Class A Self Biasing

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by somedude, Aug 20, 2006.

  1. somedude

    somedude Member

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    Is it possible to swap a set of 6L6s into a class A amp designed for EL34s? Will the amp still bias correctly? Will I potentially damage anything?
     
  2. John Phillips

    John Phillips Member

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    It might be OK. It depends on the plate voltage and how hot the EL34s are being run - ie the operating point, or whether the amp is actually Class A. If you put a different tube type in, the operating point will be different, which might end up running the tubes too hot (or too cold). In a fixed-bias amp you can't simply put 6L6s in an EL34 amp without them running much hotter since they need quite a lot more bias voltage. It isn't as simple in a cathode-biased amp though, since if the tubes run hotter the bias voltage rises and so tends to limit that - but where they finally end up will depend on the rest of the circuit.

    If it matters, you can put 6L6s in a Mesa Blue Angel - designed for 6V6s, which have a similar bias voltage to plate voltage requirement to EL34s - without adjusting anything. It does work fine, and the 6L6s run slightly hotter than the 6V6s, though proportionally slightly cooler (a higher number of watts, but a lower proportion of maximum dissipation), if that makes sense.

    I would guess that there's a risk of running the 6L6s too hot if they're going into an EL34 amp which is already designed to run them at the limit. 6L6GCs are rated for 30W compared to the EL34's 25W, so it might still be OK (it might not be with 6L6GBs, which are only 22W). I'd probably try 7581As (35W) or at the very least sturdy 6L6GCs as a test, and measure the dissipation. You should do it at both idle and full power, since there's no guarantee the amp will still be operating in Class A... even if it really is with the EL34s anyway.
     
  3. Blue Strat

    Blue Strat Member

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    Even under the best conditions (same tube type) "self biasing" does not necessarily mean CORRECT BIAS.

    You should always check the idle power (idle current X plate voltage-cathode voltage) to be sure you're running "in spec".
     
  4. TheAmpNerd

    TheAmpNerd Member

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    Also,

    Considering what everyone has said,
    it is safer to put 6L6s in an amp designed for EL34s,
    then the other way around.

    Two key reasons:

    ONE
    What OTM said, in a circuit designed for EL34s
    where the Plate voltage runs higher then Screen voltage.

    When you stick a 6L6 in there it shouldn't damage a thing.
    Kevin O'Connor and many others tout running 6L6 type tubes
    with greater screen protection (Vp > Vs).

    In amps designed for 6L6s where Vp = Vs and you stick an
    EL34 in there...Well pop goes the weasel...or at a minimum
    you are really stresssing them and the amp.


    TWO
    The Heater current requirements for EL34s vs. 6L6 are greater.
    That is EL34s draw 1500 Ma vs 6L6 draw of 900ma.
    So, again sticking a 6L6 in an EL34 designed amp causes
    less stress.

    Sticking an EL34 in a 6L6 designed amp causes more stress.


    ADDITOINALLY
    This does not negate anything Mike nor John nor OTM said.

    Examples of the cathode biased amps like this are the THD Uni Valve
    and Bi Valve amps. Whether they are really class A is another post.

    I think it was John Phillips that posted and excellent method of
    determining amp class from over all current draw. Or was that
    you OTM.

    Trying to find it now because I wanted to print it out.
     
  5. Swarty

    Swarty Member

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    I would expect EL34s to run ice cold in a circuit optimized/designed for 6L6s.
     
  6. TheAmpNerd

    TheAmpNerd Member

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    Yeah, that was it.

    Hey Swarty

    Give it a shot and let us know what you find out.

    Thanks
     
  7. Reeek

    Reeek Member

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    Let's also not forget that the amp should (better) have pin 1 tied to ground or the EL34's will self destruct.

    6L6's do not use pin one but EL34, to my knowledge do. Many 6L6 amps have pin 1 tied to ground for general purpose but if pin 1 is NOT grounded, to my knowledge, an EL34 will fry in seconds
     
  8. somedude

    somedude Member

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    Well, the amp in question is an Orange Rocker 30 (EL34s stock). I have no idea if it's "Real Class A" or simply the "Cathode Biased Amp Being Passed Off As Class A" type class A.

    That said, most of the safety concerns seem to be towards putting an EL34 in an amp engineered for 6L6.....I'm getting from this that it's generally safe to run 6L6s in an amp engineered for EL34s, though they might not run optimally.

    I know that my Mesa recommends only running EL34s in the class A sockets of my MkIII. They recommend not using 6L6s because of a shortened life span and not because of any potential damage to the amp.....which is what I believe that John Phillips was refering to.

    Comments?
     
  9. conundrum

    conundrum Member

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    The Rocker 30 has 270 ohm cathode resistors for each EL34, and the plate voltage is down around 370 I think. I'm thinking 6L6's would work.
     
  10. somedude

    somedude Member

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    Cool. It's more for the fun of experimenting, but I suspect I'll stick to the EL34s.

    Is the Rocker 30 a "real" class A amp?
     
  11. Blue Strat

    Blue Strat Member

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    ,,,
     
  12. glosie

    glosie Member

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  13. somedude

    somedude Member

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  14. ausguitarman

    ausguitarman Member

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    What about a Fargen Miniplex.

    It's supposed to run EL-34's, 6L6's and 6V6's without the need to re-bias.

    I'm currently running EL-34's but would like to swap around.
     
  15. TheAmpNerd

    TheAmpNerd Member

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    You mean the one about putting
    KT88s where EL84s used to be?

    Doesn't FryedGear make an adapter for this?



    Aussy man,

    If the Fargen was designed that way from the start,
    then plug and play (read the manual first).

    REEK and OTM also point out about the old
    tie the other end of the control grid resistor to pin 1 trick.

    That gets them every time they stick an EL34 where a 6L6
    should be.
     
  16. webs

    webs Supporting Member

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    So the real question is- has anyone tried this method to see whether an AC30 is class A or not?
     
  17. webs

    webs Supporting Member

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    Good enough for me. :nono :horse :nono
     

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