Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on June 19, 2011, 04:41:38 pm
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Finally got this amp compelted. Sort of a complicated build for just 3 tubes and single ended.
I am VERY happy with how it turned out. Similar blooming and sustain to the Tweed Overdrive. Harmonics are great.
I am as happy with this as the TOS and TBM builds.
However, it has more chime and less creamy smooth than the TOS. The chime is more like the HoSo56 type chime but with lots of sustain. Very touch sensitive to play. The overdrive is ALOT of fun! The OD is comparable to the Tweed BluezMeister and Tweed Overdrive but with more chime. Possibly the 6BM8 pentode being the factor??
My guess is Geezer would really like the chime in this amp along with the solid state rectification. That feature makes it pretty touch sensitive. I think this "small" amp sounds better than the 56T with 6BM8's and VVR.
I expected the amp to be noisy being a small chassis, sloppy wiring and cramped. However, it is VERY quiet at idle. Both the clean channel and the OD are extremely quiet. Much quieter than the VibroChamp I owned.
Yet this 3.5 watt amp is quite loud when cranked! Surprisingly so. I can dime all the controls and NO oscillation!
The clean channel is reasonable clean and has plenty of contrast to the overdrive channel. So it is genuinely useful to build both and not just the OD.
The digital delay sounds excellent both in the overdrive and clean channel. My guess is that is because of the CF mosfet and the CF triode/6BM8?
THANKS for all the help with this amp design especially with the mosfet CF which I view a success.
The updated/current schematic and layout are here: http://www.el34world.com/Forum/index.php?topic=11607.0 (http://www.el34world.com/Forum/index.php?topic=11607.0)
I'll post soundbits and voltages sometime in the next several wks.
With respect, Tubenit
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I am running the amp thru the cab with a Emminence Cannabis Rex.
The digital delay makes the amp sound HUGE at 3.5 watts :icon_biggrin:
And again, despite the messy wiring and not neat layout ............. the amp is very very quiet.
Tubenit
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You can't see it but there is a 12v zener with the mosfet CF like on the schematic.
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tubenit,
Another great set of ideas, and another great build.
I was waiting to hear how this amp was going to turn out. Mosfet in the signal path, :dontknow: But.... you say it's great, so I'll have to let this sink in.
I will say this, believe it or not, one of --- the best --- live guitar tones I have ever heard was a guy named Carlos Jonson, using a SS Acoustic amp (early 70's?) with a late 50's Gibson 335 with humbuckers. He's a Chgo. blues man.
I walked in a usual haunt called B.L.U.E.S., holds no more than 150 people tops and got a beer and started talking with a buddy of mine Bob Levis who played guitar for Lonnie Brooks and Otis Rush for many years. He was very excited about Carlos' playing and tone that night. As I sat there and listened I was floored, the man was tearing that guitar up. He was bending those strings as good or better then Albert King, Otis Rush, or S.R.V. and the tone was out of this world. Another friend of ours was there too, a guy named Mark Skyer, great guitar player and singer. He played a short time with Canned Heat and played for the great Etta James for a while among others. He said to me "I think Carlos is the guy who's wearing the Chgo. string bending crown right now."
So I said to Bob whats the amp he's using? He said an Acoustic (SS) amp. :w2: I said no way, I can't believe it. So I walked up front and sure enough it was a SS amp.
I've resisted telling this story here but with you putting a mosfet in the signal path in this build and it turning out so good sounding, I thought I'd go ahead and tell it now.
Glad it worked out so well for you.
Brad :icon_biggrin:
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Looks great, T! Hope to hear some sound clips soon.... :guitar1
I'll be building this into my "new" VJr head as soon as things slow down (this fall/winter, I suspect :sad2:).
Thanks for the updates!
G
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That gorgeous curly maple on top of everything else just kills me. Beautiful work all the way around.
I'm sure that the MOSFET cathode follower is necessary to drive the effects loop, especially where there is no recovery stage between the Return and the power tube.
Cheers,
Chip
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HOLY Sh&! :worthy1:
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The maple is beautiful, really nice looking!
Where do you get your chassis?
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I don't believe that it even works. :icon_biggrin: Until I hear a sound clip, I think you are LYING! :m8
Yer pal,
Jim
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nice work mr. tubenit. can;t wat to hear some clips.
:icon_biggrin:
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Cool glad it turned out so well, I like how your always thinking outside the box and bringing new ideas around.
Thanks Tubnit cant wait to hear it.
Bill
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Just beautiful. Love the wood!
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Congrats on the build tubenit. I'm glad to hear it worked out for you. When you get around to posting your voltages it will go a long way in helping me with my 5879 Champ build.
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T I have been admiring your woodwork , Ok jealous would be the right word that is just plain fine.
