Help with amp noise issue please

[Shack]

So my Plexi amp that I built sounds awesome , but has developed a pulsating noise when the amp sits idle. it does seem to be tied to the preamp because it goes away when the 2 channels are turned down, and it gets louder when master volume is turned up.

I changed the preamp tubes today before the gig, and that didnt help. I can substitute power tubes tomorrow and see if it helps, but if not, im wondering if it could be a cap in preamp or power supply.....amp was built this spring, and I havent heard this before, I wouldnt even worry about it except it has gotten loud. Sounds like a low beating in the background .

Hope that explanation helps.

[shooter]

Sounds like a low beating in the background .

sound like you're describing motor-boating, typically a PS issue. try searching here, there are a lot of "tips n tricks".
some of the "usual" suspects, shared cathode parts, to small value bypass cap, PS dropping R's, undersized filter caps...........

[Shack]

I thought it was motorboating, but I would have expected it to be alot louder, from the way I heard it described.

I may have a culprit if its in power supply, as the filter cap board is the only thing that is original from the Peavey I built this from. The filter caps are all new, but the resistors and diode on the board are all old, and the 10k dropping resistor in preamp section  did get hot and start to smoke while I was working on it. I didnt replace it then because the value and everything was still right. maybe all the playing ive done on it finished it off.

Thanx Shooter 

[Shack]

Hmm, just goes to show what a great troubleshooter I am, lol ...I replaced the 10k dropping resistor that smoked a bit and put it back together. plugged in guitar and the sound was gone.



Plugged in my pedal board and voila......there it was. I figured because it was still there with all pedals turned off that it had to be the amp, im wondering if ill ever learn.

I suspect the voodoo labs tremolo pedal lol.

[Tony Bones]

At least that 10k resistor won't be keeping you awake at night anymore.

[Shack]

yup, im happy