6v6 Tweed style - check my numbers?

Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, Sep 1, 2021.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    After getting finished with the Tweed Pro restoration project, I had some time on my hands and decided to mess with my Harvard style amp - my "College Dropout". Based on a Harvard but with a parallel input and normal #1 Fender input. So, the voltage table below has a shared cathode on V1, and parallel inputs.
    Thinking back to another thread about optimizing the 12ay7 I changed a couple things inside, and I would appreciate other eyes checking that things still look ok. The tube lineup right now is 5y3, 2x 6v6, 12ax7, 12ay7.

    The V1 bias resistor is a 1k shared, so 2k. The V1 plate resistor is 82k instead of stock 100k. I have 1500R grid stoppers, and 470 screen resistors across the sockets.

    Wall voltage actually was in the 120.X range when I started.

    Thanks


    upload_2021-9-1_21-15-25.png
     
  2. jsnwhite619

    jsnwhite619 Friend of Leo's

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    For reference, it's the closest match. Obviously, nowhere near stock levels, but just looking for a "Looks good" or "I wouldn't do that if I were you" feedback.
    upload_2021-9-1_21-18-23.png
     
  3. D'tar

    D'tar Friend of Leo's

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    Houston,

    We are clear for launch!

    IMO of course!

    #s seem reasonable
     
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  4. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Thanks. I thought so, but I've had a couple of brain fog moments on this one - a little out of practice on the guts lately - and wanted a second opinion.

    Now that I'm hopefully not leading anyone down a path of destruction, I'll tell you my first impressions. I changed the plate & bias resistor on V1 at the same time, so I can't give a report for just one, but I think it made a pretty big difference in the clean headroom. Even running a 5y3 rectifier instead of the 5u4 I started with (gave the one I had to a friend a while back), it's very clean most of the way up the dial. Even the parallel input with single coils just gets thicker & fatter until 8-9 or so on the dial, and it's a tighter & glassier clean than before I think. When I was building that last 5881 5f2 earlier in the year, the one thing that finally clicked and made it all work with humbuckers was dropping that 100k plate resistor down to 82k on the V1/12ay7 - huge difference in everything after that - he was wanting it to go along with humbuckers, so I did everything I could to tighten it up.

    I think a couple people chimed in on that thread about similar results on 5f1/5f2 amps - raise the plate voltage. It made a big difference on that one and on this one. I'll get some sound clips when I can. But, I wonder how much of the "issues" that are modded on these old style tweed designs that get coupling caps and bypass caps changed in & out, and stoppers, and blockers, and extra filtering, and chokes, etc., would be helped or reduced with a higher plate voltage on a 12ay7? That's kinda where that older thread I linked to was heading, optimizing & increasing the performance to get the most out of a 12ay7 instead of going to the higher gain tubes - 12ax7.

    I think the next test now that it's so glassy clean at V1 now will be the NFB. If I've got the first stage as loud and clean as I can get, maybe tweak it next in the second stage to dirty it up a little farther downstream.
     
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  5. andrewRneumann

    andrewRneumann Tele-Holic

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    Commendable 12AY7 explorations! Thank you for taking the time to share them.

    The 12AY7 has about 40% the resistance of a 12AX7, so all other things equal it will draw about 2.5 times the current as the X. Higher current means that if we use the same value plate resistor, we choke the tube off (relatively speaking). This is not necessarily a bad thing unless taken to the extreme.

    Higher value plate resistors increase stage gain though. So by reducing your plate resistor you are taking a double hit on gain—the major hit is switching to the Y which has about ~50% the gain of the X. Switching to 82K from 100K load and you lose another ~15% gain.

    Unless you have set V1 bias ridiculously high or low, it is rarely the first tube to overdrive. So your increase in headroom is probably due to lower signal levels further down the chain. Both the X and the Y in the V1 position will run clean with guitar level signals as long as they are biased appropriately.

    So how to dirty up the amp downstream of V1? Reducing or eliminating NFB sounds like a worthy experiment. The other option I can think of is to reduce voltage to the 6V6s. You will pay with lower output power, but maybe it’s a trade off you can live with. Sounds like you’ve already gone the right direction with the 5Y3. You could increase that 470R screen supply dropping resistor to lower the screen voltage. Or add another resistor and capacitor (RC filter) between that 470R and the screens if you didn’t want to lose voltage on the preamp. You might then re-adjust bias to further sculpt overdrive tone.