Bill
:worthy1:
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Nice work... It's far from sloppy!
You may need to move this to the solid state forum due to that Mosfet. :icon_biggrin:
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Very nice build
thanks for share schematic
Kagliostro
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You may need to move this to the solid state forum due to that Mosfet.
YIKES! Maybe that wasn't such a good choice for someone called "Tubenit". :l2:
Guys, thanks for the nice comments! The inspiration and help from this forum made this build (& many others) feasible for me.
I'll get soundclips and voltages up soon.
With respect, Tubenit
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Looks great , anxious to here sound clips. Tubenit where do get the solder lugs riveted under the eyelets on your board ? I'd like to try this and can't seem to find them at the usual places. Thanks, Jeff
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where do get the solder lugs riveted under the eyelets on your board
I have no idea? That was a cheapo tag board. When I have built Hoffman's turret boards, I find Hoffman's boards are MUCH MUCH sturdier. Even using a solder sucker on the solder lugs can rip them right off. That's how flimsy they are.
with respect, Tubenit
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I discovered an "issue" with this amp that I want to resolve. Ironically, I would play the amp with NO digital delay .... OR I would plug in the delay prior to turning the amp on. Well, I finally did a A/B comparison and discovered that the delay pedal in the passive effects loop lowers the volume somewhat. The amp without the delay is impressively loud for a 3.5 watt amp.
The digital delay in my other two amps with LTPI does not lower the volume at all. I am thinking the 220k grid resistors to the power tubes which are after the digital delay in those amps prevents a problem?
So, I was puzzled by that? Then I got to thinking that either the Mstr Vol on the clean or the OD2 pot on the overdrive was acting like a psuedo grid resistor to ground for the 6BM8 pentode when the delay pedal was not in place.
So when I insert a delay pedal between the pentode and the Mst Vol or OD2 pot, I think that maybe this is messing up the needed psuedo grid resistor (at some level)?
So now I am thinking I need to add a grid resistor to ground AFTER the passive effects loop? Does this sound like I am headed the right direction in this thinking?
Thoughts??? I realize that the two amps with LTPI have an additional gain stage following the passive effects and this amp doesn't. So it could be that maybe? However, it is plenty loud without anything in the passive effects loop.
I've also listed an option #2 that would move the passive effects loop prior to the master volume and prior to the 5879 OD which could also act as a recovery in the FX loop?
Thanks, Tubenit
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The effect may just be loading down the power tube.
Just guessing here... add a grid leak to the power tube. Try something between a 220k and a 1m.
If you turn the amp volume down and play through the effect is the volume drop as noticeable?
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If you turn the amp volume down and play through the effect is the volume drop as noticeable?
Great question! I'll check that out. And I'll try a grid leak resistor. I am thinking since the OD pot is 500k, that I'll start with a 470k and see what happens?
Thanks for your help! It is greatly appreciated.
With respect, Tubenit
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This is just an uneducated guess, but IIRC a higher grid return resistor should produce more gain from the power tube. Is is possible that the end of the delay effect circuit has something lower than 500K going to ground and that's what is reducing the volume? If that is the case, is there a way you could put an AC coupled cathode follower after the effects loop return and then have a 470K grid return resistor after that cathode follower? You've already let the genie out of the bottle, so what's one more MOSFET?
HTH
Chip
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Well, I am learning that a SE amp is different than a push/pull amp when it comes to a passive effects loop insertion point.
:l2:
I tried numerous grid leak resistor values to no avail. The Boss digital delay still had less volume and after careful listening I noticed it colored the tone with some slight distortion. In other words, the tone did not sound as transparent to me. Ironically, I could NOT tell any difference adding a grid leak resistor from the psuedo grid resistor of the master volume and/or OD2 pot.
So, I tried the delay between the guitar and amp. It sounded nicely transparent but it had too much white noise for me. I am somewhat picky on this issue and really like a quiet amp. It also had no volume loss.
So I thought, I wonder if I bypass the V1a input gain stage and put the passive effects between V1a & V1b if it would work?
It does !!!! Sounds fantastic to me! Very transparent with no volume drop at all. It adds delay to the tone without coloring the tone that I can tell. Works perfect on both the clean and OD. The white noise is cut way way down on the delay after the 1st gain stage. I can't tell that it adds any more noise between V1a and V1b then it did inserted prior to the 6BM8 pentode?
When I lowered the 220k down to 22k going into the grid of the 6BM8 pentode, that opened up the amp some also. I think it added to the transparency.
I am super pleased with the results of all of this! This is an ideal bedroom type amp for me. I can get the blooming and sustain at very reasonable levels. It is a very fun amp to play.
