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Vintage SS Peavey Amp Troubleshooting

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  • Vintage SS Peavey Amp Troubleshooting

    I've got a '70s Peavey solid state Series 400 head and something funky is going on with the output stage. It sounds unnaturally fuzzy through speakers... the preamp out sounds just fine. I've been able to isolate the problem to the power module panel on the back... swapping it between my other series 400 confirmed that. It seems the louder I play, the clearer it sounds, and when I play softer, it sounds fuzzier. Almost like one of those boutique fuzz pedals with a voltage starve or bias knob... Which leads me to suspect an issue with transistor biasing? No parts on the board seemed fried or broken just by looking, but I noticed some trimpots that might need adjusting? Also possible an output transistor might be shot i guess. I had a very reluctant amp tech say it might be an issue with an output transistor, and that it wasn't worth fixing. Some other folks on a diff forum said it might be the capacitors. What should I do? And please, don't say "just buy another one, they're only $200"

  • #2
    Well, people are generally reluctant to help people with DIY amp repairs, but having said that, you're on the right track. Your problem DOES sound like 'crossover distortion' and yes, it's caused by mis-biased output devices (transistors in this case). I bought a 200 watt Crate bass head with this problem, and yes I was able to fix it with the turn of a trimpot...however it's critical that you have a multi-meter when making this adjustment, or you could damage your amp in a major way.

    Right here is where we'd normally cut-and-pasted the ADMONISHMENT about non-professionals working inside a powered-up amplifier. Although not quite as dangerous as a tube amp, SS amps still have line voltage and often high DC voltages of 60 or 80 volts that can BITE YOU, burn you or even kill you. Be safe and read up on safe procedures if you're not already familiar with them.....

    Get a schematic diagram for your exact model of amp. There's usually handy test point voltages called out on the schematic.

    You'll notice looking at the schematic for the power amp that it's symetrical - the circuit is "mirror imaged". One side amplifies the "+" signal, the other side amplifies the "-" side of the signal. This keeps the power transistors "off" for nearly half the cycle, which keeps them cooler and helps them last longer. However, the trick to biasing is we want the transistors to be "a little bit ON" and not completely off. Crossover distortion is what you hearing when one side's transistor suddenly switches on, it makes the signal have a little jump that you hear as crossover distortion.

    ...to be continued.
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    • #3
      Those are nice heads when running right but they are old school designs which shouldn't be run when they develop issues because they don't have any safeguards. I had the same bass amp and mine blew its power transistors and voltage regulator sending 120V AC to the speakers and blowing my prized Altec Speakers. The power caps used in the power section are super low budget and given the age of that head should have been replaced by now, otherwise you're running on borrowed time.

      The fuzz sound you're having is nearly identical to an issue I had on a black faced Peavey head from the same series and has the same design with the preamp on the front panel and the power amp on the back panel. The problem was common to all channels which saved time narrowing it down. The distortion was being caused by a bad coupling cap which was passing DC between stages. I still have that head but I don't run it because I know the power caps are leaking and I know I'd simply blow it up plugging it in.

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