of tweaking of gain/drive & tonestacks....no "unobtainium" here!
YES, ........ I AGREE!!!
This amp the Tweed Overdrive Special has waaaayyyy better harmonics and sustain and blooming then the cathode biased D-style HRM amp did. The conversion was easily worth it.
Comparing the "Tweed Overdrive Special" to the original COS which has a Dumble tone stack on the clean channel. My initial impressions are:
1) the 5E3 tone stack gives better cleans than the Dumblish tone stack and it is more articulate than the Dumblish tone stack
2) it seems like the VVR on the original does not eliminate the "blooming" effect like the PPMIV does at lower volumes. The
VVR cranked down to as low as 1/3rd still seemed to allow the blooming to work great. The PPMIV seems to eliminate
some of the blooming effect at lower volume but it works great at around "6" thru "10". I will probably convert the amp
to have VVR at some point.
3) The 5E3 tone stack on the clean channel and the hybrid 5E3 tone stack on the OD channel sure make dialing the tone in
MUCH easier than the Dumblish tone stack and the HRM tone stack did. The original D-HRM was difficult to dial in.
4) the 5879 adds a dash of crunch and chime that the 12A_7 won't match in the OD
5) The clean and OD on the simplified match better tone wise than the D-HRM clean and OD which had a more midly clean.
6) The volume level of the clean and OD match better on the "Simplified". My guess is the Dumblish tone stack had high insertion loss making the balance of tone from clean to OD more difficult to match.
I am currently using a 5751 for V1, 12AX7 in the V2a position, into the 5879 ........ into a 12AY7 LTPI. This seems to be working just great.
DaGeezer thanks for the Tweed Overdrive Special design ............ it worked out perfectly.
With respect, Tubenit
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Interesting observation on the tone stack. The gig last night was with a fellow guitar player whom I built a similar amp for earlier this year and the only difference was the tone stack--his was done with a James and my newest was the _umble stack. FWIW, the James stack noticeably more lower mids and a little lower overall stack loss. It depends on what floats yer tone boat. My boat apparently is the James.
4) the 5879 adds a dash of crunch and chime that the 12A_7 won't match in the OD Amen.
Thanks again for the sharing. I will post a schem of what I end with as soon as the home computer is rehabilitated. Regards
dennis
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T,
I don't think you need that bright switch on V1 do you since it effectively has this when the tone control is turn up and you're placing the 500p cap across lugs 1 & 2? That would leave you w/ 3 switches.
I like options 1, 3, & 4 but haven't heard or done the second. What does that sound like?
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The bright cap switch on the vol was a left over from the D-HRM build so I just left it there. I actually like it & surprisingly, I can hear a difference so I may just leave it. I think it would help between playing the Tele with strat type buckers vs. the Tele with traditional humbuckers.
The local NFB gives sort of a compressed sound that is a little cleaner in tone. Kind of hard to describe.
Thanks for the feedback. I am leaning towards switches that make the clean channel more clean and the OD channel more OD.
With respect, Tubenit
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Ok, more experimentation........
As suggested, I tried paralleling the un-used 1/2 triode with different sections of the circuit.
1st, I paralleled it with the triode just before the 5879 pentode in the OD section. It sounded pretty good, but it made the OD super compressed & lost all dynamics, so no good there.
Then I put it in parallel with the 2nd triode of the clean section. That fattened up the tone ALOT & gave it an almost pentode flavor. I can still get it spanking clean, but it is FAT & toneful. Also easier to get a great bluesy grind & the clean section balances with the OD section much better now. Another bonus is that it evened out the mid hump just a bit.
Of course, this also affects the OD section.....it has much more "range" of overdrive now, with better dynamics & punch (without added compression).
So, it's staying right there (paralleled w/ V1b).
Geezer
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Q1)
Did you change the plate and cathode resistor on that V1b when you paralleled it ........OR did you leave it the same?
Left the plate the same (shared by both triodes) but re-used the old cathode resistor/bypass cap leftover from the HRM (1.8k/1uf), so they share a plate resistor, but have separate bias resistors.
Q2)
Is the paralleled triode from the same tube or the left over triode on the V2 tube (tube in first gain stage of OD)?
The leftover triode in V2. The parallel triodes are in separate tubes......
G
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OK, I did what DaGeezer did and paralleled the second gain stage of the clean. At least on my amp, that is quite a boost because I have it on a switch and can compare it.
Initially, I had a 5751 in V1 and paralleled a 12AX7 triode. It sounded better 12AX7 to 12AX7 triodes paralleled.
It makes the amp even more touch sensitive to play when engaged. I like it.
It's gotten messier with the conversion/rebuild but here is a gut shot of the chassis. It still is a quiet amp.
I'll update the schematic and layout in the SCH when I get a chance.
With respect, Tubenit
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that is quite a boost because I have it on a switch and can compare it.
Oh yes, it is a BIG boost in volume!
I may put a pot on the input of the paralleled triode input grid to control the amount of boost & make it a variable/footswitchable boost feature.......or not....I would probably just leave it on all the time anyway (it sounds so good when "engaged"). I'll have to think about it a bit........
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Tubenit, Geezer,
Thanks for all your R&D work!
I have one chassis left over which should be large enough to house 4 noval and 5 octal sockets and 7 pots on the front. I also have a Hammond JX 372 and a TAD Blackface Bassman replacement OT so I could do one that can use either 4 6V6s or 2 5881, 6L6, KT66 whatever. I could then install a half power switch to switch from 4 6V6s to 2 6V6s and if that's not enough I can do the Carr 1/4 power switch (that would work with the duet of large tubes as well). The 5th octal socket could be for a GZ34 rectifier (the power tranny has a rectifier winding). However I could opt for SS rectification and use the 5V winding for the overdrive switching.
But that project will have to wait until the baby boy is a bit bigger (just six weeks old and does not need an amplifier to be heard).
Cheers Stephan
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I would probably just leave it on all the time anyway (it sounds so good when "engaged").
To be honest, I am somewhat anticipating leaving mine on almost all the time because it also sounds so good.
What surprised me is that while it does increase the gain somewhat, it does NOT lose any clarity in expression. It still articulates just as well as without the paralleled triode.
In other words, the tone is not as "clean" but it is as "clear". Hopefully that makes sense. Some boosting mods seem to just muddy the tone but this doesn't to my ears at all. Instead it still articulates well and just increases the touch sensitivity in the playing.
Only disappointment so far with the paralleled is the 5751 triode paralleled with the 12AX7 triode didn't sound that great for some reason? And I liked the 5751 in the V1 position really well.
With respect, Tubenit
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From the looks of your sch. your using 275-0-275 with 150ma power tranformer so a hammand 270fx would be about right for this build which out put trannie are you using the one with 6600ohms for 35watts or 40watts. just checking what I can get. Thanks Bill
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I'm using a 20w 1620 OT. The other original COS has a 1650 OT and larger PT.
With respect, Tubenit
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Tubnit what would a pair of 5881 like to see as the optimal impedence for the output transformer on this, then what would same question be for a quad of 6v6,s
I really don't know. But both amps have 8 ohm speakers and I hook up the 16 ohm wiring to them. And I like the way they sound & have had no issues.
I think DaGeezer used Fender replacement trannies such as Hoffman carries if I remember correctly.
Here is my updated schematic. I may change the paralleled resistor switch out for something different.
With respect, Tubenit
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Another slight tweak.........
I put a 250kA pot going to the paralleled triode.....the extra 250k load only decreased the volume/gain very slightly, but it really sweetened up the tone (taking just a slight "edge" off).
With the pot on the front panel (labeled "Fattness") I can go from stock to fat (or anywhere in between) with the twist of a knob.
I suspect it could be helpful when changing from single coils to buckers, but it does seem to be a useful addition.
Again, this amp is FULL of TONE! The cleans are sweet & full, & the OD can go from crunch to smooth with easy adjustments. It's a real winner!
I have found I don't even really need a Master Volume with this amp (I've been leaving it all the way up), as clean & OD masters work just finefor getting great tone @ lower volumes.
Geezer
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This amp is such a WINNER!
The tonal variations available are amazing, and all of them are the same at low volumes or cranked up loud. I just leave my PPIMV all the way up now, & use the clean & OD MV's to control the output & balance.
I have a switch to lift the ground lug of the "Fattness" control (250k gain pot for the parallel triode) & that kicks it up even further when I want.....makes the PAF bridge pup sound like a scream'n hot Tele!
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I used a Super Reverb PT & the OT is from a '60s HiFi, but I have run the preamp into 2xEL84's & there's not a significant difference in tone, as most of the tone in this design comes from the preamp.
So, any PT rated high enough to support the tubes you are running & any OT with a suitable primary for the type of output tubes you're using will work fine.
G
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Im starting to build a thirst for this amp too. I still have my Bogen CHB-35A waiting for guitar tweeking. My first concern is, will the PT heater winding of the Bogen handle the extra currant of replacing the 6C4 PI (0.15A htr I) with 12AX7 PI (0.3A htr I) and adding the 5879 (0.15A htr I) Total increase of 0.3A? Total htr I of the existing Bogen with the power tubes is 2.35A, the increase would bring it to 2.65A. Im guessing the htr winding can handle 3 amps????
I sure dont want to burn up the heater winding in that Bogen tranny.
Am I right in guessing the htr winding can handle 3A? Thanks Punky
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My experience with many Bogen amps is that their stuff had some room in the ratings...I'd guess that a small increase like that isn't going to do any damage, unless the load is already pushing the PT to overheat (which I seriously doubt)
BTW....sound clips are done & being processed....stay tuned!
Here is one of the sound clips: http://www.el34world.com/Forum/index.php?topic=9726.0
Geezer
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Here's another version of the above linked clip, but this one demos just my amp & what it sounds like.
http://www.zshare.net/audio/786814058ef04507/
The guitar is an Ibanez Blazer strat (97) HSH config, with Dimarzio 59 PAF's in the neck & bridge (middle pup not used)
Here are the details, by section:
Opening = Clean/Neck & Bridge
Section 1 = Clean/Neck
Section 2 = Clean/Neck & Bridge
Section 3 = Clean/Bridge
Section 4 = OD/Neck
Section 5 = OD/Neck & Bridge
Section 6 = OD/Bridge
Section 7 = OD, "drive" tweaked up a bit/Neck
Fade out = OD, "drive" tweaked up a bit/Neck & Bridge
Geezer
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"My experience with many Bogen amps is that their stuff had some room in the ratings."
Thanks for the input Geezer. I feel better about adding that extra tube. Your amp sounds great and I love your playing style. Hot & spicy. Punky
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When the amp was 1st built, node "F" supplied the 5879, but I'll have to recheck to see if I changed that when I rebuilt it.......I'll check & post asap (but may be a while, busy day)
On the MV, I have 2.2megs where Heinz has 470k (to keep as close to 220k as possible when the pot & resistors are parallel), as in the "LarMar" version.
G
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Here is the latest schematic (accurate as of today) of the Tweed Overdrive Special.