Would I rebuild this with the mosfet CF and the 6BM8 triode as a CF after the 5879. Yeah, I would. I don't know if the mosfet CF matters that much? I'm thinking it allows a somewhat clearer clean channel but I don't know for sure. I am fully convinced that the CF after the 5879 OD stage is pretty important to get the smoothness in tone that I want.
Updated schematic and layout are here: http://www.el34world.com/Forum/index.php?topic=11607.msg107840#msg107840 (http://www.el34world.com/Forum/index.php?topic=11607.msg107840#msg107840)
With respect, Tubenit
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Hi tubenit,
I wonder what it sounds like if you would jumper across the mosfet CF, to bypass it? Maybe it does'nt need it now that you have moved the FX loop?
Thanks, Brad :icon_biggrin:
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So I thought, I wonder if I bypass the V1a input gain stage and put the passive effects between V1a & V1b if it would work? It does !!!! Sounds fantastic to me! Very transparent with no volume drop at all.
Now you're thinking inside the box and outside the box :wink:
Congratulations on your amp T!
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Hi tubenit,
I wonder what it sounds like if you would jumper across the mosfet CF, to bypass it? Maybe it does'nt need it now that you have moved the FX loop?
Thanks, Brad :icon_biggrin:
I was wondering the same thing.
So you like having the delay before the overdrive section?
Cheers,
Chip
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i was thinking you'd want the FX loop global as well - as in either OD or clean then the FX loop.
attached is what i was thinking about...
respectfully,
--DL
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This is all very interesting good work T.
Bill
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I think I'll try bypassing the mosfet CF and see what tone change (if any) there is? And report that.
This amp is somewhat chimey-er than I prefer. I prefer the real creamy smooth tone more. It's more EL84-ish chimey, IMO. And I have typically not been a fan of EL84 amps for some reason. I think my son & son-in-law would love the chimey tone of this amp alot. I think both the 5879 and the 6BM8 bring some chimeyness to the table?
I may eventually possibly put the mosfet CF behind the 5879 and convert the power tube from 6BM8 to a 6V6? Not sure? I'll record it as is prior to converting IF I do change it.
With respect, Tubenit
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i was thinking you'd want the FX loop global as well - as in either OD or clean then the FX loop.
That's what I was thinking also D/L.
In prior (beginning) posts T said he really like the clean tone one way and the OD tone another way regarding the ss CF approach. This was when only one of the switching positions he stated that he liked and was perfect w/out the ss CF. Then when he developed/perfected the ss CF stage further, he incorporated both stages to the ss CF. I questioned this (why mess w/ something you like and say "is perfect") but he decided to do it his/this way anyway. I'm curious how this will all play out especially w/ T being the master of trial and error that he is!? :dontknow: :bravo1:
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The reason I changed the insertion point of the FX loop from after the clean/OD to before the clean/OD is that after the clean/OD distorted & colored the tone somewhat & the volume dropped compared to no pedals plugged in.
Moving it between the V1a & V1b has maintained the same volume, much better transparency and less distortion.
Not sure why that is the case, but it is?
I had it wired exactly like DummyLoad suggested with the exception of the coupling cap after the passive FX loop prior to the grid leak resistor. Maybe that cap is needed & that's why I was having problems? I tried a variety of resistors and none made any difference to the volume drop & colored/distorted tone. Maybe that additional coupling cap would resolve that concern?
I think this is what you're referring to?: On my TOS with the tube CF after the 5879, I discovered the delay pedal sounded excellent with the OD on. However, on the clean channel of that amp with the CF, the delay doesn't sound as good. That is what prompted me to try a SS CF on the clean and use the 6BM8 triode behind the 5879.
However, doing that didn't work on this mini-TOS SE amp for some reason with the Boss digital delay? I don't know if the LTPI on the TOS made the difference or not?
With respect, Tubenit
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Yes, I understood why you changed the effects loop point and also said "way to think inside the box" referencing the possible loading affect by the effect. :) I thought this was very creative on your part.
The coupling cap "may" not be needed or make any difference but it's always a good idea to include and to be safe - not for safety's sake but for any possible DC that could find it's way from something being plugged in to the return jack. Effects boxes usually do this on their inputs & outputs as a matter of "good form" so that it also prevents any small DC from messing up something that it's plugged in to. Also this is similar to a coupling cap used sometimes before a tone stack or even before a pi stage when the signal is coming directly from a tone stack for two examples. There should be no need for the extra coupling caps used in those places but they don't do any harm.
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Jojokeo,
OK, thanks! I appreciate the info about the coupling cap post the FX loop.
I thought I'd post a sound clip before I start messing with really experimenting with the amp. Excuse the mediocre playing, please.
:icon_biggrin:
http://www.soundclick.com/player/single_player.cfm?songid=10784031&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=10784031&q=hi&newref=1)
I had a 12AV7 in V1 in this clip. A 5751 gives it more sustain and blooming.
