60's Twin Reverb 'popping'

[Jonny toetags]

I did a last minute tube swap for a buddy before a show and his backup Twin was having this weird popping issue.
Here are the symptoms:
- With the amp on, it was making a random loud 'pop pop' sound.
- When looking at the back of the amp, when the pops happened, you could really see the speaker move and the 2 tubes farthest left would flash.
- I quickly tried new tubes and it did the same.
- When I took a bias reading, it was around 13 mA.

[soundguruman]

You gotta know, "just what" model twin, what do the numbers on the tube label say?
The tube label is glued inside the wooden cabinet. The numbers on it tell you what year exactly...
Bad tube (burned) sockets, bad screen resistors...
bad tubes, could be a preamp tube
bad plate resistors in preamp or phase inverter.
But you know, without looking inside, it's just an educated guess.

I would start by looking for a burned output tube socket.
The POP may be arcing on the socket(s).

[Jonny toetags]

You gotta know, "just what" model twin, what do the numbers on the tube label say?
The tube label is glued inside the wooden cabinet. The numbers on it tell you what year exactly...
Bad tube (burned) sockets, bad screen resistors...
bad tubes, could be a preamp tube
bad plate resistors in preamp or phase inverter.
But you know, without looking inside, it's just an educated guess.

I would start by looking for a burned output tube socket.
The POP may be arcing on the socket(s).

I was really pressed for time when I looked at it.
It was a transitional model...silverface with blackface interior.
I know ti wasn't the power tubes as I tried a few good quads.
Sockets all 'looked' good and i cleaned themm but I would have liked to really look at it on my bench.

[pdf64]

A quick test may be to remove the phase splitter tube, to see if that stopped the pops.
If it carries on, an intermittant open circuit on the screen grid resistors is another possibility.
If replacing these, it would be beneficial to use a metal oxide type with a flame retardant coating.
Pete

[Chuck H]

Yup. An interior inspection is necessary. Cold joints, arched sockets, dirty pins, a failing bias circuit and a few other things I forgot could be causing problems. Need to get it under test to find out what it is.