I have made several slight changes since the last sound clips, such as lowering the clean channel plate resistors to get a bit more useful rotation (headroom) from the clean gain pot. It worked extremely well, & also improved the OD tone as well.
I also was feeling that the amp had a bit of a "choked" feel to it, so I removed the 220k/750p network from the clean section & that really helped....has a much more "open" feel to it now.
I will do an updated clip as soon as I get time..
Geezer
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Geezer, some questions:
1) could the 250k pot for the paralleled triode be on the back panel? Meaning
that it's not adjusted very much & doesn't need to be on the front panel. YES
2) I am wondering if we could have the two triodes that are paralleled share a
cathode resistor of 1k and a cathode cap of 2.2uf and still have a similar
tone? We could, but I have a 12AY7 as the V2 & it really is important to the OD tone on mine (the cleaner the signal going into the 5879 pentode, the better)....so, that means that the parallel triode is also a 12AY7.
3) Please note that I have V2a paralleling V1-b for convenience on the layout.
Is that OK with you? Yes
4) Is the OD level pot really 1M or is it 250K? I am using 250k and I don't
turn it more then 3/4 of the way up. It is 1meg....comes in handy if you want to use the OD as a clean(er) boost instead of all out overdrive (turn the drive down & the level up to compensate). It does normally stay pretty low....could make it an "optional" choice, since it doesn't affect the Hoffman board layout(?)
5) I wasn't sure how to accomplish the node F on the Hoffman style layout
board? Does this method look acceptable with the wire running under the
board? I figure the 1k/3w could be above the board. Looks fine to me
With respect, Tubenit
Also, note that I have the OD tone wired different than you have it.....mine is the same as the clean tone control.
A few other things I saw:
*The connection from the clean gain pot to the 2nd triode is missing.
*V2, pin 8 cathode resistor should be 2.7k, not 1.5k
*the 250p snubber on the 1st OD stage triode is missing
*the screen resistor for the 5879 should come from point "F", not after the plate resistor
*5879 plate resistor should be 56k
Thanks for all you work on this!
Geezer
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Is the jury still out on the usefulness of the PPIMV in this amp, or have they reached a decision?
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I definitely need it (PPIMV) on my amp to get the tone and overdrive at the lower volume I want. However, I still am convinced the VVR is a better way to go which is what I have on the original Carolina Overdrive Special (vs. the Tweed Overdrive). But it's just my personal opinion and preference. IF I want singing sustain that holds notes almost til it feedsback, I need either the PPIMV or VVR to keep it at what is a reasonable volume for me.
With respect, Tubenit
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In my experience, it's not needed unless you want the amp at a whisper.
I can get basically the same tone & response from the PRE PI MV's that are already in the circuit. I'm sure a VVR would be best, but I'm a little shy of them myself, after I had one fry on me @ a gig a while back....the 1st time in 35 years of gigging that an amp failed on me @ a gig.
As soon as the boards are ready, I'm going to build an 2XEL84 version into a gutted VJr head and I will leave off the PPIMV (no room on the front panel)....the amp will be for a very talented kid @ my church that I've already converted over to be a tube-head! :grin:
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Please forgive me for a basic question: where are the filter caps in this layout?
One of the options Hoffman offers is placing the filter caps between the pots/ground bus and the circuit board. Personally, I like the concept of having the preamp filter caps as close as possible to their respective circuits so that the current loops are minimized. See "Method Two" 5th photo down HERE (http://www.el34world.com/charts/filtercaps.htm). I've actually zipped-tied the filter caps on the edge of the board, but also committed the heresy of attaching the ground bus to the board instead of running it along the back of the pots (and using radial electrolytics), as you can see in this pic:
(http://i222.photobucket.com/albums/dd298/Chipster457/PR%202008/PR2008gutspreamp.jpg?1280325115)
Ooops, hadn't got to the point of using zip ties in that build... but I got the idea from Doug!
Just a thought.
Chip
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Question. In Geezer's latest schematic he has added a .001uF cap bypassing V2A's plate resistor. There is also a 250pF cap from plate to cathode on that same triode.
Quoting Merlin's book, Fig. 2.23, page 54, he shows both these two arrangements, plus a third where the cap runs from plate to ground, and states "All three circuit variations give identical attenuation characteristics".
So my question is, do we need both the .001uF across Ra and the 250pF from plate to cathode, or would just one suffice? What was the design decision here?
-John
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where are the filter caps in this layout?
Wherever you want them to be........either build cap farm/board, or use multi-section cans, or mount them directly onto the turret board (which I actually like to do).
Question. In Geezer's latest schematic he has added a .001uF cap bypassing V2A's plate resistor. There is also a 250pF cap from plate to cathode on that same triode.
So my question is, do we need both the .001uF across Ra and the 250pF from plate to cathode, or would just one suffice? What was the design decision here?
The 250pF cap from plate to cathode is from the original Dumble OD design (certain non-HRM models), and once I got the amp up & running, I felt I needed a bit more of the hi-end hash removed, so I tried a .001 across the plate. It acomplished what I intended, so I simply left both caps in place. I guess I now have an effective .001250 cap(?)
I know there have been comments about these "smoothing" caps being bandaides, or fixes for inferior designs, but I care not about that.....I'm interested in the resulting tone, & if I found (thru real world experience & experimentation) that I could solder an 1887 Indian Head penny across the plate load to get the tone I was looking for, I'd do it, regardless what the cork-sniffers said. :angel
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A question on v2b were you see the 68k going into #7 pin back up and look on the board it looks like its going to the wiper on a 50k pot by the way its drawn. Bear with me im a newbie is that what it is . if so should it be on the back or am I just not understanding what Im looking at . Im waiting for some parts to start building the turret board. Thanks Bill
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Hey Bill, good question.
That is pot is termed a "trim" pot, trimming the signal (to taste) entering the OD.
Dumble (& most other clone builders) put that pot inside the amp as a "set & forget" type thing.
I have mine on the front panel, labeled as the OD "gain" pot, along with the OD "drive" & OD "Level"....I like to be able to tweak it, but that's just my preference. I can get many more good tone variations from my amp by being able to adjust all the pots "on the fly".
The choice is yours where you wish to mount it...inside, on the back panel, or on the front.
G
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I put mine on the back panel.I don't like too many controls on the front of an amp.But that's just me. YMMV.
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When I build this circuit into the JVr carcass, I will put everything I can onto the back panel (due to limited space on the front)....will probably have just:
Input - Clean Gain - Tone - OD Drive - OD Level - OD Tone
OD Trim & PPIMV will go on the back.
I think I'll just wire the Fattness (parallel triode level) at a set amount via an internal voltage divider(?). It sounds best to me when the level of parallel boost is slightly below the full effect (tuned down ~~ a 1/4 turn from full up).
G
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I'll try the preamp into my 6BM8 power amp this evening & let you know how it sounds.....I've been wanting to do that anyway, as I haven't tried it since completeing all the re-builds, tweaks & upgrades
I've run it into 2xEL84's & it is stellar! A bit less mids & alot more chime. That's what will be in the VJr rebuild.
G
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OK, the 6BM8's have almost zero headroom, at least with any substantial volume. It's like "Overdrive" on the clean channel, & "MORE Overdrive" with the OD engaged. It may be that tweaking the PI could yield more headroom. I may try that (maybe raising the plate voltages to the PI, if possible)
This preamp definately likes a clean poweramp.
NOTE: Work is being done to try to come up with a more suitable PI for 6BM8/ECL82 type tubes>>
http://www.el34world.com/Forum/index.php?topic=9751.0
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Just tried the T-ODS into my HoSo56 poweramp & it sounds great, so there's enough grunt in 2xEL84's for it to work, but the 6BM8s just ain't got what it takes, at least not with the PI I have pushing them at this time.
G
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Observations of the amp in a live setting (Dance cover band I play in) thru a 1x12 closed back cab, ported, w/ an Eminence Legend 1258.
The amp sounds better live than I hoped, especially the overdrive!
The "Fattness" control basically serves as follows;
*Country/Clean Rock/Motown stuff = Fattness pot all the way OFF.....this gives a nice mid dip that is very Blackface-like. Makes my PAF HB equipped strat sound like a Tele (a Good Thing).
*Blues/Classic rock = Fattness pot turned up ~~3/4 gives a nice mid boost that helps cut thru the mix. Nice growl when digging into the strings.
The Overdrive is quite unbelievable.....the tone I've had in my head all these years (& have spent countless hundreds of $ on OD pedals searching for it) is MINE!
We do a pretty good version of "China Grove" & the OD nails that tone, both in the opening chord progression and in the solo. Pinch harmonics just scream.
Basically any OD tone you want is there (except Metal, of course) & easily obtainable with simple adjustments that are easy to replicate/remember. Reponds perfectly to the guitar volume & tone, too.
It's really hard to describe just how much I enjoy playing this amp! :grin:
I'll try to get more soundclips up this weekend.
Geezer
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I have not grounded 2 & 6 on of the 5879 tubes in any 3 of my amps. Seems to work just fine without that. I'll try grounding them and see if there is any difference. They're just "open" pins on mine.
Then don't worry about it....it had been said before that it probably wasn't needed, but I had done so on my builds. You have proved it is not needed! :wink:
Thanks for all your work! As soon as they are available from Doug, I'm placing an order for at least 2 (for now)...one for the VJr rebuild & one for a 4x16GK6 (16v EL84-type) version I'm going to do.
G
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How well, in your opinion does this type of control suite this amp, as i have heard its not that great. But in saying that a single tone control is great for the overdrive.
Remember that the Tweed Overdrive Special came after the Carolina Overdrive Special with the main distinguishing difference being the Tweed had a simpler tone control between V1a & V1b. The COS has a typical D_mble typology in it's tone stack. I have both the COS and TOS amps.
The Tweed Overdrive is MUCH easier to dial in a tone that you want. Having said that, the COS can give more of a mid-scooped tone & the PAB with mid-boost offers some variance of tone and overdrive. I played both amps yesterday comparing them & decided each had enough strengths of their own NOT to convert the COS tone stack into the Tweed.
I would say the TOS is a little more touch sensitive to play and the COS has a smoother overdrive.
As a side note, the PAB on the COS boosts the overdrive as much as switching in the paralleled second triode on the TOS
IsoTone, thanks for cleaning up the schematic! It looks alot better.
With respect, Tubenit
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IMO, the basic topology has been established by Tubenit (that it works & fantastically so), so add whatever tonestack you want to the front end (clean pre).
I have considered trying some of the other single & two controls styles myself, as I sometimes find myself wanting to be able to bring the mids down just a "touch".
Any fairly low loss stack should be just fine.
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Do you think that using a Bogen CH-35 as a base for this amp would sound ok? I have one that I converted to 6L6's laying around which I would like to gut and try. What do you think about using those tranny's?
I think it would work fine......what you need is a good, fairly clean poweramp section, which the Bogen has.
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Doug posted this picture someplace else but I thought I'd post it here to give you guys an idea on what a Hoffman turret board might look like for the Tweed Overdrive Special.