The volume was at about 5.5, the OD1 was about 5.5 and the OD2 was 6. There was plenty of room to increase blooming or sustain.
I don't think this amp is as smooth as the TOS or TBM amps? It's pretty chimey live. I have some friends that I'd think might prefer the chime more, but my personal preference is to get it a little smoother in tone. So I'll experiment a little more.
With respect, Tubenit
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I don't believe that it even works. :icon_biggrin: Until I hear a sound clip, I think you are LYING! :m8
Yer pal,
Jim
Ok, I guess I believe you now.... :laugh:
Sounds really nice! What geetar were you using? I'd also like to hear some chords blasting outa that thang!
Yer NEW pal,
Jim
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I think the problem with the fx-loop right before the power tube is that 6BM8/ECL82 tube needs driving voltage of 9,5Vrms for full power. Your delay probably gives the standard +4dBu line level output which is only about 1,2Vrms. So you need a gain stage to up that voltage. In push-pull amps LTP PI usually has enough gain for the passive loop to work.
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Ok, I guess I believe you now....
Sounds really nice! What geetar were you using? I'd also like to hear some chords blasting outa that thang!
WHAT !!!! A compliment??!! Gee, I know you must be up to something now. I will be vigilantly on my guard. :l2:
Well, prior to my smashing the guitar into smithereens at the end of the show .............. I was using the Tele (Note it was NOT a strat) on the left. However, I discovered people are less impressed smashing a guitar into a 3.5 watt amp then they are a 100 watt amp. Go figure!
:dontknow:
With respect, your sort sometimes pal back ...........Tubenit
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Oh....a tele......
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That's a very compact wiring set-up tubenit! (You might think your own work looks cramped, but its actually very tidy).
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I've drawn up a mini-TOS with a 6V6 version. This would use the mosfet CF following the 5879 pentode.
Haven't built it (yet). The editable version with a layout is here: http://www.el34world.com/Forum/index.php?topic=11607.new#new (http://www.el34world.com/Forum/index.php?topic=11607.new#new)
With respect, Tubenit
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T, what does the 470k + 250pF in parallel w/ the 250k trim pot & treble bleed cap being used for or how does it help and/or work exactly? I've seen this w/ the HRM & other dumble-ish builds also.
My initial thought or theory is that it's bleeding a tad bit of highs to ground through the 470k resistor thus helping to shape the frequency response of the trim pot for the signal going into that 1st OD stage?
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Is it possible to add negative feedback to this mini-TOS amp? I am wondering how that might work doing it as I have
drawn it? Any thoughts?
With respect, Tubenit
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it will become + feedback when you kick in the OD. maybe switch it out in OD.
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tubenit - if you read Keen's "MOSFET Follies" which discusses using a MOSFET as a cathode follower, he was careful to note that the solid state device would be more linear than a tube triode and would not provide the compressive effect of a tube triode cathode follower. It's worth trying, but I'm guessing that you won't get the nice compression that you noted with a tube triode in the CF spot after the 5879.
Regarding NFB, if you used a tube triode for the CF after the pentode in the 6V6 circuit, then you would have an additional gain stage which could be used as an insertion point for NFB. A low gain, cold biased triode might add to the amp anyway. Maybe a 12AY7 or 12AU7 in that spot?
Just a Sunday morning thought...
Chip
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Is it possible to add negative feedback to this mini-TOS amp? I am wondering how that might work doing it as I have drawn it? Any thoughts?
Are you trying to lessen gain? Maybe try a conjunctive filter accross the OT primary and see if this helps w/ what you're looking for?
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OK, I tried a few things. Let me start with what did NOT work out. What didn't work was removing the mosfet CF after the V1-b. I did not like the tone on the clear OR the OD ............. so the mosfet CF went straight back in. The CF seems to add some smoothness.
I changed the V1-6 plate coupling cap from .02 to .01.
I changed the 5879 cathode resistor from 820R to 410R.
Biggest improvement was a 22pf from 6BM8 pentode plate to pentode cathode. It cut down on the chimeyness somewhat.
With respect, Tubenit
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tubenit - thanks so much for going through this process with us! Knowing what didn't work for you is just as important as knowing what does work. Most amp designers never mention all of the things that don't work out - all we see is the end result of their experiments.
Cheers,
Chip
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T - I agree w/ Chip, but I'd personally would like to see what you think of the conjuctive filter instead of the 22pF cap? It would be a real quick and easy change-out to see how it works or not regarding your chiminess issue - that's part of what they're for and making things more linear which is also what NFB does (and you mentioned wanting earlier)?
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I'd personally would like to see what you think of the conjuctive filter instead of the 22pF cap?
