Amp sputtering question

[rhaigh]

I have a marshall plexi sytle tube amp that occasionally sputters and crackles. It happens with low frequncies notes and when you physically tap the amp with my fist. I pulled the phase inverter 12ax7 and the problem is still there when you tap the amp. I pulled the 2 power tubes (el34's) and the problem goes away. Would you say that the problem is located in the post-phase inverter/pre-output section? Oh, and its not the tubes as this problem has occured through many tubes changes.

Thanks.

[Jazz P Bass]

My guesses:
-Bad soldering job on the output tube pins.
-Bad connection at socket.
-Bias supply is acting up. (bad solder, bad cap).
The fact that you can get the amp to act up at will is a tremendous advantage.
I would start by monitoring things.
-The B+ voltage. (Vdc & Vac)
-The bias voltage. (Vdc & Vac)
For a start.
Once you narrow down where the problem is, then you can go about fixing it.