With respect, Tubenit
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I have a question about the pot labeled "Master" on the layout, not the PPIMV, but the located above the DPDT OD switch on the schematic.
If I'm reading the schematic correctly, it looks like when that switch is in the OD position, the wiper is disconnected and that pot becomes simply a 1M resistor to ground, Therefore it would have no effect on signal level. Is that correct?
I plan to build one of these, and in drawing up my own version of the layout for it, if that pot only effects the clean "channel", then I will locate it next to the clean tone control and label it something different. I would just like to get clarification from you guys who are actually playing thru the amp, that my perception of how that control functions is correct.
John
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Hopefully you can decipher this illustration. The Mstr vol affects only clean channel
With respect, Tubenit
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If I'm reading the schematic correctly, it looks like when that switch is in the OD position, the wiper is disconnected and that pot becomes simply a 1M resistor to ground, Therefore it would have no effect on signal level. Is that correct
Yes, it affects only the clean section (as Tubenit said)
if that pot only effects the clean "channel", then I will locate it next to the clean tone control and label it something different.
That's what I did....labeled it "Clean Volume"
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DaGeezer,
How important was it to you to have the 250k pot for the parallel triode in achieving the tone you want?
I currently just have an on/off switch there. However, I have a mini-pot that I can fit on the front of the chassis that is 500k. I could either use it at 500k or put a 470k resister on the outside lugs.
I am debating whether to change out the switch for the pot? Sounds like you found a "sweet spot" using the 250k pot. Am I understanding that correctly?
I changed the first .02 coupling cap to a .01 & then used a cap substitution box & found a "sweet spot" with paralleling a .001 cap. It sounded quite a bit better to my ears. This is the coupling cap going into the clean tone stack. I have no idea why paralleling the .001 made a difference like that but it did to my ears. And it sounded more expressive than the .02 which almost sounded compressed/muted in comparison. I used the cap substitution box to parallel about 7 different caps with the .01. A .002 did not sound to bad either paralleled but I settled on the .001 and put a Mallory in there.
With respect, Tubenit
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Is there supposed to be a "third" lug for the 50 KL trim pot?
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Right, but isn't that a 50KL pot on the board with the wiper going to a 68k resistor to pin7 of V2? I thought that was the pot on the BOM that was posted Hummmm...
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The schematic I am looking at shows a wire going from the wiper right to pin 7, so you don't need a lug
I see what you are asking about on the layout diagram, there is a 68k in line, but that's not what shows on the schematic, so I don't know
The creators will have to answer that.
Jeff, I sent you a board and the lugs for you to examine
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Yes, it's cathode biased power amp. DaGeezer uses a 430/10w resistor & I use a 360/10w with my lower voltages than his on the 5881's. Both use 150uf cathode cap & I substituted Hoffman parts 100uf & 47uf/100v filter caps.
The wiper of the trim pot goes to a wire (shrink tubed) to a 68k (shrink tubed) right on to the grid .............
OR you can do like the D-style amps and use a terminal strip. And have the trim pot wiper go to the terminal strip lug and the 68k attach to the terminal strip lug to the grid of the 12A_7.
The SCH library with the schematic, layout, bill of materials and turret board template show a 68k grid resistor.
http://www.el34world.com/Forum/index.php?topic=9636.0
With respect, Tubenit
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Note:
Any document references in the post above have been deleted in order to have all the current info on one page.
Just a heads up in case you see a reference to a attached document and there is no document.
The Tweed Overdrive special documents page is here
http://www.el34world.com/Forum/index.php?topic=9636.0
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Just one last question Ok Im looking at the BOM. and it says 430k 10watt resister and on the layout it shows that one as 430r 10watt
It's 430 ohm or 430R not 430k. The Bill of Materials has been corrected and updated 8/2/10
Tubenit
EDIT: With over 119 posts on this thread, I decided to edit out some. I approached this thread from the perspective of someone perhaps wanting to read all of it months or a year from now. So, if one of your posts about " I want to build one of those also" or "sounds good" or "I agree" got deleted ............ sorry. I wanted to keep it to the core essence of the evolving conversation and design of this amp.
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Good idea, Boil it all down to the essence :laugh:
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Billjade was asking about a reverb version of the Tweed Overdrive Special. So I drew one up. Check for errors and make sure the schematic and layout match. Hoffman is not making a board for this so you will have to make you're own.
The SCH schematic and layout is here:
http://www.el34world.com/Forum/index.php?topic=9636.msg88942#msg88942
The predecessor to the Tweed Overdrive was the Carolina Overdrive which uses a different style layout but does have reverb and VVR.
The Carolina Overdrive uses a D_mble style tone stack instead of the tweed tonestack. You can find SCH info on the COS here:
http://www.el34world.com/Forum/index.php?topic=8387.0
With respect, Tubenit
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well I ordered the hoffman board, gathered Iron and other parts and found a piece of aluminum to brake into a chassis.
I'm wondering.. what kind of speakers are you guys driving with this amp?
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It seems in all the consolidation of files for this amp, that all the .gif versions got removed.
I may be geekier than most, but just last month on was on a camping trip, sitting around the campfire, reading this forum on my iPhone. What's a brother to do when JSchem doesn't offer an iPhone app!
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.gif of what?
There's a .jpg layout on the docs page
What do you want a .gif of?
It can be added to the docs.
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what kind of speakers are you guys driving with this amp?
I'm running a 12" Eminence Legend (1258) in a 1x12 closed back, front ported cab....a very run-of-the-mill speaker, but fairly "neutral" to my ears, which I like....IOW, it doesn't flavor the tone of the amp with piercing highs or massive lows or mids, but lets the natural tone of the amp shine thru. I like the punch of a closed back cab.
I used to use Eminence "Private Jack"s, but found the highs to be a bit brittle sounding.
I think Tubenit uses Eminence "Red, White & Blues", but I may be mistaken.
Tubenit (me) has used a Red,White and Blues & also a Cannabis Rex
(Emminence). I like both of them with the amp real well. With respect,T
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Doug, sorry I wasn't more specific - a gif version of the final/latest schematic would be appreciated. thanks!
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Here's a gif for'ya.......
EL34 edit: I added Geezers .gif schematic to the documents page here
http://www.el34world.com/Forum/index.php?topic=9636.msg87475#msg87475
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Thanks Geezer,
I have 1 Eminence Wizard and about a half dozen Celestion Super 65's, Both speakers I find very neutral to my ears.
The folks over there at the AX site helped me a while back design a great cab for the Wizard, it's a bit big, but its convertible back makes it very versatile. I can run the back open or put a ported panel to fill the back or seal it completely.
the port seems to make the cab boomier and the sealed gives it a compressed voicing. myself I like it open.
do you think a 2x12 would be in order? I would use the Celestions there. or if I run the 1x12 I would copy the wizard cab again, both are projects I wanted to do anyway (and a 4x12 1960a cab filled with the 65's)
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Guys,
I added a flipped SCH layout into the SCH library that has all the current information.
Same layout but the view is flipped meaning it's more like the tradition Fender layouts with pots on top & sockets on bottom.
I got my board, turrets and parts from Hoffman & hopefully will start populating the board this wkend.
with respect, Tubenit
NOTE: I posted another soundclip of the Tweed Overdrive trying to demo some of the sustain that it has right here:
http://www.el34world.com/Forum/index.php?topic=9789.msg89093#msg89093
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Geezer or Tubenit,
Have you tried a switch out or pot on the neg. feedback? Just wondering if there would be any worthwhile changes at lower volume....
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There are two newly recorded Tweed Overdrive clips here:
http://www.el34world.com/Forum/index.php?topic=9789.0
http://www.soundclick.com/player/single_player.cfm?songid=9490676&q=hi&newref=1
There was some debate on this thread about using a VVR or a PPIMV. I'm gonna state that I think the VVR is a nice match for this amp (to my ears)! I recorded this tune with the amp volume NO louder than my computer speakers at medium.
http://www.soundclick.com/player/single_player.cfm?songid=9488477&q=hi&newref=1
Excuse my sloppy playing but check out how easily the amp sustains.
IF you get a pop up that asks if you want the window closed ......... just click "NO".
With respect, Tubenit
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Man that sounds good Jeff the sustain is Killer. Which switch mods did you end up going with for those two switches Im getting the board finished and need to look in that direction next. Bill :grin:
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Thanks for the nice comment. This is the first soundclip that really captures the tone of the amp that I am satisfied with. My recording system doesn't work very well but it's passable if I use a VVR dialed way down. The sustain is pretty amazing on this amp live.
I have four switches and only two were on.
The OD switch was on of course.
And the paralleled triod switch was on engaging the triode.
That's it.
The other two switches add a bright cap to the clean volume & then parallels a resistor on cathode cap. The later is only marginally useful.
Building it again, I'd only have DPDT (clean/OD) ........ a triode switch or do DaGeezer's 250k pot ........ & a bright cap on the clean volume.
With respect, Tubenit
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Cool I will definatly go with the clean/ OD a must obviously. I already know from past builds I dont need the switch on the parallelled tube Ive loved everyone Ive had, the 250k pot is a must have, and a push pull on the 1meg clean volume for the bright cap sounds perfect. Thanks Guys . Bill
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Does anyone have a drawing or schematic detailing how you are implementing the VVR for the Power Amp and PI. I have some drawings here and they all have additional capacitors. I just want to make sure I save room if I add this feature.
Also wondering how different this amp would be with a resistor in place of the choke? I'm going on vacation which gives me time to draw my layouts. I'm hoping to be ready to start this and another build when I get back.
Thanks
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Does anyone have a drawing or schematic detailing how you are implementing the VVR for the Power Amp and PI.
Try this thread>>>>
http://www.el34world.com/Forum/index.php?topic=8387.0
wondering how different this amp would be with a resistor in place of the choke?
I think a resistor would work fine......I only used a choke on mine because I has one on the shelf, otherwise I would have gone with the resistor.
G
Mine has a resistor instead of a choke. Tubenit
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Does anyone have a drawing or schematic detailing how you are implementing the VVR for the Power Amp and PI.
and if you check the ARCHIVES ........ there are schematics, layouts, & photos and comments on doing this:
http://www.el34world.com/Forum/index.php?topic=6899.0
With respect, Tubenit
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Thanks Geezer and Tubenit, I should have checked the archives first! Sorry for being Lazy.
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Just a quick question here,
On all the various layouts that's been posted here for the TOS, I keep finding "S1" between the tone and drive pots.
I keep re-reading the posts and I'm clueless to its purpose.
Heck I even went so far as to find my readers and put my glasses on.
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That S1 switch was originally from my personal Tweed Overdrive (schematic and layout) & I use it to switch the paralleled triode on or off. (Geezer has a 250k pot in that function) You could also use a spst for a bright cap on the vol pot or to parallel a cathode cap or resistor.
