Fender pro 300 farting

[pbman1953]

I have a Fender Pro 300 - 300 watt tube amp. Recently after using, for about 10-15 minutes, it starts to fart/sizzle with each note. The sound goes up and down with the volume control. I've even swapped caps to eliminate speakers. The sound went to the other cab swapped with o it's the amp.


I have a newer set of JJ -6550's which have been fine. When this happens there is a significant loss of power.


I made some some tests because each pair of tubes has a fuse next to it and the fuse can be pulled to take them out of the circuit. I've run each set alone and could not duplicate the sound.


I know I'm the early stage of detection but was wondering if my story rings any bells.


Here's the manual- It shows Sunn but all Fender did was change the nameplate.




http://www.sunnamps.com/pdfs/manuals/bass/300T.pdf


Thanks

[shooter]

I have a newer set of JJ -6550's which have been fine.

Need some clarification, do you mean with the JJ's in the problem is gone?

[pbman1953]

I really don't know if it is the JJ's. Using in pairs I couldn't get the amp to exhibit the issue, only when running all 6. I thought maybe I would see some flickering in any of  them but I saw nothing.

[shooter]

You can probably use the send return to split the amp in half, then you only have 1000 things to check!

[jjasilli]

I really don't know if it is the JJ's. Using in pairs I couldn't get the amp to exhibit the issue, only when running all 6. I thought maybe I would see some flickering in any of  them but I saw nothing.

Exactly how are you splitting the pairs?

[Tony Bones]

Recently after using, for about 10-15 minutes, it starts to fart/sizzle with each note.

As it seems to be a thermal issue I would check the resistors and look for bad solder joints. With the amp on and warmed up, gently tap each resistor with a chopstick or something nonconductive. Listen for pops from the speaker.

[kagliostro]

The Service Manual

http://music-electronics-forum.com/attachments/6721d1256437284-300t-11x17.pdf


Franco