Can you point me to a schematic with the conjunctive filter for a SE amp?
With respect, Tubenit
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OK, I found a website with some great information on a conjunctive filter for a SE amp. It has some excellent soundclips giving a clear demo of the differences. I like the smoothness I am hearing that it adds in the soundclips.
http://www.cs.cmu.edu/~ehn/ax84/ (http://www.cs.cmu.edu/~ehn/ax84/)
I will give this a try and report back.
With respect, Tubenit
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Is it something that's only limited to pp amps? I would think you could put a cap or cap+resistor across the pri tranny B+ and plate. I still think it'd be easy to try don't you think?
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The link I posted above gives both schematic and soundclips of SE amps with conjunctive filters. Yes, I plan to try it.
With respect, Tubenit
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Thanks for the link as I'm not on my home 'puter. BTW, good sound samples & I like the difference it made.
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OK, I have played the 6BM8 version for a few hrs and it's too chimey for me. It's more like an EL84 & I've gotten rid of every EL84 amp that I've owned or built. Just not my tone. I am gonna skip the conjunctive filter at this point.
I'm going a significantly different direction with this.
1) While the passive FX loop sounds good between V1a & V1b, I think it would sound even better prior to the V2 gain
stage. So I plan to move it.
2) I also discovered with the Mini-TOS that I can dial the controls with the OD engaged to get a very clean tone. Not
much difference between the clean and "OD dialed clean". However, the OD can also be dialed to get that blooming
sustain that the clean doesn't have.
3) So I am going to eliminate a switchable OD gain stage. Instead, I'll do some cathode cap switching to go from a
cleaner tone to a more overdriven tone.
4) I decided to go back to a PAB on the tone stack.
5) I am convinced some of the chimey-ness is from the 6BM8 pentode & the 5879 pentode also. So, I am going to
go with a 6V6 for a smoother tone. And I am going to lose the 5879.
6) And I am going to use a 12A_7 with a CF in the V2 position. I did this on my friends "Double C" amp and it sounds
excellent. This way I can use everything from a 12AU7 to a 12AX7 & dial in different gains with different tubes. I
think it will actually make the amp more versatile from clean to OD than the switchable clean to OD approach.
Don't know if I will have to get it all done this wkend, but I'll keep you guys posted.
With respect, Tubenit
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http://www.cs.cmu.edu/~ehn/ax84/#filter (http://www.cs.cmu.edu/~ehn/ax84/#filter)
Same link I posted a few posts up. Tubenit
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This started off as the Mini-TOS using a 5879 in overdrive with a 6BM8 triode CF going into a 6BM8 pentode. The mini-TOS was too chimey for me ............... so I converted into the Carolina SongBird.
This amp is much smoother tonally. Still does the sustain and blooming thing but it has a clearer tone. I think the 6BM8 was being somewhat overdriven & the tone was not as clear as on the Carolina SongBird. It does NOT have the jangly chimey tone of the mini-TOS.
This amp may be the most touch sensitive to picking attack of any that I've built. Without changing any setting on the guitar or amp, light picking will almost sound like I switched to a clean channel and turned off the overdrive. Diggin in, the overdrive returns. It has very nice harmonics to it.
While the amp doesn't have a switchable OD, you can dial the amp from clean to light overdrive to more overdrive by chaning the pot setting and engaging the PAB and/or midboost. The midboost switch would be a nice feature to have
footswitchable. The midboost increases both volume and sustain.
With both V1 and V2 being 12A_7 wiring, you can substitute all kinds of tubes for different tonal options.
I am using a 6K6 in the amp currently and love it! It's about the same loudness as the 6BM8 was.
The passive FX loop between the V1 and V2 seems to have the best balance of transparency that I've tried (since I tried 3 different placements).
Between the mini-TOS and Carolina SongBird, this amp is much closer to the tone that I like. I definitely prefer a smoother tone to a chimey-er and more jangly or distorted tone of an EL84 tube.
Editable SCH schematic and layout are here: http://www.el34world.com/Forum/index.php?topic=11889.0 (http://www.el34world.com/Forum/index.php?topic=11889.0)
With respect, Tubenit
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Man tubenit, you are an idea machine! Hats off to you sir. :icon_biggrin:
Thanks for sharing not only the finished thing, but also the whole thinking and build process.
Great name too. Look forward to hearing the sound clips. (BTW, FWIW, I think your a better player than you allow yourself credit for.)
Brad :icon_biggrin:
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And now for something completely different!
I love the fact that our friend tubenit has no hesitation about saying "I just don't like this, so I'll change it by..."
Really interesting circuit evolution.
Cheers,
Chip
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Good job T Im glad you found the sound you really like. and gave us interesting things to think about along the way.