So, essentially ........... it can be for whatever switching purpose that you would want. Use a spdt or dpdt there ............ whatever
With respect, Tubenit
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Hi guys, This amp has got me buzzing. I'm waiting on parts to arrive so a couple of questions. Can someone tell me when calculating current draw for the transformer size, what am i looking for on the tube data sheet or what is the current draw for this amp?I just want make sure i have the right size tranny for this build.I have a 50w plexi i built awhile ago and was going to do a few tweaks and a bit of fine tuning after this project. I have a Hoffman "hot switch" added which was posing a couple of small problems.This ment opening it up and doing some surgory.The question is,if the 5F6A bassman and the plexi are of similar build, if the hot switch was removed and the overdrive inserted(i had thoughts of not having a tone on the overdrive so that the only colour comes from the plexi)could adding the overdrive to the plexi make it sound similar to the Tweed Overdrive Special.Thanks
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could adding the overdrive to the plexi make it sound similar to the Tweed Overdrive Special.Thanks
My guess is NO! But I could be dead wrong. We tried adding this overdrive to the HoSo56 preamp topology and it just did not work out for some reason. IF you try it .......... let us know and record some sound bits, please.
I think any trannie that is 140ma to 200ma sound work OK if the heater current rating is also high enough.
With respect, Tubenit
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Guys, I used a hammer (not a drill press cause I don't own one) & Hoffman's staking tool to install the turrets/lugs on the Tweed Overdrive Special Layout board. Tutorial on how I did that with pictures is right here:
http://www.el34world.com/Forum/index.php?topic=9800.0
Only took me about 55 min to install 66 turrets including putting tools up.
With respect, Tubenit
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Tubenit two questions my friend. On the one tube reverb does it have an on off switch like other reverbs. Ive checked the archives found good info you put there just not that , and the Dwell for the reverb looks like a pot do you guys put that on the back or should i move it out to the front. Thanks Bill
Bill, the reverb doesn't have an on/off switch & I always put the dwell on the back. With respect, Tubenit
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Well, played the amp for 3 long sets last night @ an outdoor gig.
Ran reverb & delay thru the effects loop.
Had a distortion box (Bad Monkey) hooked up ...........DIDN'T USE IT ALL NIGHT!
I got every tone I needed from the Clean/OD sections & by varying the Volume & Tone controls on the guitar.
I set the amp up on the "hot" side (fairly gainey when the guitar volume was dimed) then just rolled back the volume pot for a beautiful clean tone.....more "bluesy"?...just bring the volume up a bit.
I played Motown, Skinnard, beach music, Neal Diamond ( :rolleyes: ), Rare Earth, Vince Gill, Righteous Brothers, Doobie Brothers........all with the exact same amp setup (clean/OD ONLY).
Got lots of (good) comments from the band members (& guys in the crowd) on the tone....best yet!
G
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Just to echo what Geezer said ..............
I played my Tweed Overdrive with the band & got some very nice compliments on the amp's tone from the band and audience.
It cut thru the mix with no problem even with a loud keyboard player. Whether on clean or overdrive & no matter whether playing rhythm or lead .......... it cuts thru very clearly even with the OD pushed. I had the paralleled triode on all the time. Harmonics are very nice even with chording/rhythm.
This is the first time I've played an amp with a relay switching OD. I like that feature. Very quiet switching from clean to OD.
I have found switching out 12A_7 types, such as using either 12AX7 or 5751 or 12AY7 or 12AT7 in different positions, that every combination has sounded good to me and you can sure tailor your tone to a wide variety doing that.
Only thing I am not too keen on is the PPIMV which doesn't sound good dialed below "6" to my ears. Above that sounds fine to me.
With respect, Tubenit
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Only took me about 55 min to install 66 turrets including putting tools up.
With respect, Tubenit
Man, I never realized how much time my drill press saves me. I just did an AB763 110 turret board on it in 5 minutes.
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I just did an AB763 110 turret board on it in 5 minutes.
WOW! That's quick! You did 22 turrets a minute which is one turret every three seconds. Blazing fast.
With respect, Tubenit
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Well the drill press is set up for nothing but turreting and I load all of the turrets at once. Then, I can jump from one turret to the next about every 2-3 seconds. Every once in a while one hangs up and causes others to pop out with the release jolt. But preventing that is all in the angle of the board when each turret is pressed. I don't know how many thousands of turrets I've pressed with this setup but it's a lot. I used to do a lot of time and motion studies when I was in the Hotel business and I have a tendency to analyze every process to death for time savings.
I had to use a hammer on the turrets for the bias trimmer since they're so close together and I can see how that would take a long time one by one.
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Another soundclip posted HERE>>>> http://www.el34world.com/Forum/index.php?topic=9817.new#new
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When Geezer changed the internal 25k trim to an external panel mounted 50k trim .......... I thought "nah, that probably wouldn't be useful to me" ................ & then he changed the on/off switch to the parallel triode to a 250k fattness pot stating that there was a "sweet spot" in there somewhere just back off full on. And I thought "yeah, right (sarcasm) it can't be that much different or better".
You ever hate it when someone else is SO right compared to your preconceived notions!! :angry: :cry: :laugh:
OK, Geezer you're right ......... there is a "sweet spot" with the fatness pot.
And you're also right in that the 50k external pot that you can adjust is a "must have" now that I've tried it.
Turns out that I can get an even smoother sounding overdrive by turning the 50k trim pot to about "7" and lowering the drive pot to about "2" & having the OD level at about "7.5". I mean it gets super smooth OD that way withOUT losing any useable harmonics and sustain. The 50k trim pot also allows an incredible amount of distortion/OD to be used IF someone wanted that (trim at "7", drive at 7.5 & OD level at 2).
OK, I admit it ........... you told me so & you were right!! UGH! :wink:
In fact, the pot that I change the least is the clean volume pot. It has the least adjustment when I try dialing in a particular tone.
Sorta kinda with respect, Tubenit
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Another soundclip posted HERE>>>> http://www.el34world.com/Forum/index.php?topic=9817.new#new
Updated the soundclip....I had the tracks off time from each other, that's why it sounded so BAD! :angry:
Give another listen........please
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Attached is my stab at a faceplate and control names. Ignore the rear plate as it is not for this amp.
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Barry,
Superb job! I like it. Nicely done.
Any chance of fitting on a manual spdt switch to go with the relay?
With respect, Tubenit
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Now that's an overdrive special on steroids. I've never seen so many gain, drive, and level adjustments and so few tone controls in one amp. Nicely done you guys.
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I've never seen so many gain, drive, and level adjustments and so few tone controls in one amp.
GOAL! :laugh:
Yeah, all we have to worry about is the amount & type of distortion/overdrive, NOT fiddlin' with the tonestack all the time! :wink:
I absolutely LOVE this amp!
G
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Geezer - love the soundclip
Bnwitt - great job on the plate design
You all have created a thing of beauty in my humble opinion. Thanks!
Chip
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I really like the names you have on the face plate there perfect , I would only add two things on the back the effects in and out and the footswitch plug in for the channel switch. Bill
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Yeah, all we have to worry about is the amount & type of distortion/overdrive, NOT fiddlin' with the tonestack all the time! :wink:
What a novel concept, it's about time something like this comes along. This one's going to be hard to top? BTW, good playing G :)
Turns out that I can get an even smoother sounding overdrive by turning the 50k trim pot to about "7" and lowering the drive pot to about "2" & having the OD level at about "7.5". I mean it gets super smooth OD that way withOUT losing any useable harmonics and sustain. The 50k trim pot also allows an incredible amount of distortion/OD to be used IF someone wanted that (trim at "7", drive at 7.5 & OD level at 2).
I have found that combined with higher amounts of gain/overdrive and the more raw signal and/or mids dialed in, the smoother the inherent drive also (in my DumbleWatt). To describe it is to think of the lead in Bob Segar's "Down on Main Street", or many of Santana's solos.
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Barry,
Superb job! I like it. Nicely done.
Any chance of fitting on a manual spdt switch to go with the relay?
With respect, Tubenit
It's getting a little tight on the 17.5" chassis but I'll do some squeezin'. Any other suggestions?
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I really like the names you have on the face plate there perfect , I would only add two things on the back the effects in and out and the footswitch plug in for the channel switch. Bill
TJ, I haven't even started on the rear plate yet. The one shown is from another amp. I'll be sure to put the effects in/out on it. Probably in place of the bias jacks since this amp is cathode biased.
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Barry,
Superb job! I like it. Nicely done.
Any chance of fitting on a manual spdt switch to go with the relay?
With respect, Tubenit
I've been giving this some thought on how to have a panel mounted clean/od switch and a foot pedal too
Here's what I came up with
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You guys make it easy for us newbies so much thought with so much detail and diagrams . Ive learned tons from you guys thanks for the good work . my soldering iron is a smokin so I better get back to work. Bill :grin:
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Good afternoon Gents:
Aint been around in a while, it's not surprising to see Tuber and the Geez are still a couple of the driving forces over here. Seems I'm building more than I hang out on forums any more, looks like I've been missing out.
I checked back looking for a flexible amp that would do country cleans and bluey OD and this thing looks perfect. I've read through the post and no one seems to think or feels it needs a traditional "full" tonestack. Chicken pickin Tele to rock Les Paul and this amp works, that great. I've got a chassis and transformers for a fixed biased 6550(or whatever) set-up. What do you guys think about this preamp driving that type of power amp? The amp would be used on big casino stages and outdoors mostly, in both a country band and a classic rock band.
BL
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I've got a chassis and transformers for a fixed biased 6550(or whatever) set-up. What do you guys think about this preamp driving that type of power amp?
Because the TOS has it's gain mostly in the pre & OD sections ........... I think it would work OK with a fairly clean power amp that was fixed bias. The large 147uf cathode cap we use is to emulate more of a fixed bias tone.
IF you give it a shot let us know how it works for you?
With respect, Tubenit
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Tubnit a question on the 10 watt 430 ohm resister you use on the output tubes would 400 ohm do the job properly or should I double up and make 430 out of 2 resisters. Bill
PS I also was going to use 6l6 tube,s should I change anything for this or will this be fine it just happens I have them laying around and cant afford 5881,s right at the moment.
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ToneJunkie,
Don't know the answer to that? I use a 10w/360 ohm resistor but my plate voltages (348v) are less than Geezer's.
The 5881 tubes are only running at 38ma.
My Carolina Overdrive Special (same amp essentially but with D-type tone stack) has higher voltages & I use a 360 ohm with that one also & have not had any problems. My COS has 6L6's not 5881's.
Calculate your ma after trying it. I would presume a 400 ohm will work but do the math. Measure the voltage across the power tube cathodes and divide by 400 ohm. For example: If you measured 32v across the cathodes divided by 400 would be 80 divided by two tubes would be 40ma per tube.
With respect, Tubenit
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Thanks Jeff you just tought me how to bias by voltage which I can relate to having mostly done variable bias amps.I will print this out and add to my wall of important knowledge, you have a couple things up there. Bill :wink:
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I'll do that and take some pics along the way.
BL
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I got the Tweed Overdrive Special Hoffman turret board finished. Turned out nice!
