NOS GE 6V6GT Plate Voltage in my SE homebrew is 425v... too high?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Axeplyr, Jul 28, 2008.

  1. Axeplyr

    Axeplyr Supporting Member

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    Hey amp gurus...

    My recently finished Dave Hunter Two-Stroke (classs A, single ended) is running 420v on the plate (pin 3). Cathode (pin 8) is at 19.5v, for about 400v actual plate voltage (pin 3 minus pin 8).

    The amp is cathode biased. While it sounds stellar, I'm just worried that this is a little too much voltage. Do I have anything to worry about, and should I try to lower the plate voltage a bit? Or just crank it and enjoy?



    Thanks! :)
     
    Last edited: Jul 28, 2008
  2. Blue Strat

    Blue Strat Member

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    The voltage the tube "sees" is the plate to cathode voltage. In AB circuits this is the plate voltage (since the cathode is at zero volts), in cathode biased circuits you subtract cathode voltage from plate voltage.

    Plate to cathode voltage: 420-19.5= 400.5 volts

    This should be fine.

    Next step is Idle Power Dissipation: Plate to cathode voltage X Plate (or cathode) current

    Cathode current = 19.5/cathode resistor value/2

    Assuming a cathode resistor of 250 Ohms (a commonly used value):
    Cathode current: 19.5/250 = 78mA, 78mA/2 = 39 mA

    Idle Power per 6V6= 400.5 X .039 = 15.6 watts.

    The max power dissipation for 6V6s is 12 Watts 15.6 is obviously too high.

    IF your cathode resistor IS 250 Ohms, it should be increased until the idle dissipation is right about 12 watts. It's impossible to calculate this so you'll have to take a few stabs. Something around 325 ohms should get you close.
     
  3. Axeplyr

    Axeplyr Supporting Member

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    Thanks, Mike. It's actually a 500 ohm cathode resistor, and only a single 6V6 tube.

    Pretty much a Fender 5F2A Princeton circuit with some tweaks, slightly more versatile. Hammond 373CZ power trannie (325-0-325).

    Where does that put me? Seems the idle power would still be too high?
     
    Last edited: Jul 28, 2008
  4. Blue Strat

    Blue Strat Member

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    Puts you in exactly the same place because the number of tubes is halved and the cathode resistance is doubled.

    800 ohms to 1K for the cathode resistor is a standard swap.
     
  5. Blue Strat

    Blue Strat Member

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    BTW, if it was REALLY like a tweed circuit, a better choice for a power transformer would have put the plate voltage at around 355V just like the original amps. Then you wouldn't be out of spec or as far out of spec.
     
  6. Axeplyr

    Axeplyr Supporting Member

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    True. Unfortunately, the transformer I got was the only one available here in Australia. So I went with it.

    Sounds like I need to change a couple of things. Though the amp sounds really good, it's worth trying. And I don't want to start killing NOS tubes unnecessarily.

    So I can swap out the cathode resistor easily enough. Is it easy to drop the plate voltage as well? I might do it just to see what happens.
     
  7. sdgvintage

    sdgvintage Member

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    What rectifer are you using?
     
  8. Axeplyr

    Axeplyr Supporting Member

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    Sovtek 5y3gt I believe. I've read elsewhere I can drop 30v or so just by going with an NOS 5y3?
     
  9. Blue Strat

    Blue Strat Member

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    Not sure if it's that drastic, but a NOS 5Y3 will have a lower cathode voltage so it's worth a try.

    There's no other way to constructively lower the plate voltage other than this swap or a different PT. You could install a series resistor, but it will totally mess up the feel of the amp by adding major sag.
     
    Last edited: Jul 29, 2008
  10. Axeplyr

    Axeplyr Supporting Member

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    Thanks. Guess I'll 1) get an NOS 5y3 and remeasure, then 2) adjust cathode resistor accordingly.

    Thanks for the help everyone!
     
  11. Axeplyr

    Axeplyr Supporting Member

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    Tried an NOS Jan 5Y3WGT, only gave me a 10-15v improvement. Now reading 410v on pin 8 to grnd, 387v or so pin 3 to pin 8. I was hoping for better.

    Picked up a 1kΩ 5w resistor, I'll sub the 470Ω job that's in there tomorrow.
     
  12. JJman

    JJman Member

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    Note that the plate voltage will go up when you increase the cathode resistor. However, the wattage will go down and the tube will be happier.
     
  13. Axeplyr

    Axeplyr Supporting Member

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    ThankJJman. It did.

    Took an 820 ohm resistor in the end.

