Wavebourn, I'd like to replace the guts of a SS Fender practice amp with your 'micro-champ' but currently have no high-mu small-signal tubes. Can I get away with ECC81 in the same circuit while looking for the 'proper' tube (I believe 6n21 has mu of 70 and ECC81 is 60 so not that far away)? I can only find 6n16 on epay - do you have a source for 6n21? Also, did you ever post the latest schematic with the 'patent-pending two-pot trick'?
Since it has been quiet.
Not the best thing that I done but it is getting there. Bit of work to do on the other side of the circuit board and then start wiring up to the tubes. I have jumper wires in place for the plate, cathode, and screen resistors for the preamp pentode, still have to play with the values and it beats swapping parts on the board. The couple of parts that are standing off the board are also temporary till I pick up the right ones.
Sorry, but I do not agree! If you tidy up the wires it will look very seksi. Chassis colour looks sooooo 70's!
Still missing a lot of wires, I'll do what I can to keep it neat. The color was a tough one, I stood in front of the paint display for quite a while deciding on which paint can to pick. The color is off in the picture due to the light, it is much brighter, more like the tone in between the board and the over exposed bit.
Played around with some 6AU6's in a 6V6 SE amp and think I found values I like. Lost the pentode/triode switch, it worked but you can roughly do the same thing with the volume control. Now I am thinking the versatility of the amp will be more with a bass and treble control rather than a Tweed type of tone control. The question is whether to put it before the 6AU6 or after it? Any suggestions?
Oops, cut off the left side of the circuit. Negative feedback from the output and a filter capacitor missing.
Oops, cut off the left side of the circuit. Negative feedback from the output and a filter capacitor missing.
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In that schematic, I think I see the tone controls inside your feedback loop. That is not supposed to work. If NFB is doing it's job it will "correct" out whatever the tone stack does. Same goes for putting a master volume control inside the loop.
So to answer your question about were to put the TS. Just before where the NFB loops back.
See what happens when you do a cut and paste. Good catch. Alright then, with no NFB (I was debating whether I really wanted it anyway) where do you see the tone stack?
Amp competition -- tone controls -- a design to submit!!!
I agonized over where to put the tone stack for weeks. For one thing, I am working on a three-tube design (five gain stages in the bottles) that includes a spring reverb tank--a REAL tank, not an inventive arrangement of speaker parts.
And if there is no distortion control, then the amp is less than ideal, so . . .
Reverb has to go after distortion. Tone has to go after reverb, it seemed to me.
Most of my original ideas had one too few stages to pull it all off. That was before I decided to trust one stage to do two jobs (actually, three jobs, if driving the tone stack is one of them), and then proved the concept workable.
I considered all sorts of novel ideas but most didn't really appeal to me.
The most "out-there" of the potentially useful ideas was to control only the bass (and possibly mid) in the dry path around the reverb send-return and put the treble after the reverb recovery stage.
The reasoning was that the reverb send had to deliver current. So it could also drive a bass boost/cut circuit, and reverb is effective only at the mid and upper mid anyway, so perhaps putting the bass controls in the bypass around the reverb would be okay. But I later rejected this idea. All tone has to be in the same stage--after the reverb.
Well, long story short, and an announcement!:
I've got a design to submit. No circuit diagram yet, but the prototype is made and it sounds good!
Maybe it will make the price limit. Have yet to tally it all up, but it's close.
I'm using a 6EB8 (my one-tube amp tube) as a first stage (triode) and distortion driver, (with a Gain control between the triode and pentode). The 6EB8 pentode is the reverb driver and distortion producer. Out of the 6EB8 pentode with a low value plate load, the signal splits just past the Master Volume pot. Through a high-pass cap to the MOD reverb tank (4FB3D1B -- no tranny) on one side, and through a dropping resistor to tame the direct dry into the tone stack on the other.