I will admit I am NOT a strong detail person (or that patient) so the wires aren't nice and straight. :undecided:
However, this type of board seems very sturdy to me & was relatively easy to do. The parts that Hoffman sells fits the board just fine.
1) I numbered the turrets so I could track easily what went where.
2) I connected the appropriate turrets first.
3) Then simply populated the board.
With respect, Tubenit
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I will admit I am NOT a strong detail person (or that patient) so the wires aren't nice and straight.
Jeff - we've seen your cabinet work! :grin: So what if you were in a bit of a hurry on this board. You've got to be plenty patient to get your cabs to look as sweet as they do.
As far as details go, you and Geezer figured this baby out through how many iterations???
Chip
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Ok more dumb questions sorry :angel With your hundred ohm resister going to the cathode of the power tube Im figuring one side of the heater goes to one tubes cathode and the other side to the next one do I still ground my heater center tap. Sorry for the newbie question. Bill
PS. being that this is a rebuild i already have it grounded but didnt know if that caused problems to that type of bias circuit
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You do NOT use a heater wire ground IF you are using the 100ohm resistors (as an artificial ground) per Hoffman's Library of Info.
I did not ground those 100 ohm resistors to the power tubes shared cathode, but may try that at some point. I followed Hoffman's grounding scheme from his Library of Info and grounded the heaters that way.
With respect, Tubenit
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Hey Geezer and Tubenit,
Why did you guys choose a cathode biased power section?
I have this PT that Magnetic Components says is a Super Reverb replacement, but the secondary voltages are higher than spec'd on the schematic.
The flea bay Sovetek 6L6WXT's are here and Thant tranny has a 53V bias tap.
So I'm thinking fixed bias might be in order here. the OT is rated between 50-60 watts and the Celestion I have is too.
What would be the pitfalls of going this rout?
Will the PPIMV need modified too?
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Hey Geezer and Tubenit,
Why did you guys choose a cathode biased power section?
I have this PT that Magnetic Components says is a Super Reverb replacement, but the secondary voltages are higher than spec'd on the schematic.
The flea bay Sovetek 6L6WXT's are here and Thant tranny has a 53V bias tap.
So I'm thinking fixed bias might be in order here. the OT is rated between 50-60 watts and the Celestion I have is too.
What would be the pitfalls of going this rout?
Will the PPIMV need modified too?
Just to start Ray off:
How loud do you want this amp to be? Sounds like you could use the entire poweramp including PI of a Super Reverb, adjust the dropping resistors on the power rail to get preamp voltages where Geezer & tubenit found sweet spots, and be good to go.
Just a couple of days ago there was a discussion of PPIMV circuits in a Plexi context. The Trainwreck "type 2" is virtually identical to what the guys have used here, except that the volume pots are "grounded" on the bias tap instead of at chassis ground. IOW you just insert the bias supply at the junction of the two 2.2 meg resistors instead of grounding that point. Done.
That's going to be one clean, loud power amp.
Cheers,
Chip
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I'd love to hear one of the sound clips of this amp (especially the one with the band) but none of the links work for me. Maybe because I'm using Firefox on a Mac?
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Why did you guys choose a cathode biased power section?
Because I like the sound of cathode biased amps alot. It is easy to do than fixed biased, IMO.
Because I don't need the extra volume at all. I need to preserve the hearing I have left.
And because I discovered with all 3 of my cathode biased amps, I can simply switch between 6V6 & 6L6 tubes with no problem. I like being able to do that. No re-biasing is needed.
with respect, Tubenit
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I'd love to hear one of the sound clips of this amp (especially the one with the band) but none of the links work for me. Maybe because I'm using Firefox on a Mac?
Try this link>> http://www15.zippyshare.com/v/93652381/file.html
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Just a couple of days ago there was a discussion of PPIMV circuits in a Plexi context. The Trainwreck "type 2" is virtually identical to what the guys have used here, except that the volume pots are "grounded" on the bias tap instead of at chassis ground. IOW you just insert the bias supply at the junction of the two 2.2 meg resistors instead of grounding that point. Done.
Like this?
I've not built a fixed bias amp yet, and things seem to keep pushing me to on this build.
And where would I put the 1 ohm resistor to measure current draw?
Thanks Tubenit,
3 very valid points and I do agree on all 3. If this doesn't work for the best it should be easy for me to revert it back to cathode biased.
Ray
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I'd love to hear one of the sound clips of this amp (especially the one with the band) but none of the links work for me. Maybe because I'm using Firefox on a Mac?
Try this link>> http://www15.zippyshare.com/v/93652381/file.html
That worked, thank you. Nice sound, nice playing.
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Stingray - I think you nailed it. The 1 ohm current sensing resistors go between pin 8 of each power tube's cathode and ground.
I believe that it's important to use shielded wire going to and from the Master Volume pot since these grid leads carry relatively low signal levels but are in the poweramp end of the amp. IOW the potential for lots of nasty interference in the neighborhood.
Chip
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I used different component values & PT than Geezer and am VERY happy with this amp.
I do NOT like the PPIMV that I had in the amp and so I installed a Dana Hall VVR which I like real well. I could not turn the PPIMV below "6" and still get a good tone. I can dial the VVR down to "3.5" and still get a great tone, IMO.
Just thought I'd share how I did it. In case, anyone else is interested in trying that.
I should mention that I installed two Dana Hall VVR's in two different amps in Sept '09. The amps have been used regularly and had NO issues with the VVR at all.
I should mention that there is a MORE dramatic contrast between clean and OD with the VVR then there was with the PPIMV.
With respect, Tubenit
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Another soundclip here>>
http://www.el34world.com/Forum/index.php?topic=10305.new#new
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Man oh man , that is a kickin' lil amp. Great sounds!
love it!
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Posted by: mresistor
Insert Quote
Man oh man , that is a kickin' lil amp. Great sounds!
love it!
Roger that!
Hey Mr Geezer, what speaker/s were used in that recording?
Much difference between 6L6's and EL34's?
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Speaker is an Emi Private Jack in a 1x12 closed back, front ported cab.
The tubes are actually E34L's, which are "sorta" like a cross between the EL34 & 6L6 tone, and added more "punch" & clarity.
It could just be that all the 6L6/5881 variants I have are fairly well used & have lost their punch (past their prime), but I really like what the E34L's did for this amp.
I will post an updated (tweaked) schematic as soon as I have time to work it up.
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Hello,
This is my first post here. I've been reading up on the TOS and it sounds like a killer amp. I'm pretty stoked about building one. My question is: Does anyone know of a source for the 250K dual-gang pots?
thanks,
Gary
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I purchased an Alpha for my TOS from Antique Electronics Supply: http://www.tubesandmore.com/
Part # R-VA-8MM-2A
Between Doug's store here and AES you should be able to get everything you need.
I highly recommend to buy what you can here at Hoffmans first, especially the turret board.
I guarantee you won't be disappointed with Doug's service.
Ray
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Thanks Stingray,
I buy everything I can from Doug. His service is top notch!
Gary
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Geezer that sounded real nice wish I played that well, Have you ever got around to another schem. from where your at right know i think those el34L,s sound real nice. Bill
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This may be a dumb question, but I can't get my head wrapped around this master volume issue (not the PPIMV). I understand that the master only affects the clean channel. If that's the case, what's the difference between the master volume and the clean volume? Is it there so you can crank up the clean volume and get some clean drive and then control the overall level with the master?
thanks,
Gary
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This may be a dumb question, but I can't get my head wrapped around this master volume issue (not the PPIMV). I understand that the master only affects the clean channel. If that's the case, what's the difference between the master volume and the clean volume? Is it there so you can crank up the clean volume and get some clean drive and then control the overall level with the master?
thanks,
Gary
Hi Gary,
Basically you've got it.
Just to be on the same page (the controls have had various names put on them as this keeps evolving)
on the clean channel there is in order of process,Volume, Tone, Fattness, Master Volume, PPIMV
On my build I labeled the first Volume as GAIN. this sets the overall signal that is seen from here out.
Tone is self explanatory
Fattness I labeled as Depth, this controls how much signal is inserted into the second parallel gain stage
Master Volume I labeled mine as Clean Level.
OK
So when I set this amp up, I put the GAIN on 2 or 3
Tone and Fatness to taste
Clean Level around 5
Now the OD side.
I set the OD Drive to about 3
Tone to flavor
OD level around 2
NOW strum a few chords switching between OD and Clean channels.
set the Gains and Levels (master volume on clean side) so that there is no noticeable volume difference between channels.
from there, I can set the gain higher on the OD channel and use it like a lead boost pedal.
at least thats how I've set it up every time I let some one use it, LOL I'm not a musician.
In a nut shell the Master Volume as Tubenit labeled it and the OD level I use to balance the perceived volume of each channel. the PPIMV then is used for absolute volume levels for both channels simultaneously
Hope I haven't confused you more :)
Ray
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Thanks Ray,
Very nice explanation. I get it now.
Gary
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What are people using for the OD switching? There's a few mentions of relays through out the thread, as best as I can tell a regular DPDT switch would work. Are there any kits anyone can recommend for a drop in relay board?
Thanks
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Are there any kits anyone can recommend for a drop in relay board?
I use the Weber relay boards (low power version).
http://taweber.powweb.com/store/chansword.htm
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Wow that is the kind os sonic beauty I have dreamed of owning. That is some really fabulous fretboard work I must say!
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Thanks Geezer! I also found one on the london power site, it's like 4x as much though.
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What are people using for the OD switching? There's a few mentions of relays through out the thread, as best as I can tell a regular DPDT switch would work. Are there any kits anyone can recommend for a drop in relay board?
Thanks
REALLY??
C'mon Nate! if you can build the power supply for a tube amp, a switching relay will be a walk in the park!
I build mine on a proto board from radio shack, relay can be bought from Doug if you like dependable relays and fast shipping.
LOTS of builders here would walk you thru it.
The TOS I built had MANY firsts for me, a switching channel was one of them, most likely the easiest part of the build.
Ray
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Ha! Nothing like a swift kick in the pants to get the old gears spinning. A quick google search got me sorted on how relays work, I don't know why it never made sense to me. :rolleyes:
Stinray, have you tested that diagram you have further up the page for using a panel switch and footswitch? I'm worried about shorting the control voltage to ground, or is it too low to pop circuit breakers?
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Nate,
I implemented that schematic exactly.
You aren't shorting the control voltage to ground, That is a coil, one side to + the other to ground. You're simply switching the ground side of the coil.
Ray
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Hello everyone,
I'm in the middle of my TOS build and have a question about where to use sheilded cable. Is it necessary to use it on all the runs from the pots to the board or is that overkill?
thanks,
Gary
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I tend to only use it on long runs & not short ones from the board to pots.
