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I am working on a Peavey Classic 30 amplifier. It is completely dead, no sound. All the tubes light up. The 12ax7 tubes don't seem to glow as bright as the L84 power tubes.

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Hi directmusic

You can get a circuit diagram from here.

The 12ax7's will not glow as bright as the EL34's . Is just the nature of the tubes. The latter being higher powered and the 12ax7 low power. The EL84's as output tubes will always glow brighter, and often even with a dull blue glow about the plates from the higher voltages made available to them. If the plates them selves are glowing red(inside the EL34's). then you have a bias problem. Be careful running the amp in this condition. Do some static checks with the amp off looking for failed components about the bias and screen control stages. .

Start by checking all the DC voltages first. The circuit diagram i supplied the link for has voltages marked for you to check. A common problem with this amp is the 390 ohm 5 watt resistor going open. A faulty output tube(s) may also have caused this. I would replace the output tubes anyways for a good repair.

I am more than happy to answer any other questions you may have about it, just give me a hoy :) Thanks for using FixYa, If you consider the advice I have given you as sound, a FixYa rating is a great reward for my efforts to help you. Cheers.

regards
robotek





Posted on May 29, 2008

  • Graeme Ross
    Graeme Ross Jun 05, 2008

    Hi, just a quick followup to see how you got on with your
    problem. If you still havent got it sorted, let me know, I am happy to
    do some more research and offer some more information and solutions. If its fixed...
    excellent!! A FixYa rating from you will close this unrated solution
    for me. Thanks for using FixYa. I am happy to answer any other
    questions you may have.



    regards

    robotek


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Such a long delay and a slow return to volume is puzzling. If the return came back in say 30 seconds, I would suspect 1/2 of the V1 tube (12AX7) might have a failure of the heater. When it has failed see if this tube has TWO points of light in it. The cooling down and warm up would give a 15 second ramp up or down in volume. This tube has a path around it that would sustain a very low volume. Try swapping one of the other 12AX7's into this spot. If the sound totally goes away replace the one that was in V1 at first place and see if problem goes away.

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* ALWAYS UNPLUG YOUR AMPLIFIER BEFORE REMOVING TUBES *

If you can see that a tube is definitely not lighting up then it may be blown. If this amp has seen regular use then it may be time for a new set of tubes. Try replacing the 12AX7s first before committing to a new set of 6L6s. Though, if you have the cash available, a new set of power tubes isn't a bad idea either. I would advise you to do the preamp tubes first and play through the amp for a few days first before changing the power tubes.

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Hi Mike,

If you switch on the amplifier, do you hear nothing at all?
Or do you hear low level noise or hum out of the speaker?
Don't know if you're technician.
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Check also fuse 3 and 4, thats the neg. voltage -27V and -14V.


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Please update this with what you mean by "PT" and we'll continue from there.

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