Bill :happy1: :happy1: :happy1: :guitar1
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Here is the Carolina Songbird amp played reasonably clean. Only about 3 watts with 6K6 tube. Nice low volume. The song did not record real well but you can get the idea. There is some sustain and overdrive towards the end of the sound bit. The clean to the OD shows the picking dynamics because I never changed the volume. It sounded smoother live, IMO.
:icon_biggrin:
http://www.soundclick.com/player/single_player.cfm?songid=10829392&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=10829392&q=hi&newref=1)
I like the sustain at the end of the soundclip. That's with a 12AV7 in V1 and a 12AY7 in V2. Obviously with a 5751 or 12AX7, you'd get ALOT more.
And we are all gonna pretend that I didn't really do this one (same amp played pretty clean):
http://www.soundclick.com/player/single_player.cfm?songid=10829410&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=10829410&q=hi&newref=1)
:l2: :laugh:
And with the mid-boost switch engaged. Excuse the very mediocre playing on this one.
http://www.soundclick.com/player/single_player.cfm?songid=10819072&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=10819072&q=hi&newref=1)
With respect, Tubenit
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Well I would post a few soundclips, but while I don't mind you all snickering at me bumbling my way through the build, I'm not brave enough to put up my playing. You want mediocre, by gum I'd show you mediocre! :laugh:
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Tubnit I like the bite and pick attack when you dig in the amp sounds good in all 3 segments. I always Liked the lead sound with a good mid boost .
Thanks Bill
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Tubenit --
Watching you go from great design to great design has my mind spinning -- thanks for sharing it all with us.
Have couple of small questions - I looked carefully but could not find the primary impedences for the mini-TOS and Carolina Songbird.
Thanks again for the great ride!!
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I think it's 7k primary to 8 ohm speaker.
With respect, Tubenit
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Tubenit -
Checking my trafo stash I came up with a couple of Hammond 269GX PTs - they are 225-0-225V 75mA. If I used them, I guess I could put in a SAG resistor and/or choke to get back down to 269EX voltages. What DCR would you recommend I shoot for? thx much!!
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Just read the ongoing thread about dropping PT voltage with a zener or power resistor on the center tap. Think I am more interested in a SAG resistor on the B+ unless it is too big and causes too much sag in which case I will probably use both methods until the right SAG level is reached.
Tubenit -- Do you have any idea how much current to use for the calculation for both the mini-TOS and the Songbird?? thanks much.
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What power tube? Use tube rectification if needed
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I would like to try both the 6v6 and 6BM8 designs you have posted. My reasoning (or madness) for using a sag resistor instead of a tube rec is that in Class A my understanding is that tube rectifier SAG cannot not happen. The SAG resistor is the only way to simulate the effect on a Class A amp. Thanks for the response Tubenit.
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Just an update.
I've got one of Doug's 12AY7 in V1, an old 5751 in V2 and now I've got an NOS RCA 6K6 in. With that combination, I can dial in both a reasonably clean or very smooth overdriven tone. The digital delay makes this small amp sound huge at a very low volume. This is working even better than the VVR'd amps for playing at home.
This is proving to be one of the most fun amps to play that I've ever plugged into. It just has a really sweet musical feel & the RCA 6K6 gives this amp a really smooth tone. Again, the picking dynamics really change the clean to OD tone significantly. It is very touch sensitive,IMO.
I am convinced the mosfet CF helps with the digital delay and the 12A_7 triode helps with the OD tone control allowing it to be less edgy and somewhat smoother.
FWIW, I think the 6K6 is a smoother tone than the 6V6 tube.
With respect, Tubenit
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Tubenit,
I have been planning a dumble style se amp for a while now but I think this may be a better way to go. Just a couple questions. If I have room is there an advantage to adding another tube and replacing the mosfet with a tube or would you stick with the mosfet?
I really would like to be able to retain the clean channel. I know you said that you could dial in the overdrive channel to get clean but I really want to be able to go cleaner with a foot switch. Should I try to add this back in or would you recommend staying away from the switching?
Thanks for your design
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I think it would be fine to use a tube for the CF instead of mosfet, but it's certainly not necessary. The reason I am not doing the switching is that to get the tone I wanted using a passive effects loop with my Boss digital delay, I found I needed another gain stage prior to the power tube. IF you are NOT using a passive loop, then I'd definitely go for the switching.
Having said that ................. this recent tweaked version has more clean headroom but still maintains excellent sustain and nice harmonics. In other words, it is a comparable sustain but has a cleaner less slightly distorted tone.
With respect, Tubenit
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I finally had time to do thorough tweak experiments. This version has more headroom & sweeter harmonics. However, I lost a little of the blooming feature at the previous volume. I can recapture the blooming by notching the volume pot from 6-7.5.
