Here's a bit of a rarity. Normally I don't like solid state stuff because it gives me a headache! This time only a minor one. Why wasn't it working? Symptoms described were, power amp works if you plug into effects return but the preamp is just a white noise generator.
Someone was in there, gave up and didn't even bother tightening the pot nuts. I did that and to my surprise got a signal through on the scope. The pots were all scratchy but reacted well to a spray clean. So I thought it was fixed. However, through a speaker all I got was white noise, hum and the faintest signal at full volume.
Why did the signal generator output work when the guitar didn't? I couldn't find any loose grounds but signal tracing from the input I found the first OP amp wasn't passing the signal on. Luckily I have a small stash of LM1458 chips. There's a knack to getting the 8 pin units out and I always put a DIL socket in there just in case they blow again. I guess someone foolishly overloaded the input at some point.
Anyway, it all sounds good now and full output can be measured through a dummy load before clipping. The power transistors run very cool on that massive heat-sink!