This photo is NOT a very good example because it's not clear. The "thicker" black wires are the shielded. I used quite a bit black wiring to ground to a buss bar so it's a little confusing, IMO. The thick black wires on the socket side of the tagboard are B+ wiring that has shrink tubing around it (probably overkill).
http://s28.photobucket.com/albums/c216/tubenit/?action=view¤t=completedchassiswiring.jpg
With respect, Tubenit
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Anybody know if this is the kind of dual-gang pot needed for this build? http://www.mojotone.com/guitar-parts/Guitar-Potentiometers/Fender-CTS-250K-Dual-Stacked-Potentiometer-TBX-Tone-Control
Thanks!
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This is what I used>>
http://www.mouser.com/ProductDetail/Alpha-Taiwan/RV24BF-10-15R1-A250K/?qs=R1h7DEsyLZi9EhQqk47wow%3d%3d
http://www.mouser.com/catalog/specsheets/TW-700160.pdf
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Thanks Geezer! I've spent all morning trying to find a single distributor that would have 3 parts I need, to save on shipping, but since I've had no luck, it looks like I'll be placing a couple orders anyhow.
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I'm still a little foggy on the relay wiring, what's on the schematic is clear but how exactly to wire the AC to the solenoid isn't. I'll be using the LED/bezel combo that Doug sells and attempting to do the switching al'a Ray's diagram. I think I tie one leg of the 6.3v supply to one of the solenoid pins but ahhhhhhn what about the other one? Or do I have it completely wrong. Here's what I have so far...
(http://i254.photobucket.com/albums/hh118/nateflanigan/relay.jpg)
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Is that Doug's 5 volt DC relay? If so, you gotta build a power supply for it. See this...
http://www.el34world.com/projects/relay_switch.htm
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Wow, that is something else. Couldn't be more clear now, thanks!
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I used a 6vdc wall wart for the power supply I just took the case off wired it to the power switch and done! Bought mine at the .99 store LOL. I also used a Linrose Superbrite LED PN 3990a5 with a 220 ohm resister in series wired right across the relay coil terminals and now have an OD indicator light you can see across the room.
Tony
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How critical is the filtering in that circuit?
http://www.el34world.com/projects/relay_switch.htm
I've got a bunch of 1000uf 10v caps around i'd like to use.
Thanks!
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It's not critical. I'd use those 1000µFs in a heartbeat.
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Thanks, I didn't think it would be an issue. Now if I could just find some available space in the chassis...
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I have been keepin an eye on this magnificent project since Tubenit and Geezer started it. I want to have a go at it and build one. I downloaded the schematics and layouts but can´t open the .sch files. Can you recommend a program that can? I have Microsoft Windows 7 Enterprise on my computer.
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http://www.expresspcb.com/ExpressPCBHtm/Download.htm
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I have been studying the SCH´s and the postings on this very interesting amp. Looks like a masterpiece. I will definitely order from Doug soon, what it takes to build one. What is the Wattage rating of this amp with 5881?
My "ordinary" english is OK but I still struggle with technical terms and especially abbreviated technical terms. Abbreviations are not used in my language that much.
What does PPIMV and VVR stand for and what is the basic difference?
Please don´t laugh much, but it took me awhile to figure out what IMHO meant, and that´s not a technical term :grin:
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Throstur,
Don't feel bad, I was born and raised here in the states and I don't always understand what their talkin about. :laugh:
I'm 52 and I don't remember abbreviations being used very much, some things but not many, in every day conversation. I think it's more of a computer/internet driven thing. I think this internet thing kinda has it's own cluture, I'm not knocking it, but it's not quit the same as talking face to face with someone or on the phone. BTW (by the way :angel) it took me a couple of months to figure out what IMHO was too. :rolleyes: I don't know where your from, but you post/write better than I do, I'd never guess that your not from the US or any other english speaking nation. For What It's Worth IMHO you'll do fine.
Anyway, Post Phase Inverter Master Volume and Variable Voltage (I should know this 3'rd letter, but...) Resistance? It's for lowering your Out Put/amps volume while keeping the "same sound" of an amp thats turned up. There's a thread going on for the last few days on it. Blues Bear, If I Recall Correctle, started it.
All the guys here are very nice, so if you don't understand something just ask, they'll be glad to help.
Here's the thread...... http://www.el34world.com/Forum/index.php?topic=10977.0
Brad :smiley:
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Well Brad here,s were Im at I just had to google it to find out what it meant.(LOL) IMHO Im behind the times. :laugh:
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Like I said ALL the guys here are very nice. I feel right at home.
BTW, LOL? :laugh:
Thanks, Brad :smiley:
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VVR is variable voltage regulator.
Tony
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Thanks guy´s, a good laugh is worth 30 minutes of jogging and a lot more easier :laugh: I have been hanging around this forum for many years on and off and have often be led through problems by bnwitt, prr, sluckey, tubenit, da geezer to name but few.
I´ll definitely go for the VVR then. There has be much talk about the overdrive and other nice features in this amp. I´m also very interested in how it works clean.
Has it around 40 Watts output?
Does it have good headroom?
Can the clean sound be compared to some amps we know, like Fender, Marshall, Vox e.t.c.
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Throstur What i liked about the cleans on this was you cant compare them to any of those amps probably more like a dumble or train wreck clone it had blooming which those other amps dont really have in there cleans. depending on the setting of the first volume knob, then with the setting on your guitar volume itself. turn my guitar volume down to 3 and it has an almost jazzy sound turn it all the way up and you get a great blues clean with a little bite how much depends were you set your first volume knob. That said you could also dial in the cleans of those other amps also depending were you had everything sitting.
But if you look at some of the other layouts use the one that parallels v1b with the unused triod you will have thats the one I built and wouldnt do it any other way. Bill
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VVR is variable voltage regulator.
Dhoo! :BangHead: Thanks Tony, winter brain freeze. :icon_pale:
Brad :smiley:
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Has it around 40 Watts output?
I am using a 20 watt OT & 5881's. My guesstimated wattage is 23watts measuring across speaker terminals. It can get loud which makes the VVR very helpful to me. I think this amp cuts thru a band mix really well, IMO.
With respect, Tubenit
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Throstur What i liked about the cleans on this was you cant compare them to any of those amps probably more like a dumble or train wreck clone it had blooming which those other amps dont really have in there cleans. depending on the setting of the first volume knob, then with the setting on your guitar volume itself. turn my guitar volume down to 3 and it has an almost jazzy sound turn it all the way up and you get a great blues clean with a little bite how much depends were you set your first volume knob.
Sounds great Bill :smiley: I see that Tubenit uses 20W OT. I seem to remember that Da Geezer uses Super Reverb iron? Could be wrong though. What kind of PT/OT did you use Bill?
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I used the weber clone of the super reverb Iron. Same voltage as Geezers amp. all I had were some jj 6l6,s but they do the job. I keep thinking Im going to spring for some new 5881,s just hasnt happened yet.
Bill
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I wanted to give the fixed bias idea a try just for fun. I built a little board and everything but once I got it hooked up I realized the bias pot I had around from a fender was only a 5k pot. The bias circuit is just in the chassis, it's not wired to the amplifier circuitry itself. At the wiper I'm measuring about -15v to -18v. I'm guessing that's high since I have less available resistance than ray's schematic calls for. What range of voltage do I want coming out of the bias circuit?
Thanks
Like this?
I've not built a fixed bias amp yet, and things seem to keep pushing me to on this build.
And where would I put the 1 ohm resistor to measure current draw?
Thanks Tubenit,
3 very valid points and I do agree on all 3. If this doesn't work for the best it should be easy for me to revert it back to cathode biased.
Ray
bias supply.GIF (14.73 KB, 455x472 - viewed 93 times.)
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Another question...
Can I pop a pair of kt88's into the TOS? In trying to research this all of really been able to determine is that it depends on the amp.
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I switch back and forth between 5881's and KT77's all the time I would think if you can handle the extra heater current it should work just fine. I put a second transformer in just for the fil's on the output tubes. The KT77's are much louder than the 5881's and I like the sound better. I would think the 88's would sound nice also.
Tony
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Thanks, I'll do the heater current math and give it a shot.
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Just to interject real quick, my TOS is using the Tungsol KT66 -which has 0.9A filament current, less than a true 66. It compares favorably to the 5881, to my ears. But, I also changed the clean side to 5E3 component values, 12AY7 and all. Plus the paralleled triode 2nd stage which I agree is key.
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The heater current would add up to ~4.4a, my transformer is speced for 4 amps. Probably ok to try it out and see if I like the sound, but if it's a keeper I'd add a separate filament trannie.
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two things...
First the bad, I tried setting up a fixed bias supply and messed something up. The amp is converted back to the original cathode bias but theres some problems. If I play quietly (like a comfortable living room volume) things are ok. If I bring up either the clean master volume, the first input volume or the volume on my guitar I cross some kind of threshold and crackling starts. Sounds a lot like scratchy pots (only I'm not adjusting pots) if I try to ignore it it slowly gets worse, then the power starts to fluctuate. When I tried out the fixed bias, it was putting out about -70v, the power tubes lit up like light bulbs, the bias wouldn't adjust down, so I turned it off. I've tried out a variety of tubes, the crackling happens either way. What do you think I fried?
The good, despite the above issues I thought I'd try out the KT88's. Played at low volumes I am loving them, they're really getting me closer to what I want from this amp. Really big and articulate. I cranked up the drive channel just to see what it sounded like, again huge, if you wanted to get this amp to do metal or modern heavy stuff this was lots of gain but still tight.
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Hello,
This is my first post here, thank you for a great forum and project. I am building a TOS, the rest of my parts should be here today. This being my first tube/amp project, I have a couple of (stupid) questions that I haven't been able to find the answers to:
1. On the schematic, there is a "fake" center tap created with two 100 ohm resistors off of the heater wires, it says to connect their ground to the cathode of the power tubes. Do I connect the ground wire directly to pin 8 of the sockets or just to the same ground point that ties to the 10W resistor and 150uf cap coming off of the sockets?
2. All of the pots I have are audio taper, except for the 50k trim pot, which is linear taper. Is this correct?
3. There is .001 cap in parallel to the 180k resistor on the schematic (between D and V2 pin6) that is not shown on the layout. Was I correct in following the schematic?
That's all I've got right now, I'm sure I will come up with more dumb questions when I start wiring things up later. Thanks guys!!
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This being my first tube/amp project
Quite ambitious for a 1st project......
1. On the schematic, there is a "fake" center tap created with two 100 ohm resistors off of the heater wires, it says to connect their ground to the cathode of the power tubes. Do I connect the ground wire directly to pin 8 of the sockets or just to the same ground point that ties to the 10W resistor and 150uf cap coming off of the sockets?
Directly to the cathode (pin 8)....you are "elevating" the heaters slightly for improved quietness (theoretically)
2. All of the pots I have are audio taper, except for the 50k trim pot, which is linear taper. Is this correct?
OD "drive" is also linear, but won't make too much difference.......
3. There is .001 cap in parallel to the 180k resistor on the schematic (between D and V2 pin6) that is not shown on the layout. Was I correct in following the schematic?
Yes, the .001 snubber needs to be there.
Good luck!
