Amp Running Hot

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mightypudge, Jun 27, 2006.

  1. mightypudge

    mightypudge Member

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    Tonight I played my Rivera K100 at practice for the first time. I installed brand new power tubes on Sunday, a matched set of GT E34L's rated #4, and biased the amp just under factory specs. Rivera calls for 40 mA, I biased to 38 mA.

    After practice I went behind the amp to unplug everything and noticed it was pretty warm back there. It doesn't appear the paint on the tubes wore off, and I could still touch everything (except the tubes, of course), but it sure seemed pretty hot.

    So, assuming the bias is set correctly what would cause a tube amp to run hotter than expected? I used my 8 ohm cabinet and set the amp accordingly. I used good speaker cable. Maybe this is all in my head?
     
  2. Texasamp

    Texasamp Member

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    Probably because they are new power tubes. You biased it a bit colder (38mA), this also will cause a higher plate voltage. Did you change the rectifier tube as well?

    Darrell-
     
  3. mightypudge

    mightypudge Member

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    Hi Darrell, Rivera amps don't use a tube rectifer. They are all solid state rectifiers.

    You think maybe because the tubes are new they might be running a little hot? Should I try bringing the bias up to 40 mA? I didn't think 38 mA would make such a difference.

    Thanks for your response!
     
  4. Texasamp

    Texasamp Member

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    It should not make a difference so that you would notice it being hot.
    When you are playing it, is the PT getting hot to the touch? Also, if you let a tube amp idle, it will feel warmer than normal. Do you know how to check the power tubes plate voltage?
     
  5. John Phillips

    John Phillips Member

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    If you biased the amp correctly to 38mA per tube it's actually running cooler than at 40mA. The fact that the tubes are new makes no difference - the amount of heat being produced is purely a function of the tube current and the plate voltage. The difference in plate voltage is absolutely negligible for a bias change of 2mA.

    Could it be simply that you've never run the amp cranked before? You mentioned that it's the first time you've practiced with it. The amount of heat dissipation from both the tubes and the transformers goes up substantially when the amp is under heavy load - tube amps are not very efficient, and if you normally use it at a few watts output (at idle, effectively), the difference between the power used then (at a guess, around 150W), and with it cranked could be at least another 150W or so of wasted heat, ie around double. You'll notice that for sure.

    (I can show you the math for that if you like :).)

    Also, was the practice room hot, compared to playing at home? The hotter the air, the less well the amp can cool itself.
     
  6. mightypudge

    mightypudge Member

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    Hi John,

    Good to hear from you again. I'm pretty sure I biased the amp correctly since I have biased Rivera amps before with good results. Using a DVM the black contact to ground and the red to pin 8 of one of the tubes, my reading is 38mA. Let me know if I've done something wrong there.

    This was the first time I cranked the amp. It's completely possible that I'm making a big deal of nothing. I'd gotten used to my Mesa Dual Rec which probably runs cooler than any amp I've ever used. So perhaps the heat is not abnormal, just more than I'm used to.

    Would checking the plate voltage tell me anything valuable? If so, how do I check that? Remember, I'm a rookie with a DVM so use small words. :D
     
  7. John Phillips

    John Phillips Member

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    I wouldn't worry - in order for the plate voltage to be so much too high for the tubes to run over-hot at the correct bias current, there would have to be something major wrong in the power transformer, and the amp would most likely not work properly at all. In any case, you probably wouldn't be able to even get a correct current setting, so that alone indicates that everything is OK. It sounds like you've biased it correctly.

    Measuring the plate voltage directly is fairly dangerous, and I wouldn't risk it unless you really need to - some DVMs don't even have enough voltage rating to be safe. The plate voltage is in the 450-500V region, and I wouldn't try measuring that with anything less than a 600V-rated meter.

    If you've only used the amp at home before, I think you can probably put it down to the greater heat from actually running it loud - you'd maybe be amazed how little power you use at home, on a good MV amp... probably no more than 1 or 2 watts output, which is still surprisingly loud through any normal cab. That's effectively the same thing as idling, to the amp. But in a band situation you need a lot more power, and every extra watt of output adds about two to the heat the amp produces.

    Mesas are 'notorious' for being biased very cold, BTW.
     
  8. mightypudge

    mightypudge Member

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    Hi John,

    Thanks for helping to put my mind at ease. I guess unless they're glowing red or melting the tolex :))) the tubes are probably running OK.
     

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