    Plate to cathode voltage: 415-24= 391 volts

    Idle Power Dissipation: Plate to cathode voltage X Plate (or cathode) current

    Cathode current = 24/cathode resistor value

    Cathode current: 24/820 = 29mA

    Idle Power for 6V6= 391 X .029 = 11.35 watts.

    The max power dissipation for 6V6s is 12 Watts - 11.35 is good. :BEER

    Thanks again for the help!

    Hopefully this won't mess me up when I want a 6L6 in there. :munch
     
  14. Axeplyr

    Axeplyr Supporting Member

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    Here's the schematic of the amp as I have it right now. I drew it with MSWord believe it or not - no schematic of the Two-Stroke existed. I've also tweaked a couple of things from the original Dave Hunter design.

    Amp sounds really 3d, complex, and responsive. Very dynamic. Barely broken up is nice as well. Only complaint - it gets really brash/harsh when cranked up. Not at all smooth.

    Ideas?

    [​IMG]
     
    Last edited: Jul 30, 2008
  15. Blue Strat

    Blue Strat Member

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    Nice work. How, if any, did it affect the tone?
     
  16. Axeplyr

    Axeplyr Supporting Member

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    I think it scooped out the mids a tad and made the bass a little deeper/tighter, the highs a little brighter, and the amp maybe a bit less compressed. I may have liked it just a little better before. The harshness when cranked seems to have maybe gotten just a little worse.

    Of course I may be imagining things. The amp still sounds great overall - this is fun! :dude

    My Tele and Strat sound excellent overall, clean or dirty, as long as the amp isn't cranked past 60% or so on the volume. Makes the Tele and Strat sound exactly like they should, with plenty of snap. Gets a little brash if cranked over 60% though.

    The Les Paul sounds great clean, but doesn't distort well at all.

    I still feel like there is work to be done.

    Here's an old pic of the innards, quite a few changes to the wiring have been done. Same components mostly though.

    Maybe much of the harshness is due to the lack of carbon comp resistors and SoZo caps?

    [​IMG]
     
  17. stratman_el84

    stratman_el84 Member

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    I notice that the NFB circuit and "Impedance" switch doesn't look correct. Not sure if that's just the drawing.

    However, the other thing I see is that there's no coupling/D.C. isolation cap in series with the 68K resistor going to V1b. That will put V1b's cathode directly across the secondary of the OT, which means that V1b's 820-ohm cathode resistor is in parallel with the DC resistance of the OT secondary.

    That might very well be where the harshness is coming from. Throw a 0.1uF coupling cap in series. You might even try it with no NFB at all, or switched in/out. Good luck!

    Cheers!

    Strat
     
  18. Groovey Records

    Groovey Records Member

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    Is this the Victoria Kit or did you gather the compnents yourself?

    The original Two Stroke was a dual class A

    I love THAT Two Stroke with 10" and an 8" speakers, maybe some of the brashness could be addressed by looking at the Speakers and giving them a chance to break in?
     
  19. Axeplyr

    Axeplyr Supporting Member

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    The impedance switch is definitely drawn incorrectly. I need to fix that.

    Just so I understand... place the coupling cap in series with the NFB resistor? I'll definitely try it.

    I have tried reducing the NFB value with a 56k resistor in parallel (about 30kohms). It made the amp a tad darker and a little spongier, but there wasn't a huge improvement in the harshness factor. I'll try removing it as well.

    Here's the layout from the book. This is basically what you get to buid the amp from - and also why I drew the schematic!

    [​IMG]
     
    Last edited: Jul 30, 2008
  20. Axeplyr

    Axeplyr Supporting Member

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    I agree - I love the amp. Definitely inspired by the 5F2A Princeton with some tweaks, the Two-Stroke is a great design. It sounds much better than I expected, but it's not quite 100% yet. He no longer offers it as a dual-octal setup as he felt the single tubes sounded better anyway. When I built mine I followed his addendum, which removed the second tube circuit from the design.

    I'm playing it thru a 65Amps 2x12 cab with a Cele Gold and a Cele G12H-30, cab sounds stellar with my Marshall JCM-800 and 65 SoHo head. It's pretty well broken in.

    No kit. I built this from a mishmash of parts bought from miscellaneous sources. Hammond trannies, Hammond aluminum chassis, and whatever resistors/cap/pots and other stuff I could acquire.

    It was challenging and probably much more rewarding, but I think the overall results would have been much better had I bought a kit.

    Here's a clip from a few weeks ago:

    http://www.tonecentral.net/gearpics/homebrew/twostroke/Two_Stroke_Mar08.MPG

    Another pic:

    [​IMG]
     
    Last edited: Jul 30, 2008

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