popping noise from Marshall Jubilee

[Bluefinger]

I have a problem with a Jubilee Head. There is a popping noise that sounds kinda like a bass drum. sometimes it's gone, sometimes there's a pop every now and then and sometimes it goes pop, pop, pop, pop ... It doesn't make a difference if the amp is hot or cold. All tubes have already been replaced without a change. The pop must come from somewhere after the PI because only the left pair of EL34s reacts to the pop (the blue cloud inside flashes with the pop. everything looks fine inside ... any ideas what could cause this?

thanks
BF

[pdf64]

What happens if the left pair are swapped with the right pair? Does the blue flash follow the tubes or stay with the sockets?

[Jazz P Bass]

I would suggest pulling the preamp tubes one at a time to see which circuit is popping.
Without the proper test equipment you are gropping blindly in the dark.
A list of possibilities:
A bad plate resistor.
A leaky coupling cap.
A faulty bias circuit.

[Bluefinger]

All tubes have been swapped already ... it's always the left pair of tubes, no matter which tubes are in the sockets.

[Bluefinger]

Switching circuits and pulling preamp tubes doesn't make a difference. It must be somewhere around or after the PI I guess.

I'll try replacing coupling caps and plate resistors tho ... it can't hurt ...

[Jazz P Bass]

It would be nice to have all new coupling caps & plate resistors.
Except....
You may be introducing other problems in the process.
Does the amp still pop if the phase inverter tube is pulled?
Do you have access to a decent volt meter?
You may be able to narrow down the problem if you do.

[pdf64]

Might also be a bad screen grid resistor or 2?
Have the tube socket contacts been retensioned?
Billm Audio » Re-tension, resolder your tube sockets

[acorkos]

There is a popping noise that sounds kinda like a bass drum. sometimes it's gone, sometimes there's a pop every now and then and sometimes it goes pop, pop, pop, pop ... It doesn't make a difference if the amp is hot or cold. All tubes have already been replaced without a change. The pop must come from somewhere after the PI because only the left pair of EL34s reacts to the pop (the blue cloud inside flashes with the pop. everything looks fine inside ... any ideas what could cause this?

Could be arcing inside one of the power tube sockets. might need replacing

[Bluefinger]

Problem seems to be solved. I cleaned the PI and power tube sockets and resoldered all the connections around the PI and power tubes and the popping seems to be gone ... thanks everybody!