G
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Thank you for the response
This being my first tube/amp project
Quite ambitious for a 1st project......
:icon_biggrin:Not my first project, just my first "from scratch" tube build. I have built a ton of studio gear and have recapped and performed mods to a couple of tube amps, I'm just trying to learn as I go (so, again, I really appreciate everybody's willingness to share their expertise here). That being said, I have a problem...
I finished wiring everything up last night, powered on, light came on, heaters lit up. I took the amp off of bypass and pop goes the fuse immediately. First stupid mistake, when I screwed the cap can down one of the keps nuts caught the edge of the B+ wire going to the power tubes and grounded it. I felt really stupid for a couple of minutes and then proceeded to remedy the situation.
Second power up, lights came on, took the amp off of bypass, lights remained so I plugged in my fiddle and began to raise volume knobs. I got sound, it sounded kind of distorted and then pop goes another fuse. Time to inspect wiring again. Found nothing.
Feeling ballsy, I went for a third try, this time with the gtr plugged in from the getgo. Taking the amp off of bypass and playing immediately yielded a great sounding gtr tone. But now I noticed that within about 10-15 seconds the plates in the power tubes would begin to glow red so I quickly bypassed it. Same thing happens with two different pairs of 6L6's (RCA/Sylvania).
I used the BOM and schematic found here http://www.el34world.com/Forum/index.php?topic=9636.0 (http://www.el34world.com/Forum/index.php?topic=9636.0)
The only difference is that I added the relay switch circuit found in Hoffman's library page. Another strange issue is that when I throw the channel switch, the LED indicator comes on bright and then dims down about halfway within a second or two. I used a pretty large red LED that I clipped from some random board I had laying around, so I don't know any specs on it.
Measuring voltages coming from the filter caps gave me readings from 30-50V below what was listed on the schematic:
A = 380V
B = 374V
C = 304V
D = 262V
E = 186V
F = 185V
I will trace out the circuit again and do some more research when I get home from work later. I know I must be close, the amp sounds great for the few seconds that I have it on, and all of the controls seem to function properly. I just keep getting the red plates within 10-15 seconds. If any one could give me any pointers or advice I would really appreciate it.
Thanks guys.
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What voltages do you measure on pins 5 and 8 of the output tubes?
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And what size power tube cathode resistor do you have and the voltage across that cathode resistor also, please?
With respect, Tubenit
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Voltage on pins 5 and 8 is 0 without tubes installed
With tubes, around 50mV on one and -50mV on the other. The reading never quite stabilized enough to be accurate before I had to hit the bypass due to plate lighting up.
The cathode resistor is 430R 10W as per the schematic, and measures a steady 0V with or without tubes.
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"plate lighting up" and "The cathode resistor is 430R 10W as per the schematic, and measures a steady 0V with or without tubes" are two inconsistant symptoms. Red plates means too much current is flowing through the plate. Plate current must flow through the cathode so there should be a healthy voltage dropped across the cathode resistor while the plates are glowing red. Unless something is really messed up in the wiring. Can you post some hi rez pictures of the output circuit?
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I'm wondering if the cathode resistor is actually grounded? Any way you could check that out. Measure the "positive" end and then put the other probe on the chassis.
With respect, Tubenit
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Wow
What a humbling experience...
I think I am in the clear now. I reread the response to my first post and realized I must have misunderstood the answer or just poorly worded my question regarding the heater resistors.
I had connected the resistors directly to ground, and then also connected pin 8 of the power tubes directly to ground. Common sense should have told me sooner that I was bypassing the cathode resistor and caps.
I removed the ground wire, no more red plates, and now I get 40V across the cathode resistor. And the amp sounds very nice, too. Hopefully I'll get a chance tomorrow to really open this thing up and see what it can do.
So now I'm wondering if I should have connected the 100 ohm resistors from the heater wires to pin 8 instead of to ground?
Anyway, I have to say thank you one more time for all of the fast, helpful responses. And also a big thanks for those of you who put this great project together, and to Doug Hoffman for his excellent service.
(On a side note, last week I just finished wiring a new theatre system at a college down here, and out of well over a thousand connections made, testing revealed only two faults - pins 1 and 2 reversed on a couple of mic jacks. I had been gloating a little, but now this whole amp experience so far has helped knock me down a couple of notches, which I obviously deserved.)
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So now I'm wondering if I should have connected the 100 ohm resistors from the heater wires to pin 8 instead of to ground?
Either should work fine. You can try both and see what is quieter. I just connect my 100 ohm resistors to ground.
I am willing to bet all of us have made comparable or goofier mistakes at some point. Just part of the process.
With respect, Tubenit
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Hi,
don't mean to hijack the thread...I'm planning a TOS build with 4x6AQ5/6005W and would like to include VVR as well, however I have never done it and was wondering if NTE2973 will withstand current consumption for a quad of 4x6AQ5/6005W. I will follow tubenit directions about vvr'ing power plates, G2 and P.I. Also, I searched mouser for NTE2973 and got a message saying: "Not recommended for new desings"..Does it mean NTE will discontinue this part? Are you aware of a suitable replacement/upgrade for it?
With Respect.
Best Regards.
Rzenc
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Should the 250k drive pot be audio taper also? Thanks!
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...wondering if NTE2973 will withstand current consumption for a quad of 4x6AQ5/6005W.
searched mouser for NTE2973 and got a message saying: "Not recommended for new desings"..Does it mean NTE will discontinue this part?
It should easily handle the current comsumption - it's rated for 14A continuous, and No, ignore the message.
Should the 250k drive pot be audio taper also? Thanks!
No, use a linear taper pot since it's a votage divider
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Thanks!
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What are people using for the OD switching? There's a few mentions of relays through out the thread, as best as I can tell a regular DPDT switch would work. Are there any kits anyone can recommend for a drop in relay board?
Thanks
REALLY??
C'mon Nate! if you can build the power supply for a tube amp, a switching relay will be a walk in the park!
I build mine on a proto board from radio shack, relay can be bought from Doug if you like dependable relays and fast shipping.
LOTS of builders here would walk you thru it.
The TOS I built had MANY firsts for me, a switching channel was one of them, most likely the easiest part of the build.
Ray
That's what I thought as well but I fried a power transformer by trying to get the control voltage from the heaters (center tapped) using a bridge rectifier (grounded). I am yet to successfully implement channel switching into an amp - after six successful complete single channel builds. I would really appreciate more info on this.
My TOS build is getting closer - I already started to drill the chassis. But it will only become a TOS if I get the channel switching down. BTW I will use a Hammond 372JX transformer which has a 5V-0-5V secondary winding. How would I go about using this with a 5V relay - just use one 5V and the 0 tap or both 5V taps? And do I connect the 0V to ground?
Thanks and kind regards,
Stephan
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IF you do a search on the AmpGarage forum for "relay switching" ............ you will find ALOT of information.
http://ampgarage.com/forum/search.php (http://ampgarage.com/forum/search.php)
And in Hoffman's Library of Information, there is 5 pages of schematics, layouts and info:
http://www.el34world.com/projects/relay_switch.htm (http://www.el34world.com/projects/relay_switch.htm)
With respect, Tubenit
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Thanks, T. The Hoffman library is very instructive.
Cheers Stephan
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How would I go about using this with a 5V relay - just use one 5V and the 0 tap or both 5V taps? And do I connect the 0V to ground?
If you will not be using that 5 VAC winding for anything else, I'd use it for the relay supply. BTW, that PT is not 5v-0-5v. It's 5VAC with CT, which is to say, 2.5v-0-2.5v. Wire it like the circuit in the pic. Don't use the center tap. It's OK to connect the negative ground side to chassis. The 7vdc output will drop some when you connect the relay coil. If it doesn't drop enough to suit you, put a resistor in the positive line.
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Swapping out the transformer was a breeze, probably the easiest "mod" that can be done to an amp, just pop it in and solder a few wires - nothing to it. Well, the result was simply spectacular! Gone were the nasal, thin sounding BAAAD over-driven sound that I remembered. With the new OPT, I can easily dial in a number of great sounding "patches" - from clean cleans to growling Marshall-y overdrive. Immediately, the amp moved from the bottom of the heap to the very top, just like that.
Excellent! That's good to know. Thanks for sharing it.
With respect, Tubenit
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Hi folks,
What dimension chassis would be appropriate for the TOS? I am interested in putting it in a combo cabinet. What similar sized Big Name chassis/cab would be perfect?
Regards.
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I used a 20" x 8" x 2.5" blank chassis. It worked fine and would work just fine for a combo amp also. There was quite a bit of room.
The blank was supposedly for a Marshall or Bassman head. There are obviously other options that would work as well. You could look at the Fender Super Reverb style chassis if you want to go more that route. You'll need to make you own front panel plate of course.
with respect, Tubenit
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Occasionally, you can find blank "amp chassis" on ebay. This is an example, not sure about the thickness of the aluminum though?
http://www.ebay.com/itm/Tube-Amp-Chassis-19x8x2-12-gauge-/171015101106?pt=LH_DefaultDomain_0&hash=item27d14b5ab2 (http://www.ebay.com/itm/Tube-Amp-Chassis-19x8x2-12-gauge-/171015101106?pt=LH_DefaultDomain_0&hash=item27d14b5ab2)
With respect, Tubenit
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The title says 12 gauge I think? Right? :w2:
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Thanks for the cab specs, Tubenit. It's the thought of that odd millimeter here or there that would complicate it's fit in the cab or is not as big a deal as I imagine?
1. Super Reverb is the cab size to go for, or is a generic 12" speaker type sufficient?
2. Is there room for ears or should the side folds be bent inwards?
The thickness of the aluminum would be 2mm, at least in these parts.
A little personal background: With the help of an engineer friend and after a year or so of reading up, I put together a 50 W plexi type and a chimp er champ but the enclosures left something to be desired. This project I want to look its best. From all the raves about the design it would be a darn shame not to.
I also had an experience similar to Jazzbo8's:
A guy spun me the trannies but with "hifi" in mind. After taking them out and putting in some old iron ("Levant"?!) the 50 watter sounded much better.
BTW: An ancient pair of Telefunken EL34 also sounded better than the (caveat emptor!) TubeStore.com "matched" set.
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I have always built my own cabs ............ so matching the cab and chassis is probably something you'll have to figure out on your own OR
use something that is already a match like Super Reverb chassis and SR cab. My chassis has the ears out.
The TOS is a "tweaker's amp" which means that you may or may not like the "original" design of it? Be prepared to build it & then tweak things like coupling caps, tone stacks, cathode values ............. to get it to suit your style of playing, speaker, guitar .....etc
With respect, Tubenit
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Does this sound right? The 3 H choke at 120 hertz offers 2.25 kOhms impedance. To swap the choke with a resistor would one use 2.2 Kohms?
Assuming a current of 160 mA this makes 60 watts on the resistor. What is the safety factor here? ie: what power capacity should it have?
Re the cab discussion: after shipping, customs, etc. the box would cost almost as much as the rest of the amp!
