traynor ycv20wr eating tubes?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mikeyp123, Jun 28, 2005.


  1. mikeyp123

    mikeyp123 Guest

    Anyone have any idea how long power tubes should last in a class-A, EL-84 bassed amp design, running about 2 to 3 hours nightly? I'm getting about 3 months out of my tubes... although I'm a little concerned that it could be something else.

    The amp is sounding dull, and the volume drops significantly with some crackling like noise. Athough it will work fine for about 15 minutes when I first turn her on.

    Does this sound like the tubes need to be swapped or something more serious.. given that I swapped the power tubes about 2.5 months ago and preamp/inverter about 6 months ago.
     
  2. unclej54

    unclej54 Guest

    you're probably going to need to take it to a qualified tech to have it checked out..sounds like the bias could be set way off or the bias resistors are going bad.
     
  3. Blue Strat

    Blue Strat Member

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    Check out the top of the EL84 section of my website.

    "Self biasing" amps (especially EL84 amps) are often biased to the point of destruction.

    Why amp designers do this is a mystery. All it takes is a higher value cathode resistor.
     
  4. John Phillips

    John Phillips Member

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    +1.

    Three months at 2-3 hours a day sounds like about 200 hours - not untypical for modern-production EL84s run too hot, but you can do much better than that.

    If you don't want to adjust the amp, just be thankful that EL84s are the cheapest power tubes... but don't waste good NOS ones trying to improve the tube life.

    I think the reason designers do it is because they may be under the (wrong) impression that this gives Class A operation, which 'is desirable' because the 'Vox AC30 is Class A' (also wrong, although they do run very hot). This is a classic case of wrong assumptions producing market demands which produce design changes which then cause further problems... and actually give the opposite of the real original intention (the characteristic tone/feel of an AC30 is specifically that of a cathode-biased Class AB amp).


    The PI shouldn't need changing much if it's a good one to begin with.
     
  5. mikeyp123

    mikeyp123 Guest

    thanks for the info gentlemen. What would you consider a normal life span for preamp 12ax7 tubes? given the super hot EL-84 situation.. I've had my current set of preamp tubes in there for 6 months now, maybe those need to be swapped as well?
     
  6. John Phillips

    John Phillips Member

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    If they're good old-production types they're just about run-in now ;).

    Seriously, I've worked on many 70s and even some 60s amps with their original preamp tubes in, working perfectly. The lifespan of those old high-quality tubes is in the tens of thousands of hours. Even the power tubes last much longer than modern ones as Mike will tell you too - I've also worked on many 80s amps and a very few 70s ones with original power tubes.

    If they're modern preamp tubes you should still get a couple of years out of them (more than a thousand hours). No need to change them unless they're showing signs of going bad in any case.
     
  7. Blue Strat

    Blue Strat Member

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    What John said. A general benchmark (average) for preamp tubes is about 4:1 lifespan of power tubes. Even under PROPER biasing (unlike most EL84 amps) power tubes take much more abuse from heat than preamp tubes do. Heat is the enemy.
     
  8. mikeyp123

    mikeyp123 Guest

    excellent info. I just wish there was some way to verify that the power tubes are indeed "bad".. I guess the old shaking a light-bulb trick won't work.

    I've got a new pair of JJ tubes on the way. Funny thing is that the supposed death of the tubes happened to coincide with a speaker change... I double checked the speaker load with a multi-meter last night, it seems ok.

    The amp does run insanely hot, my practise room eventually becomes unbearable due to the heat production... I'm tempted to just drop the entire Class-A, EL-84 fiasco and get a Fender amp.
     
  9. Blue Strat

    Blue Strat Member

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    You can always install a better suited cathode bias resistor which will set you back about $1.
     

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