The tank signal comes back to (and here's the beauty of this design, the tube!!!) a 6KE8!! This tube, which is an FM or TV mixer oscillator, has a pentode that can live on a fraction of a volt at the grid. (At less than one volt on the grid the plate current is already down in the cellar. Read: GAIN) VA is in the neighborhood of 220 when the reverb signal emerges on the output side where it mixes with the direct dry, and that mix is fed to the tone stack. A modified Fender three-way with some adjustment in the values gives a mid boost about as far as you could stand it--or drops it out completely in a nice phatt smile curve! The tonal possibilities of this amp are incredible! Especially since there is a treble bypass cap around the dry path attenuator that keeps the ching and clarity in there unless you dial it out with the treble control.
There's more reverb than I might want for some things, so there is also a pot that varies the amount of reverb return mixing back in from zero to "plenty". And it took some time and head-scratching frustration before I realized that with no dedicated reverb driver, the return was out of phase after going through the recovery pentode, which made everything oddly thin and trebly so that the reverb sounded very different than the dry. Solution: rewire the RCA out jack from the amp to put the ground in the center and the audio cable feeding the tank to put the center pin to ground at the amp end to reverse the connection at the tank end while keeping the shield grounded to the amp ground (whew!).
The medium-mu (40) triode of the 6KE8 takes the output from the tone stack to drive a 12A6 (to a 6K6 what a 6K6 is to a 6V6) single-ended into a 5W Edcor SE tranny with a super-linear screen tap.
This amp plays well with a perfectly clean acoustic guitar with a combo under-saddle piezo/soundhole condensor), when dialed in for clean, and goes far enough into fuzzy distortion to handle All Along the Watchtower, Jimmy version, or Stevie Ray's Pride and Joy. You want just enough overdrive for Johnny B. Goode? Okay. You got it.
You gotta hear this amp!
I'll have to make some recordings. Is an MP3 acceptable?
Later this week, the circuit diagram. The MP3s. The BOM.
Oh! Am I excited about having finally gotten the bugs out of this thing and realized the intent of the design!!! And just in time, it would seem.
I agonized over where to put the tone stack for weeks. For one thing, I am working on a three-tube design (five gain stages in the bottles) that includes a spring reverb tank--a REAL tank, not an inventive arrangement of speaker parts.
And if there is no distortion control, then the amp is less than ideal, so . . .
Reverb has to go after distortion. Tone has to go after reverb, it seemed to me.
Most of my original ideas had one too few stages to pull it all off. That was before I decided to trust one stage to do two jobs (actually, three jobs, if driving the tone stack is one of them), and then proved the concept workable.
I considered all sorts of novel ideas but most didn't really appeal to me.
The most "out-there" of the potentially useful ideas was to control only the bass (and possibly mid) in the dry path around the reverb send-return and put the treble after the reverb recovery stage.
The reasoning was that the reverb send had to deliver current. So it could also drive a bass boost/cut circuit, and reverb is effective only at the mid and upper mid anyway, so perhaps putting the bass controls in the bypass around the reverb would be okay. But I later rejected this idea. All tone has to be in the same stage--after the reverb.
Well, long story short, and an announcement!:
I've got a design to submit. No circuit diagram yet, but the prototype is made and it sounds good!
Maybe it will make the price limit. Have yet to tally it all up, but it's close.
I'm using a 6EB8 (my one-tube amp tube) as a first stage (triode) and distortion driver, (with a Gain control between the triode and pentode). The 6EB8 pentode is the reverb driver and distortion producer. Out of the 6EB8 pentode with a low value plate load, the signal splits just past the Master Volume pot. Through a high-pass cap to the MOD reverb tank (4FB3D1B -- no tranny) on one side, and through a dropping resistor to tame the direct dry into the tone stack on the other.