    Just some ideas to explore. No promises any of this sounds better to your ear. Hope this helps though.
     
  6. Wally

    Wally Telefied Ad Free Member

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    Have you considered using a 5751 in V1? The original 6AT6 is closer to one of those 5751 triodes than anything else, isn’t it? Of course, it appears that you are not really wanting a Harvard as it was built.
    imho, a Harvard is the 5F11 Vibrolux without the bias vary tremolo, isn’t it? Imho,that is the small tweed that moved into the more modern Fender approach, and it led directly to the 6G2 Princeton. These amps are essentially the same, and imho are wonderful amps for humbuckers as long as the player can control the guitar’s output. These amps are very Sensitive to pick attack….heavy hands don’t do well with them. Of course, I don’t agree with uncalled for heavy pick attack anyway since it takes every note sharp and also yields a lot of fret rattle. Anyway….different strokes. These amps are cleaner than a 5E3 due to the fixed Biasing. One can turn a 5E3 or one of these amps up way beyond halfway and have wonderful cleans with a controlled guitar output no matter what pickup one uses….outside of some sort of modern high output pickup, of course, which I tend not to care for anyway.
    It would be interesting to hear what you have wrought there…but I am too far away to drive over…and you have invited me anyway!!! Lol….

    and…Inhave to ask….are you using pin 1 on the 6V6s as a contact point for your screens? i also assume that you have diode protection on the rectifier tube?
     
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  7. D'tar

    D'tar Friend of Leo's

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    If I know anything of @jsnwhite619 , we're bound to have some soundfilez!!!!;)
     
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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I don't have any 5751 tubes, I don't think I ever have used one. Correct on the Harvard/Virbrolux heritage, but the 5f10 was spec'd at 300-305v B+, 5f11 was 340v I think. Definitely a cleaner amp than the 5e3. But, you're welcome any time you're in Georgia Wally!

    Yes, I'm using Pin 1 for the contact point - is that a bad thing? Yes on the rectifier diodes as well.

    Soon as I can!

    I went back into and if you check out the 5881 build thread in the first post, there was a LOT of time and discussion and load lines working toward 12ay7 use. So, I went back into it today and tried a couple things that was suggested then. I swapped the 82k plate for a 47k which is a lot closer to the 12AY7 internal resistance X2 figure of 45k, and I dropped the bias down to 1.5k. The voltage differences:
    upload_2021-9-2_20-33-52.png

    It's a pretty good difference in tone & feel with the extra voltage in there. Even with a 5y3, it is very clean and tight, more distinct and hard edged notes and bass tone at the lower volumes. It doesn't sound like the typical Tweed & 5y3 sound, and the only thing changed here is the V1 setup. It almost has a tighter 5u4 tone to it, more of the bigger piano type bass tone. It is cleaner and lower gain, but still gets much louder than I ever used to gig my Pro Jr at and louder than comfortable for just sitting at home playing with a decent modern speaker. But, the overdrive....write home to mama!

    I already had the grid stoppers, and a few things in between that are all listed on Rob's site for tightening the sound and reducing the fartiness, and phase inverter distortion, etc. But my Pearly Gates humbuckers are like butter. Yes, I know that it's reducing the signal making it into the circuit, and most people raise that resistor to hit the next stage harder, but every time I've tried that, it just caused more problems to have to tame and I never left it that way. But there is no raspiness, overtones, fuzz, swirling phantom notes... I'm not a big Tube Screamer fan, but the best I can describe it to my ears - haven't recorded it yet - is like a Tube Screamer boost. A smooth, mellow overdrive that doesn't fall apart with the amp on 12. Right now, I have a 2uf bypass on V1, and a 25uf on V2. As smooth as it is right now, I may tack on another 25uf on V1 and see what happens. If it will make up for a little bit of the lost gain & output - which is still more than I like to sit and play cranked for extended times, this might be a hot little number. The real breakup doesn't start now until around 3:00 on the dial, so I'm interested to see what that does.

    I tried it through my Ragin Cajun, 1058, and Cannabis Rex tonight. The 1058 is actually the darkest and most bass-heavy of them all. My favorites may have been the Cajun, C-Rex, and 1058 last.
     
