Addressing the issue in a 3 ch triple rectifier.

I try to figure out what is the cause; when it comes to suddenly loss/cut off in volume and only moderate audible hum dominated the sound in all channels. I try to measure the dc voltage in the plate of the phase inverter tube with the dmm, comm touching the ground/chasis and the other lead to pin 1 (and pin 6) of the 12Ax7 and a strange happen, the hum just gone away, the amp back sound normal again, same thing when i measure the 30uF/500V cap or just flip the stand by switch off and on again (those action can made the hum gone). What is really the cause of this? Is the particular tubes failure (CF or PI)? OT issue? or something with the filter caps or the the .047uF/1 KV? I measured all the component values, the B+ supply, the bias, and all the DC supplies at the point A, B, C, D, E (as in the schematic); and all of them just seems to be OK. Can someone help to tell what is the cause of the problem?

Check the pin sockets to see if they need to be cleaned. The tube may also be bad. I'm wondering if you grabbed stray electrons building up at the plate or the the DMM lead was enough to create an arc to the socket, restoring the connection.

That's a logical thing to start with.. thanks afu for your suggestion. I'll start from there then. It's freaked me out, because every time, when i play the amp it sound ok, and the next day, after a while the hum issue could suddenly popping out, and it will back to sound normal again just by flipped the stand by switch off and on. Once, i replaced with a set of Mesa 12AX7's and it has many cycle of playing, day after day but for about 2 or 3 months after then the ugly annoying hum start to disturbing again. I cannot test the tubes before install, there might some inferior or about to fail among them, they could stand in my other JVM or 6505+ head but maybe not triple.