The tank signal comes back to (and here's the beauty of this design, the tube!!!) a 6KE8!! This tube, which is an FM or TV mixer oscillator, has a pentode that can live on a fraction of a volt at the grid. (At less than one volt on the grid the plate current is already down in the cellar. Read: GAIN) VA is in the neighborhood of 220 when the reverb signal emerges on the output side where it mixes with the direct dry, and that mix is fed to the tone stack. A modified Fender three-way with some adjustment in the values gives a mid boost about as far as you could stand it--or drops it out completely in a nice phatt smile curve! The tonal possibilities of this amp are incredible! Especially since there is a treble bypass cap around the dry path attenuator that keeps the ching and clarity in there unless you dial it out with the treble control.
There's more reverb than I might want for some things, so there is also a pot that varies the amount of reverb return mixing back in from zero to "plenty". And it took some time and head-scratching frustration before I realized that with no dedicated reverb driver, the return was out of phase after going through the recovery pentode, which made everything oddly thin and trebly so that the reverb sounded very different than the dry. Solution: rewire the RCA out jack from the amp to put the ground in the center and the audio cable feeding the tank to put the center pin to ground at the amp end to reverse the connection at the tank end while keeping the shield grounded to the amp ground (whew!).
The medium-mu (40) triode of the 6KE8 takes the output from the tone stack to drive a 12A6 (to a 6K6 what a 6K6 is to a 6V6) single-ended into a 5W Edcor SE tranny with a super-linear screen tap.
This amp plays well with a perfectly clean acoustic guitar with a combo under-saddle piezo/soundhole condensor), when dialed in for clean, and goes far enough into fuzzy distortion to handle All Along the Watchtower, Jimmy version, or Stevie Ray's Pride and Joy. You want just enough overdrive for Johnny B. Goode? Okay. You got it.
You gotta hear this amp!
I'll have to make some recordings. Is an MP3 acceptable?
Later this week, the circuit diagram. The MP3s. The BOM.
Oh! Am I excited about having finally gotten the bugs out of this thing and realized the intent of the design!!! And just in time, it would seem.
I put my takeoff for reverb after the tonestack and second gain stage but before the third gain stage, and injected it right back between the second and third stage.
I've just about finished wiring all I can before adding the tagboard.
Progress has been slow as I stopped to make a cradle to make wiring easier.
The tube sockets aren't completely mounted, hence the odd angles.
I've just about finished wiring all I can before adding the tagboard.
Progress has been slow as I stopped to make a cradle to make wiring easier.
The tube sockets aren't completely mounted, hence the odd angles.
Attachments
@ Printer2:
Looks like it's between your second stage and phase inverter.
Do you want to put it somewhere else? If so, why?
Or are you asking for confirmation that this is where it best goes?
Carvin puts their tone stack right after the master volume. Most Fender and Marshall are the other way around. Carvin's design gave me the courage to give that design a try (which is really the only place the tone stack can go in my design without messing up something else).
You've got a choice. But where are you getting your distortion, if any?
You want tone to follow distortion, right?
Looks like it's between your second stage and phase inverter.
Do you want to put it somewhere else? If so, why?
Or are you asking for confirmation that this is where it best goes?
Carvin puts their tone stack right after the master volume. Most Fender and Marshall are the other way around. Carvin's design gave me the courage to give that design a try (which is really the only place the tone stack can go in my design without messing up something else).
You've got a choice. But where are you getting your distortion, if any?
You want tone to follow distortion, right?
What's your filly tranny? or Dual pots? Cool.
@ The Gimp:
Looks like maybe you've got five gain stages, too, like I do.
But you've also got five filaments. What are you using to heat all those tubes?
Also, I've considered using a dual-pot to tame the volume somewhat when gain is up. Is that what you've got there in pot #3 (left to right)? Or do I see seven pots, not six? And TWO duals (#1 and #4)?
Please enlighten us on this. (Apologies if you already did and I haven't read it.)
If there were more time, I might experiment with taming the dry signal even more and mixing it with the reverb return before going into stage 3, as you evidently are doing, but I'm concerned about fiddling with something that more or less works this late in the game.
