Old Peavey Solid State Amp "fizzes"

[Mr. Moscode]

Do you have a oscilloscope? Could be crossover distortion which is when the signal crosses the zero point one half of the output stage turns off before the other half turns on. You should be able to hear it at a very low volume and see it on a scope. The output stage could be under-biased. Does it run cold? Probably should be a bit warm. It could be more complex than that, like what is causing it if a bias adjustment doesn't make it go away. Try to find a service manual online.