fender blues junior help

[JC@]

I have a problem here. It is a fender blues junior, and as soon as it gets warm it begins making shotgun like noises. That is, a loud PAM PAM PAM PAM...
I pulled out the preamp and PI tubes and I changed the power tubes and It does more or less the same thing, and I could see that the glowing insides of one of the tubes changed in intensity with every PAM
After that I interchanged the tubes in both sockets and the thing happened again, this time ending with a really wild oscillating type howl.

All resistor values around both tubes are right. I doesn't give me enough time to read values while this is happening, but before that, plate, screen grid, and bias readings are good.

I have retensioned and remelted the tube sockets.

Any clue?
Thank you.

[52 Bill]

If it's really loud, it sounds like there is something that is shorting power supply to ground. It could be a filter cap, the output transformer or ?

Try and read supply voltages when it happens. Will it do this without the power tubes installed? (If it doesn't make the noise does the power supply voltage fluctuate?)

[JC@]

If it's really loud, it sounds like there is something that is shorting power supply to ground. It could be a filter cap, the output transformer or ?

Try and read supply voltages when it happens. Will it do this without the power tubes installed? (If it doesn't make the noise does the power supply voltage fluctuate?)

Yes it's really loud and sharp, and that makes me worry about the speaker.
ok, I will try without power tubes, and I will test with a dummy load and check voltages.

In fact those were my next moves, B+ filter caps and output transformer.
It still makes me wonder why one of the power tubes keeps the rythm with the pam pam, increasing it's glowing accordingly.
Thank you 52 Bill

[Twist]

Is this happening at one particular socket? Is it a output tube that seems to glow to the sound. If it's in the output section try this, remove one output tube and turn on the amp. If nothing happens move that tube to the other socket. If nothing happens remove that tube and try the other tube and repeat the process. This should isolate a tube or particular socket.

[JC@]

I have this amp parked for the moment while I finish other work.
I am begining to think it may be the output transformer, but I don't know if a bad OT can make one side of the power section oscillate so wildly.