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Happy Accident w/Sunn Concert Pa.

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  • Happy Accident w/Sunn Concert Pa.

    Repairing one of these for the same person with a dozen other SS Sunn amps I've fixed the past couple weeks,
    This one had a noisy preamp, all channels loud white noise.

    It seemed to be those funky 7933 opamps. Luckily I had a bunch from another dead Sunn PA, swapped out the first three and it was much better.

    While probing around I accidentally killed the +15v preamp Zener.
    Swapped in another, amp back up, then I realized the -15v supply was only like -6v! Replaced the other Zener and HEY! Preamp is super quiet, no while noise at all!

    Always check the power supply idiot! (Talking to myself) The other 7933's are probably okay, well one had a dead half.

    Output drivers are running a bit hot, I'll probably swap in some newer beefier ones. Concert PA (1).pdf

  • #2
    The output 2N3055 do run hot as they are almost run in class A and very inefficient. So if you fit monster transistors, they will run just as hot because the heatsink is what keeps them cool.
    Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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    • #3
      Yeah, that makes sense.

      The Concert Lead and Concert Bass with the same transformer coupled output run hot also.

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      • #4
        You probably have DC on the output. Check it with no load and check all the 10 Ohm 2W and 330 Ohm resistors. These may be a different value than what is shown on the schematic. The driver transformer can develop a short on one of the windings, check that as well.
        WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
        REMEMBER: Everybody knows that smokin' ain't allowed in school !

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