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  9. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I'm also ordering a new 5u4 to stick in that son of a gun. To me, I could always get a bigger & meaner overdrive with the 5u4 and even GZ34 in this case, and I assume it's because of the single tone knob and no EQ to dial in and adjust. A bigger & firmer bass response with no mid cut, only a tone knob had great lower level cleans, but cranked up and dirty, the 5u4 always completely smoked a 5y3 in the dirt department. In my mind, I just imagined the difference as the same size hose and same well, but the 5y3 is a hand pump, and the 5u4 is a Briggs & Stratton. I may be 100% wrong, but I always got way more dirt and attitude with the 5u4 rectifier than the 5y3.
     
  10. NTC

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    With the 5U4, all voltages will go up.

    You mentioned a 25uf bypass cap on V2. Does this amp have the BF Princeton/ PR feedback?
     
  11. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yes, 5.6k NFB & 47R tail resistor borrowed from the Princeton-style setup. I tried it with the NFB lifted and it really didn't make as much difference as I expected. More highs, hear your fingers on the strings, but it definitely didn't go from Tweed to Marshall that way. And that was before I lowered the plate resistor.

    From my notes, the way I had it before when I had a 5u4 on hand, B+ was around 404, 399, 322. 180v on the V1 12ay7 plate, and it is almost exactly the same voltage ratio I have now with the 5y3 & supply/plate ratio. Funny how things work out when they are right... ;-) I just went back and found that, so that is absolutely why I'm "missing something" right now. But, I've found that a 12ay7 is happy with a much lower plate resistor than 100k - it's designed for it - and when dealing with any vintage Tweed "shortcomings" in fuzz and buzz, a much cleaner path is available.

    For all the mods you see online, the plate load seems to only be mentioned to increase gain and hit the next stage harder. Higher preamp voltage and reduced signal level by lowering it is not something I've found talked about. I'm gonna have to find somebody with a reverb pedal to try this one out because it really is a wonderfully fat clean tone. I'm guessing that it will like a good O.D. boost as well.
     
  12. NTC

    NTC Tele-Holic

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    Nice work!
     
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  13. Wally

    Wally Telefied Ad Free Member

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    Re: the 12AY7 and plate resistance. Leo used the 12AY7 to great effect with 100k ohm plate resistors in a number of amps in the tweed era.
    I admire your work. However, imho, you are aiming to have something that is not a tweed amp as Leo built it; and there is nothing wrong with that. It just won’t be what a Harvard amp was. As long as it goes where you want it to go, all is good!
     
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  14. Wally

    Wally Telefied Ad Free Member

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    Oh..and regarding that NFB loop…..there is not a great deal of cancellation in that loop. You discovered that when you lifted it, though, didn’t you?
    Since you cleaned the signal up in the input stage, why not increase the cancellation in the NFB loop by decreasing the the value of that 56K series resistor in the loop? Or…put a 15k static resistor there in series with a 50K pot? Then, you could clean the signal up with a lower resistance there if so desired.
     
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  15. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Definitely more "Harvard inspired" than anything. Parallel input and second gain stage borrowed from Blackface Princeton/NFB circuit.

    For sure. In the case of that older thread I linked with the discussion on it, it became a matter of "Yeah, it works, but could it be improved?" Was the 100k there because it was best, or because it was a good compromise knowing that a 12ax7 would potentially/probably used there instead at times?
     
  16. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Here is a clip with my Les Paul w/Pearly Gates pickups running through my Ragin Cajun. The speakers you listen through apparently have a big impact on this one - my flatscreen monitor speakers make it sound thin and no bass, headphones and my sound bar both sound rich & full. This still has a JJ 5y3 that drops plenty of voltage, and I'm not touching anything until the 5u4 arrives. The last two songs - China Grove & Living Loving Maid - are both played through the #2 input first, then the #1 parallel second. I think everything else was all through the #2 input. #1 can really help fatten up single coils, but it is easy to be too much with humbuckers.

     
  17. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Video link in case Soundcloud link doesn't work for you.

     
  18. Wally

    Wally Telefied Ad Free Member

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    Edit to modify notification method…
     
  19. jsnwhite619

    jsnwhite619 Friend of Leo's

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    ??
     
  20. Wally

    Wally Telefied Ad Free Member

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    I was getting email notifications, which I don’t need, because I forgot to check the box. I wish this forum would set it up so that it email notifications we’re not a default but rather had to be selected. I use the forum notifications only.
     
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