I have found that on my Tweed BluezMeister and other push/pull amps I prefer a lower value cathode cap such as 10uf/100v. So that is what I started out with on the Carolina SongBird. However, I tried a 50uf paralleled with the 10uf and it sounded better to my ears. Then I tried a 100uf and it gave more headroom for chording, so I went with that.
A 60uf cathode cap still allowed for considerable blooming which was slightly loss with the 100uf. I think the 60uf would be a good compromise if someone played more lead and very little chording.
I changed how I did the PAB and like this version quite a bit more. The 22M resistor keeps the switching quiet. I originally had a 82p across the volume pot but changed that to a switchable 120p & 250p which I like better.
If you're looking for a bedroom type volume with sweet harmonics and a blooming sustain, this amp is worth considering. I put a SCH editable schematic and layout in the SCH library.
With respect, Tubenit
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I think it would be fine to use a tube for the CF instead of mosfet, but it's certainly not necessary. The reason I am not doing the switching is that to get the tone I wanted using a passive effects loop with my Boss digital delay, I found I needed another gain stage prior to the power tube. IF you are NOT using a passive loop, then I'd definitely go for the switching.
What if I had the third preamp tube using one triode for the cf and the other as a gain stage for the clean channel after the passive loop? Would that maybe give me the best of both worlds?
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I only had about 5 min to draw this for you. IF I am understanding you correctly ....... then yes I think something like this
would work just fine. I would use a 12AU7 or 12AY7 for that CF & following gain stage. That would sort of be like installing a built in FX loop (kind of). Since I was cutting and pasting the values of that gain stage may need tweaking?
With respect, Tubenit
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I would use a 12AU7 or 12AY7 for that CF & following gain stage. That would sort of be like installing a built in FX loop (kind of). Since I was cutting and pasting the values of that gain stage may need tweaking?
With respect, Tubenit
Yes, that looks right (all the parts are there), but as T said, after you actually get it wired in, there will probably have to be some tweaking of the values (biasing the extra gain stage & coupling caps & such)....or it may be just perfect "as is". You'll have to get it done & see if you like the results or if it needs some tweakage.
G
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exactly what i was thinking. Thanks for the help guys.
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What is the purpose of the first pot (100K)?
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Mid pot. You could use a resistor and a spdt center off for 3 different values.
With respect, Tubenit
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Mid pot. You could use a resistor and a spdt center off for 3 different values.
With respect, Tubenit
But isn't there already a mid switch? I thought the switch with the 390P cap was the mid switch? What would be the purpose 2 mid switches? Sorry for the questions but all this tone shaping is difficult for me and I'm trying to learn. Most of my builds had the traditional tmb off a cathode follower.
Thanks
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Mid pot. You could use a resistor and a spdt center off for 3 different values.
With respect, Tubenit
But isn't there already a mid switch? I thought the switch with the 390P cap was the mid switch?
Thanks
That's the mid-BOOST switch, it increases the coupling cap to the treble pot for a big boost in mids ....the mid pot gives you control over the "normal" mids
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Thanks Geezer,
Eventually I'll learn
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My goal with this amp was to increase blooming and clean overdrive and sustain.
I made 3 more changes. The sustain, blooming and harmonics are superb, IMO!!! This small amp sounds HUGE with the digital delay.
I changed the mid cap from .02 to .03. This allowed more blooming & lessened the (minor) distortion on mids. The mids bloom better and are somewhat cleaner. The .03 apparently made a shift in which of the mids are being overdriven and this sounds signficantly smoother to my ears. I A/B'd over and over setting the controls on the amp and guitar in different positions and it consistently gave a smoother tone with more bloom.
I added an 82p on the mid pot wiper to ground. This is only engaged when the midboost is OFF. The 82p goes away with the midboost on. The 82p took off some high frequencies when the amp is set to a cleaner tone. IF you like chime, then leave the 82p off completely.
I lessened the resistor going into the OD section from a 200k to a 100k. This also increased the blooming and sustain. It also made the bass notes sound clearer but changed little on the mids and treble frequencies from what I can hear. It may have boosted the mids and highs just a tiny bit?
I have never spent this much detailed time tweaking an amp. I will make a tweak or two, then play it for hrs before going to any other experiment.
I think this amp really does have the "wow" factor going for it. I'm very very pleased with it. Sounds great with 6K6 or 6V6. It is the "biggest" SE amp tone that I've heard in person.
With respect, Tubenit
http://www.el34world.com/Forum/index.php?topic=11889.new#new (http://www.el34world.com/Forum/index.php?topic=11889.new#new)
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I have been resolved to continue experimenting to see if continued tweaking really does make a significant difference and eventually end up yielding something really remarkable in tone.
I have a resistor box and a capacitor box that made tweaking a quick process.