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Does this sound right? The 3 H choke at 120 hertz offers 2.25 kOhms impedance. To swap the choke with a resistor would one use 2.2 Kohms? Assuming a current of 160 mA this makes 60 watts on the resistor. What is the safety factor here? ie: what power capacity should it have?
Usually when replacing a choke with a resistor you would chose the resistor value to be equal to the dc resistance (DCR) of the choke. For example, a 3H choke 'may' have a DCR of 150Ω, so use a 150Ω resistor. And if the current will be 160mA the resistor will dissipate 3.84 watts. Double that wattage for a safety factor (7.68 watts) and get the next common resistor wattage. In this example I would use a 150Ω/10W resistor.
But, if you're talking specifically about replacing the 3H choke in the TOS amp, there won't be anywhere near 160mA flowing thru that choke because the output tube plate current does not flow thru the choke. There's more likely only about 10mA flowing thru the choke so the replacement resistor could be lower wattage.
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10mA- of course! Not the first time I've made that mistake. I don't get the theory behind the choke's DCR. Copper resistance yes, but as to the function of the inductor I thought that it would show the 3 H at double the mains AC: X = 2piFL.
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I don't get the theory behind the choke's DCR. Copper resistance yes, but as to the function of the inductor I thought that it would show the 3 H at double the mains AC: X = 2piFL.
Inductance is not dependent on frequency, so a 3H choke is 3H at 60Hz, 120Hz, or 1000Hz.
On the other hand, Inductive Reactance (XL) is dependent on frequency. As frequency increases, so does Inductive Reactance. And that's the beauty of a choke used in a power supply. A perfect 3H choke would offer 2261Ω of resistance to a 120Hz ripple signal, but it would offer 0Ω resistance to DC. Ideally, we want the choke to offer a lot of resistance to the AC and no resistance to the DC. This is how a choke helps to 'smooth' the AC ripple into a relative 'flat line' DC. An inductor (choke) opposes any change in current (AC component) and a capacitor opposes any change in voltage (AC component). When a choke is used together with a couple capacitors in a PI arrangement, they work together to smooth the AC pulses to a usable DC.
But an inductor is not perfect. There will always be a DC resistance due to the resistance of the wire. This unavoidable resistance is in series with the choke and is responsible for any DC voltage drop across a choke. And that DC voltage drop is what you are trying to mimic when you replace a choke with a resistor. By using a resistor whose resistance is equal to the DCR of a choke, you will maintain the same voltages downstream on the B+ rail. For sure, the filtered DC will not be as smooth as it would be with a choke, but that's not always necessary or desirable in a guitar amp.
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If I understand Sluckey correctly, putting up only 6% of the choke's impedance (150/2.25K) at the offending frequency is what one gets for swapping out the choke. This effects the circuit's response, I would guess giving more sensitivity and a little more voltage but with the cost of some noise.
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Please keep in mind that I used 150Ω DCR purely as an example. I pulled that number straight out of my back pocket. It is a fairly realistic number though. Generally speaking, a 3H 200mA choke will have a smaller DCR than a 3H 100mA choke. More current capability usually means larger wire which usually means lower resistance.
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If I understand Sluckey correctly ...
Are you needing to replace the choke with a resistor in your build, or is the question just academic?
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Sluckey, great explanation. I knew that a choke was "better" but didn't know why. And of course, this is why a choke is better money spent on a SE amp rather than push-pull, because SE is naturally more noisy and the PP tends to "cancel out" the hum. <insert light bulb moment here>
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Anyone out there with a TOS sized choke willing to put a VOM on it and let me know what its DC resistance is?
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Ross,
FWIW .......... I don't think it's a critical factor at all! The COS I built had a choke. I later converted it to a TOS when I sold it. Then TOS I built for myself had a resistor. The TBM has a resistor. The D'Mars ODS has a resistor. I think those are all 1k/2w?
I think anything from 220R/2w to 1k/2w would be fine. I have switched between a resistor and choke before and didn't hear much difference. Choke maybe was a little smoother but not much.
With respect, Tubenit
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Anyone out there with a TOS sized choke willing to put a VOM on it and let me know what its DC resistance is?
I don't have one handy but the BOM lists the choke as 125C1 (probably should say 125C1A). Hammond 194B is a direct replacement and it's DCR is 108Ω +15%.
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Thanks, Slu!
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Finally getting around to this project, caught my attention a year ago and had it bookmarked since. Iv been studying the schematics posted for both the VVR and PPIMV versions, and I assume the PPIMV schemtic would be a good starting point, and I could always experiment with VVR later. Pretty excited to take on this project, planning to tweak the design to my tastes, and I have also been drawing up a quick visio layout that I can post here if anyone is interested.
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a quick visio layout that I can post here if anyone is interested.
That'd be great! And I'd love to see one.
With respect, Tubenit
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Hopefully this is the correct place for this. I figure if my layout is correct it could someone in the future.
Im building the standard version TOS and have a few questions and also was hoping to share my layout. I do my layouts with affinity Design 2 on ipad, its easier for me than anything else i have tried.
The biggest problem I'm having is with the relay. First I made a layout of the TOS with just a dpdt switch for the channel switch like in the schematic. Then i made one using The Hoffman relay boards. My Transformer does have a center tap so i made sure to keep the power supply for the relay floating, the schematics and diagrams available show the bottom left terminal of the relay board connected to ground. In my layout i connected that terminal back to the “negative” terminal, as its been described, of the relay power supply board. Is this correct? I also didn’t include the LED or foot-switch jack but i understand how that works and I’m not sure if I’m going to use the relay yet.
I have a few other simple questions. The 5879 only has two heater connections, was i correct to assume that i would connect one heater to pin 4 and the other to pin 5?
Lastly, i made a board for my filter caps. Its quite big so i plan to use tall standoffs and mount it partially overtop of my power transformer cutout. Would it be better to cut the board into two separate parts? Im not sure if raising it up off the chassis too much could cause problems or if its just better to separate it regardless.
Thanks for the help, I've learned so much from this forum but i have a long way to go. This is my 5th amp and without these discussions i wouldn't have been able to do any of them.
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You have the negative side of the 5V power supply board connected to a relay contact. That's wrong! The negative side of the 5V PS should only connect to the switch and then to the relay coil.
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You have the negative side of the 5V power supply board connected to a relay contact. That's wrong! The negative side of the 5V PS should only connect to the switch and then to the relay coil.
Thanks for such a fast response!
I had a feeling that was incorrect but i cant figure out whee the bottom left terminal on the relay board goes. The schematics and diagrams all show ground. Which i understand that the signal needs to be switched to ground but because im using my transformers center tap i wasn’t sure if it was ok to connect the relay board to ground or if it was only the power supply that you never connect to ground. So i should connect the bottom left terminal to the ground bus on my pots? Thanks Sluckey, your an incredible source of information and i truly appreciate you and your time.
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So i should connect the bottom left terminal to the ground bus on my pots?
yes
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Lastly, i made a board for my filter caps. Its quite big so i plan to use tall standoffs and mount it partially overtop of my power transformer cutout. Would it be better to cut the board into two separate parts?
On all 3 of my amps I use turrets on two strips. This has worked well for me and I like being able to get at any wiring underneath IF needed. And I use somewhat taller standoffs.
with respect, Tubenit
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So i should connect the bottom left terminal to the ground bus on my pots?
yes
Thank You!!!
Lastly, i made a board for my filter caps. Its quite big so i plan to use tall standoffs and mount it partially overtop of my power transformer cutout. Would it be better to cut the board into two separate parts?
On all 3 of my amps I use turrets on two strips. This has worked well for me and I like being able to get at any wiring underneath IF needed. And I use somewhat taller standoffs.
with respect, Tubenit
Thank you as well! You guys are the best! Ive been planning and preparing this build for a really long time and i think i am ready to start drilling my chassis.
Ill update my layout and post it just so that if anyone in the future can use it it'll be here.
Thanks again Guys your truly great.
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I haven't forgot to post my layouts and info, i’m trying to finish my faceplate still. Doing it after the build is not a great idea. I cut a piece of 16 gauge steel with a chisel, hammer, and a hacksaw. Not easy lol Ive made 4 wooden templates and can finally drill it. I have a co worker 3d printing me some stencils i designed and if that works out it will finally be complete.
For anyone looking for info before i finish i have everything on my website www.fictitiousfuzz.com/tubeamps
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Hello, I am planning on building the TOS, and I had a question about the negative feedback. Is it worthwhile to wire this as a switched NFB or a presence control?
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I would build IAW the schematic. Once the amp is up and running is a good time to play with NFB.
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Thank you!….another question about the Tweed Overdrive Special…What OT primary impedance has worked well with the 5881’s? I am thinking 6600, which is the impedance of the 018343 OT, although many clones of this OT are 4200. I don’t need any extra power…
Also, the 022798 replacement PT is a little higher voltage than shown on the schematic (355-0-355, though Hammond does make a 325-0-325)…this would be around 460V B+. The JJ 5881 data sheet says 500 plate volts max, and I think I can subtract the cathode voltage from the B+, but are there any other issues with the slightly higher voltage?
Thank you in advance for your help. Really appreciate it and look forward to building this amp.
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I think of the Tweed Overdrive Special as a "tweakers amp" meaning it's one you build and then tweak/mod to best suit your taste and tone in playing. So, in my thinking the TOS doesn't require/need precise following the original schematic to get a great tone. However, I would lean towards following the schematic 1st and then make small changes at a time to mod it to your tone preference.
I think the 6600 OT is fine. Dumble purposely mismatched OT impedance on some of his builds.
Over time, I have found myself preferring and enjoying lower voltages on output tubes. I think sometimes there is more warmth in tone playing at lower/softer volumes with lower voltages on output tubes. Just my opinion and experience and it's subjective. Others may feel the opposite.
IF I were building another TOS, I would probably use a Hammond PT with 300-0-300 and at least 140ma. I think 325-0-325 is fine also. Given the TOS is cathode biased, using 300-0-300 would comfortably allow the use of either 6V6's or 5881/6L6 using a GZ34 or even a solid state plug in rectifier.
I do think some specific brand resistors for plates and specific caps can play a factor in a smooth tone with this amp. Again that's just an opinion, personal preference and my experience. What I prefer may not suit others at all. You can check out The Amp Garage Dumble clone files to see about resistor and cap preferences with those amps.
(10) #102 Robben Ford Amp (High Plate Skyline) - The Amp Garage (https://ampgarage.com/forum/viewtopic.php?f=5&t=12873)
with respect, Tubenit
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Thanks for the quick reply! Hammond makes a PT with two taps, 355-0-355 and 310-0-310. I may try that one, which has some flexibility for tweaking, or go with the 325-0-325. I’m using a super reverb chassis, so I’m trying to fit the existing PT cutout, which limits the choices somewhat.
Thanks for the help!
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Probably not the best place but tubenit .I have found I like lower voltages on some tubes also. For a non Marshall amp I like about 385 volts on my el34,s sounds really good. Bill