Biasing EL84 Based Amps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by devin, Mar 30, 2004.

  1. devin

    devin Guest

    I have a bias meter for the larger power tubes, but how does one set bias for a Class A amp with EL84's, or do you at all?

    I have an Alessandro English, and I recently swapped the sovteks it came with for some JJ Tesla's. Terrible tone! The amp was much louder, but also brighter with a strident character. I wonder if this is due to the characteristics of the tubes, or if it might be a bias issue (if you can even bias this amp).

    Anyone know?



    Devin
     
  2. sws1

    sws1 Member

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    I don't think "Class" has anything to do with it. What you need to find out is if the amp is "cathode biased" or "fixed biased". If it's cathod biased, typically the only way to bias is (if you need to) is to change out resistor(s).

    Typically in non vintage amps, (although not all the time), an EL84 amp is cathode biased, and as long as you buy sturdy tubes, you can just swap them in.

    Of course, since I don't know much about the Allessandro, I can't confirm.

    GRATUITOUS LEGAL DISCLAIMER
    Amps can kill. Don't touch the insides. I'm not at fault if you die. yada yada yada.
     
  3. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Devin - the English is Class A, cathode-biased.
     
  4. devin

    devin Guest

    Thanks, guys. I appreciate the info very much. Looks like I'll be buying a quad of sovteks and saving the JJ's for spares.

    Devin
     
  5. redjet55

    redjet55 Member

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    either that or get some lower gain jj’s
     
  6. devin

    devin Guest

    The ones that I received are marked with a "49" on the boxes. Any guess as to what number I should request?

    Devin
     
  7. redjet55

    redjet55 Member

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    I got some lower gain ones from eurotubes marked at 40 that sounded great in my badcat cub.
     
  8. PaulC

    PaulC Member

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    The numbers on the box don't really mean anything except to that tube dealer. You will be able to tell if one set is hotter than another by those numbers, but only for that dealer. New sensor numbers don't mean anything to JJ/Euro tubes/Mojo etc...

    Cathode bias can run hot and cold from a simple tube swap. While they're known as being self biased they do suffer from bias issues. Changing the bias resistor is one way of doing it, but most builders will set the resistor around a grade of tube they like. Then they stick to the "grade 6" for that amp. This keeps headroom and break up qualities closer amp to amp. You can always call George up and ask him what grade/make of tubes he uses.

    Later - PaulC
     
  9. redjet55

    redjet55 Member

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    I agree the numbers marked on the box are only relative to a particular vendor. So if you got tube from a specific vendor probably best to ask for lower gain tubes from that vendor.

    Bob at eurotubes tests and grades his so he has an idea relative to his grades. Probably won’t mean anything to another vendor.
     
  10. devin

    devin Guest

    Those are some great points. Thanks!

    Devin
     
  11. JohnnySavant

    JohnnySavant Guest

    IMHO, Sovtek EL84s are among the worst made, if tone is a factor. They get muddy when pushed.

    If you like Russian, the old military 6p14p-EBs are much better (closer to the Sovtek EL84M), and even cost less, too.

    Right now, I prefer EI Gold Elites among new production EL84s.

    This is my first post to this site! Glad I found it!
     
  12. devin

    devin Guest

    Hey Johnny,

    Thanks for the suggestion. I'll have to try those Ei Elites if I can find which tube rating will work.

    Devin
     
  13. 1kidc

    1kidc Member

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    I gotta agree w/ REDJETT - Bob @ Eurotubes knows much about these issues. He's fast with email and can definitly give you some answers.

    Props to the Mighty Bob for making my Gibson Goldtones sound 1000% better. He Da Man!
     
  14. Tube Guy

    Tube Guy Silver Supporting Member

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    I have a Goldtone GA-30RVS. What tubes did you end up wth in yours that made it sound so good?
     
  15. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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    John, Pete Cage would perhaps be better situated to reply fully to your "why bother" question, but as I noted (with pics) in this thread:

    http://63.151.115.106/board/showthread.php?s=&threadid=38196

    we recently added a bias trim pot to my 2XEL84 circa '76 Traynor YGM-3 to allow us to set the bias exactly as I wanted it for the best (to me, of course, YMMV) combination of headroom and tone.

    One quick answer might be that instead of searching for "tubes it's happy with," we can now make the amp happy with just about any of 'em. :AOK

    Isn't that why all amps with fixed/adjustable bias are constructed that way? :confused:

    P.S. Replacing the EHs that it came with with JJ's sure improved things. Of course, tube selection is often amp-dependent.
     
  16. 1kidc

    1kidc Member

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    Unfortunately I've thrown away the tube boxes... write to Bob and tell him Dave DeMichelis sent you. He'll know for sure.

    Mine were for my ga-15's. Gave me a bit more clean headroom before breaking up. I used to have a ga-30 rvs and never seemed to have that issue.

    Hope you have roadies - that was a heavy beast of a combo! I sold it and bought two of the ga-15's. Easier to haul around and a bigger stereo spread. I do miss the effects loop and boost switch however....

    Good luck w/ the goldtone!
     

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