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classic 30 with low volume and odd sounding distortion

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  • classic 30 with low volume and odd sounding distortion

    howdy,
    i have this older classic 30 that up until now has been fairly solid. i have done a bit of work to it out of necessity. i replaced every bloody jumper wire in the thing because i got sick of them breaking. i've also modified the tone stack with some orange drops and silver mics and bypass cap on V1 in the preamp (Steve A's mods).

    over the course of the past three months, the amp has progressively gotten quieter in volume and has developed a soft distortion in both the clean and distortion channels. the distortion only happens when signal is being sent to the amp. not when the amp is silent like you'd expect from deteriorating caps. this distortion also sounds kind of like an envelope filter in a way. messing around with the tone knobs while sustaining a single note creates this "wah-wah" effect. i changed out all the preamp tubes thinking that the problem could have been with the phase inverter, but no luck. i had an older quad of el84s that i tried with the exact same results thinking that, if it were the power tubes, the older set would at least sound somewhat different. i've ordered a new quad just to be on the safe side, but i wanted to pitch the problem on here to see if anyone has ever heard of such a thing. i've tried every combination of search queries into to Google only to come up empty handed. i don't think the OT would cause such a problem... but i'm at a loss on this one. hah. any input is surely appreciated.

  • #2
    Welcome to the forum.
    I'd check all the solder joints you worked on. To me it sounds like some connection is not steady when signal is applied. Might be a ground connection (cathode, grid resistor to ground, or whatever) or signal line connection.
    What happens when you apply a signal direct to your power amp? Same result?

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    • #3
      And what happens when you connect a spare cord from send to return jacks on the FX loop?
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        thanks for the warm weclome, mr. txstrat. i lurk on this board a lot. it's about time i had a reason to pick some other brains on here.

        a spare chord running in the effects loop produced no different results. i even just plugged a guitar signal direct to the return jack which (according to the schematic) sends signal directly to the power amp. i confirmed that the tone knobs had no effect just to be sure, and still the noise persists.

        i located some jumpers and components in the power amp on the pc board. all seemed ok. i even found someones that wiggled a bit, but even replacing the jumps are resoldering them didn't offer much improvement.

        i did find that one of the .047 OD caps that i had put in was making a popping noise when i tapped on it, so i yanked it and put it the original back in. i imagined that it wouldn't solve the problem as all signs point to the power amp at this moment, but i figured that it couldn't hurt.

        the power amp seems pretty simple overall. i've got 5 caps total. a couple of .1s, .22s, and a 1pf in there. the way it seems to act as an envelope would suggest caps to me (oh yeah, my quad of el84s came in). i tested all the resistors in the power amp and they seem to be functioning fine. unless you guys can think of something else to try, my next step is to start replacing those caps in the power amp and hope for the best. any other suggestions? thanks so much for all the help.

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