I tried changing component values in about 6 different areas. Most of the experiments made no difference in tone ......OR worsened the tone. Everything I experimented with in blue was useless or worse. I simply tried different values in the coupling caps where indicated in tone stack and/or between gain stages.
However, I did find something that worked well and that was eliminating the 250p &470k to ground going into V2-2. It opened the amp up some more and gave it a more transparent tone with richer harmonics. What I did is I paralleled the 250p with the 100k & 120p.
I would say this amp is the most touch sensitive of any that I've built and has the most transparent harmonics. It's just got a very musical tone to it (for lack of a better description). I feel like the bass, mid and treble frequencies are very very nicely balanced now!
I now have added some voltages to the schematic. I am using a 12AY7 in V1 and a 5751 in V2 & continuing to use the 6K6.
I have updated the schematic and layout in the SCH library. At this juncture, I can't think of anything else to try? So this should be the "final" version.
With respect, Tubenit
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...eliminating the 250p &470k to ground going into V2-2. It opened the amp up some more and gave it a more transparent tone with richer harmonics. What I did is I paralleled the 250p with the 100k & 120p.
With respect, Tubenit
Thanks for the update tubenit. So you have a very small cap paralleled w/ a resistor and even smaller cap. I personally don't think all of this necessary as you're simply making a very high pass filter which should be able to be accomplished w/ only a single capacitor? Have you tried just doing that instead? The reason I say this is because when I used to do my tone stack bypass circuit sort of in the same way. I used a larger one for blocking all DC and the resistor to block some low end w/ another cap paralleled to allow a certain amount of high end through. But, simply using a smaller coupling cap did the job even better and I was only over-doing something and making it more complicated when it didn't need to be w/ three parts instead of one. It ended up sounding better when I simply found the right cap. (I got a cap decade box which helped also). I've been finding that the least resistors or resistance used in the signal path the better as far as "tone" is concerned.
In your circuit you're feeding the second triode w/ such a high end signal it's very similar to feeding a reverb input circuit where you're only boosting the extreme high end, but if it's giving you what you want then bueno. These are just my thoughts and experiences and giving you some input.
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Sure a single 390p cap would be better.
I simply paralleled the 250p with the 120p because unsoldering two caps and soldering back in one was more work then soldering the one leg of the 250p to the 120p. If you look at the schematics I posted, the 250p was already in there but going to ground thru a 470k.
With respect, Tubenit
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Tubenit, the switching shown in your schematic... are they simply switches or relays? I keep meaning to utilize the relays in the amp I built, but it's sounding so good I hate to mess with it anymore. Thanks again for sharing your work!
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They are just switches unfortunately. I wish the midboost & PAB was a relay because I'd definitely use it for leads and leave the midboost off for chords.
The amp chassis is really small though & it would be a challenge to install a relay.
With respect, Tubenit
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Sure a single 390p cap would be better.
I simply paralleled the 250p with the 120p because unsoldering two caps and soldering back in one was more work then soldering the one leg of the 250p to the 120p. If you look at the schematics I posted, the 250p was already in there but going to ground thru a 470k.
With respect, Tubenit
T - I did look at the schem and you mentioned the 100k resistor in there too. As long as it's in there it's not the same thing. Respectfully, J
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I personally don't think all of this necessary as you're simply making a very high pass filter which should be able to be accomplished w/ only a single capacitor?
OK, got it now. You're right, I wasn't understanding you correctly. You're meaning eliminating the resistor and the cap and just having a cap. I thought you were meaning having one cap & resistor instead of two caps & resistor. My error.
Yes, I tried that & did not care for the tone. I did try a cap there with no paralleled resistor. I felt like it was overdriving that gain stage more than I wanted it to.
However, perhaps I may have not tried it in the fashion that you are suggesting & didn't experiment enough with different caps? I used the lower value caps like 500p to 47p alone. Maybe a larger value would have worked? I don't know.
Or maybe if the grid resistor was larger, then the single cap would have worked?
With respect, Tubenit
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When cascading a stage or two, I almost always end up needing or wanting to "siphon off" some signal. Similar to what's going into the OD stages. Something you might like to try would be to use split load resistors on V1B's plate? But you have your "clean" channel to worry about and you'd lose some signal from this point and it may not drive the output tube as much. However, it's a low output amp anyway and you may be okay w/ this? You may have a lot gain in reserve already? Then you could go w/out any voltage divider all together and w/ less resistance in the signal path, you'd retain better clarity and tone into your OD section. You'd still have your gain/vol control and grid resistor's resistances which would lessen too much high end and you would still have a smoothing cap on the incoming OD stage or feedback cap to further bleed off highs in those ways if you wanted. Again, just "thinking out loud" & throwing ideas out there.