I've still got to build up some callouses and practice all the clams out of these pieces I've mentioned before the end of the week if I'm to make an MP3. As it looks, I'll likely have raw, bleeding fingers before I have a clam-free act.
Can I submit just the good sections?
Like from midway in one phrase to midway in the next?
(And just trust everyone's ADHD to smooth out the edits.)
@ The Gimp:
Looks like maybe you've got five gain stages, too, like I do.
But you've also got five filaments. What are you using to heat all those tubes?
Also, I've considered using a dual-pot to tame the volume somewhat when gain is up. Is that what you've got there in pot #3 (left to right)? Or do I see seven pots, not six? And TWO duals (#1 and #4)?
Please enlighten us on this. (Apologies if you already did and I haven't read it.)
If there were more time, I might experiment with taming the dry signal even more and mixing it with the reverb return before going into stage 3, as you evidently are doing, but I'm concerned about fiddling with something that more or less works this late in the game.
I've still got to build up some callouses and practice all the clams out of these pieces I've mentioned before the end of the week if I'm to make an MP3. As it looks, I'll likely have raw, bleeding fingers before I have a clam-free act.
Can I submit just the good sections?
Like from midway in one phrase to midway in the next?
(And just trust everyone's ADHD to smooth out the edits.)
You can look at the tone position in two ways. After your distortion stage you can shape the distorted sound for the tone you want or before the distortion you can cut the bass and cut treble so if you really crank it the sound does not turn into mush.
I picked up a small console radio from a thrift shop on the weekend and used it to play around with the 6AU6. I never used pentodes in the preamp yet (real new to this tube thing) and heard they can sound nice but would like some signal hitting them therefor my triode before it. I am finding if I have my Telecaster turned up (not a high output guitar) and want it clean I have to keep the input volume down to 1 or 2 ans still have the gain I want when turned up to 10. Probably should have built it with low and high input jacks, live and learn. Dropping down the cathode resistor to get one volt on the 6AU6 sounds nice but you can forget about any cleans from the amp then.
The allied transformer is rated at 2.5A@6.3V.
2X 0.72A for the two 6DX8s = 1.44A
3X 0.45A for three 6GH8A = 1.35A
Total heater current is thus 2.79A out of available 2.5A so I have added an additional transformer to offload one or two tubes.
It was marginal (2.49A) till I added the Reverb, which also busted the budget.
The first dual pot has section 'a' for the gain after the tonestack. The 'b' section is for the gain after the third gain stage going to the PI. This controls how sensitive the amp is to overdrive. Both of these pots have a resistor in series to give me more control over the ratio from each takeoff.
The second dual pot is the PPIMV master volume control. This allows one to control volume independent of overdrive sensitivity.
7 Pots total, plus bright switch. Gain, Treble, Bass, PPIMV, Trem Rate, Trem Depth, Reverb strength.
2X 0.72A for the two 6DX8s = 1.44A
3X 0.45A for three 6GH8A = 1.35A
Total heater current is thus 2.79A out of available 2.5A so I have added an additional transformer to offload one or two tubes.
It was marginal (2.49A) till I added the Reverb, which also busted the budget.
The first dual pot has section 'a' for the gain after the tonestack. The 'b' section is for the gain after the third gain stage going to the PI. This controls how sensitive the amp is to overdrive. Both of these pots have a resistor in series to give me more control over the ratio from each takeoff.
The second dual pot is the PPIMV master volume control. This allows one to control volume independent of overdrive sensitivity.
7 Pots total, plus bright switch. Gain, Treble, Bass, PPIMV, Trem Rate, Trem Depth, Reverb strength.
Yes, I had exactly the same thought when I considered using a dual pot for the distortion gain with b side wired in reverse to scale master volume ahead of (or across) the master volume pot to counteract the gain setting. I may still implement that in a version of the amp that isn't trying to beat the price target.
That's a lot of heat you're generating there. And I heard that "bust the budget" part of it. That's why I asked.
But then, you do have quite a few more gain stages than I do.
