cathode biased el34 amp

Discussion in 'Shock Brother's DIY Amps' started by guitjopicka, May 29, 2014.

  1. guitjopicka

    guitjopicka Tele-Meister

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    So I posted this on the Weber forum, but thought I'd double it here too as I find this forum super helpful and insightful.

    I built a Phat Ass off of the weber forum, actually my second. But this one was a cathode biased el34 version. I feel like it is lacking in the ummmph section. Pretty much clean, not a lot of power or spank. Maybe something is wrong, but everything seems to measure out about right.

    I was thinking about changing out the preamp to something else. Or modding it in some fashion. Like I said, it is a cathode biased el34 pair, being fed by a ltpi with a ppivmv. I don't get much breakup until the thing is pretty much cranked and my guitar volume is on full. My traynor ygm3 with a fixed bias el84 pair walks all over this amp.

    What should I do, trying to retain the power section/phase inverter? Not totally opposed to adding another 12ax7 if I need to. There are two as of now.
     
  2. Telenut62

    Telenut62 Poster Extraordinaire

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    Can you put up the schematic so people don't end up getting big bottom girl websites :lol:
     
  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    "Now Honey, don't cry. There's nothing wrong with your behind. I was just trying to help a guy with an amplifier problem. Really. I swear."
     
  4. guitjopicka

    guitjopicka Tele-Meister

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    True, sorry. Google is probably gonna throw a porn ad in here quick. I'll get to that tonight.
     
  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    I think the "Phat Ass" and "PA26-66" that has been making the rounds are the same thing, or close to it.

    The first gain stage runs both halves of the 12AX7 in parallel. If you split those up and use one triode to feed the other, you'll have plenty of gain. :cool:
     
  6. andyfromdenver

    andyfromdenver Friend of Leo's

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    Ooh, the "Hard Ass" mod :)
     
  7. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    :lol:
     
  8. guitjopicka

    guitjopicka Tele-Meister

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    Yes, you are right they are the same. Here is the schematic, and then a layout that includes the master volume that I used. The first one that I built was using 6v6's in a small tube pa that I got for a song. It killed, but I wanted a bigger one. So I built this one using el34's, a bigger PT and OT. After it was biased up it was supposed to be pushing close to 30 watts (i could be off, it was a while ago) but it is sounding pretty weak. I may just run both halves of v1 into each other first to see how that works for me.
    Code:
    
    
     

    Attached Files:

  9. guitjopicka

    guitjopicka Tele-Meister

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    also, before someone else says it. I know it says 6v6's, but the guy who made the schematic says he's tested this with el34's and after biasing it right it worked fine for him. Are there any changes that I would need to do to make it work with the bigger bottles that are just "assumed" i'd know?
     
  10. Jebrone Lames

    Jebrone Lames Banned

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    You could add 1/2 of an additional 12ax7 = Trainwreck Style.

    Changes to consider to balls it up:
    1) Use independant Plate resistors 220K V1(a) and 150K V1(b)
    2) Use independant cathode resistors and by-pass caps (instead of shared) on V1
    For example: V1(a) 1K + 22uF. V1(b) 1K + 10uF (or higher)
    3) Change first coupling cap to .0082uF
     
  11. laird

    laird Tele-Holic

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    EL34s can take a lot more input signal than 6V6s - this circuit needs another gain stage to produce any real overdrive. Take one of those parallel triodes and place it between the volume and the phase inverter - that's very close to a 6G3 Deluxe preamp. It will give you plenty of drive then! It'll also respond very well to tube substitutions to clean up / smooth out the gain.

    -Laird
     
  12. guitjopicka

    guitjopicka Tele-Meister

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    So I can look to the brown deluxe for wiring and values? Or will values be different?
     
  13. laird

    laird Tele-Holic

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    Measured/actual voltages will be different because of the different power supply, but for cap/resistor values, sure. The cathode voltages should be in the ballpark of 1.5v. All you'll need is another 100k resistor for the second plate, a 1.5k cathode resistor for each cathode, and another cathode bypass cap (whatever value you like). The input grid can be connected directly to the volume pot wiper and the plate connected to the .022uf cap that feeds into the phase inverter.

    -Laird
     
  14. Lindsay

    Lindsay TDPRI Member

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    Guitjopicka,
    I built the same amp last year to Scott's schematic you posted and found the same issue. Mine tested 24W at full power using a 275-0-275 PT/4K OT/8 Ohm. The voltages were a little low. A 300-0-300 would be a better choice for EL34's. It was built for a friend who enjoys the clean to mild break up that the amp produces. I was never completely happy with the amp.
     
  15. guitjopicka

    guitjopicka Tele-Meister

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    i should be fine there actually. I used the W022723 which is 320-0-320.
     
  16. guitjopicka

    guitjopicka Tele-Meister

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    ok, getting somewhere here. Pretty much have all the resistors/caps for what we're doing here. I'm using a 1.5K/25uf on each cathode. I have a 100K for the plate. But I have questions.
    The plate resistor would be connected to the power rail section that goes to the preamp, then to the plate. Laird, you said the input grid could be connected directly to the volume wiper. Do I not need to go through the .0047uf cap that I was going through previously? Should I skip that? And then you said the plate goes to the .022uf cap connecting to the phase inverter. By this you mean pin1 plate correct? I am confused as to where the middle lug of the volume would connect back to the circuit then.
     
  17. IggyT

    IggyT Tele-Afflicted

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    see my recent build thread for a PA 26-66. I used approximately 300v-0-300v PT and I kept V1 in parallel mode; cathode biased with 2 KT66s.... I am quite pleased with the amp...it sounds best with a strat in my opinion and has a lower volume JTM 45 vibe going...
    iggy
     
  18. laird

    laird Tele-Holic

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    If you're looking at the schematic, basically we're moving one of V1's triodes between the volume control and the phase inverter. You could move either V1A (pins 1-3) or V1B (pins 6-8). Below are the instructions to move V1B.

    The V1A plate (pin 1) is connected to the .0047uf cap. The V1B plate (pin 6) must be disconnected and attached to the power rail via a new 100k plate resistor. The V1B cathode (pin 8) needs to be separated from the V1A cathode (pin 3) and given its own 1.5k resistor and bypass cap to ground. Replace V1A's 820R cathode resistor with a 1.5k.

    Once that's done, the volume wiper must be disconnected from the phase inverter's .022uf input coupling cap. The V1B input grid (pin 7) should be connected directly to the volume wiper (middle lug).

    The V1B plate (pin 6) would then be connected to the .022uf cap. And that would complete the circuit change!

    -Laird
     
  19. guitjopicka

    guitjopicka Tele-Meister

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    I can't wait to finish this off and try it again. Thanks! If my phone was not on it's death bed I would be supplying pictures... But my smart phone has a head injury.
     
  20. guitjopicka

    guitjopicka Tele-Meister

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    finished up with the couple changes out of those that I wasn't sure about. Bam. Gain banger now. So much so that I totally have some parasitic oscillations going on. The lead dress is crap because I've moved the board from another chassis with most stuff still attached and modded it a couple times since. So it basically looks like spaghetti in there right now. Ran quiet and fine when it was so low gain. Now that it's pumping I get tons of breakup, but if I move the volume knob on guitar or amp at all it just cuts out almost completely. And if I have it turned up loud at all it just goes into feedback. The whole thing is pretty microphonic now. I could probably yell into the chassis and have it come through the speakers (seriously, I'd even try it but I want to have this fixed before I'm in the studio next lol)
     
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