I'm not familiar with your circuit but it is clearly much more sophisticated than mine.
Triode-pentodes do have an appetite for heater current! That was a major design limitation that I was aware of going into this. And the reason I almost went limp at the knees when I saw the specs for the 6KE8: 0.4 A! My 6EB8 is already taking 0.75A, and I wanted to use a 1.6 Amp transformer I have from which I get much of my savings. The 12A6 beam power output is a miser with heater current, using only 0.15 A!!! (Compare that to a 6L6 which uses almost an Ampere!) It's 12 volts, so that equates to 0.3 A at 6.3, but that's still pretty frugal. But there just wasn't room in there for two 6EB8s AND the 12A6. and then I saw the 6KE8 in an assortment of tubes I bought at eBay and looked up it's specs. O Happy Day!
I've got another tube that uses 0.15 at 6.3 volts that will just fit into this fillament tranny, too, and it produces a real nice distortion rich in second harmonics, but it will have to wait for later. It also could make a great vari-mu compressor stage. I overreached and tried to get it to do both. Not enough time to get that design right for this amp challenge. I spent weeks on it before abandoning it for the simpler design due to weird oscillations.
I can't imagine shaving off $25 in order to have reverb, well not unless I did all my part sourcing here and there. When I started I decided to stick with two venders in order to keep the shipping costs down. (Allied Electronics, Antique Electronic Supply). Oh I would love to have some reverb happening. Next amp. Unless something obvious comes up this is what I will wire up.
In my 5 tube (8 tube sections) design I started out with the tone stack after the gain pot which is between the first and second triode gain stages. I played with the order of things several times before settling on the tone stack after the master gain which is between the 4th gain stage and the PI. This seems to allow the best compromise between clean and cranked to meltdown tones.
Another "ingredient in the secret sauce" is an "L pad" between the OPT and the speaker. This allows the amp to run wide open at 15 watts but the power applied to the speaker can be dialed back to 1 or 2 watts to avoid the noise police!
Secret ingredient #2, I wired them all in series and hung the string across the B+ supply. No heater winding....no heater transformer.
Secret ingredient #3, I stuck a tone control in the feedback path. Not as useful as I expected, but good for some rather unusual tones.
I was still experimenting with that one when I had to leave town. I might have a little time when I get back if work doesn't expect me to sleep under my desk at night after being gone for nearly 3 weeks!
So there goes my credibility. You know when I say, 'This is the circuit I will be making'? Just believe it once you hear a clip of the amp.
I pretty much did what I wanted with my test amp and decided to dismantle it, wires and test clips all over the place. Well I noticed that I was playing with it missing the bypass cap on the 6AU6. Might as well see what it sounds like with it. Hook it up and grab the guitar... ...oh my, it has much more effect on gain than on a triode. Back to square one.
Cathode and screen values needed to be much different, decided to put a switch in to change the plate resistor between 100k and 220k just for kicks. Much more chimey. I have a problem with the low E string though. And I probably have to dump some gain somewhere. Talk about cranked to meltdown. Maybe the tone stack before so I can dial out the bass and cut some of the signal before it hits the pentode. The ears got tired so I am giving it a break for a while. And I thought I almost had this wrapped up.
I pretty much did what I wanted with my test amp and decided to dismantle it, wires and test clips all over the place. Well I noticed that I was playing with it missing the bypass cap on the 6AU6. Might as well see what it sounds like with it. Hook it up and grab the guitar... ...oh my, it has much more effect on gain than on a triode. Back to square one.
Cathode and screen values needed to be much different, decided to put a switch in to change the plate resistor between 100k and 220k just for kicks. Much more chimey. I have a problem with the low E string though. And I probably have to dump some gain somewhere. Talk about cranked to meltdown. Maybe the tone stack before so I can dial out the bass and cut some of the signal before it hits the pentode. The ears got tired so I am giving it a break for a while. And I thought I almost had this wrapped up.