Fender Blues Junior with bad distortion

[xtian]

My friend brought her Fender Blues Junior over for a tone stack mod. But when we turned it on (before we made any mods!) it had developed a bad, ugly distortion. Sound file attached.

I replaced all five tubes with known working ones, tried an external speaker, and different guitar. It's definitely a problem with the Junior's amp.

What is a likely culprit?

FWIW, it sounds like, during the 20 seconds or so it takes to turn on, there is a moment when it sounds great, but then it warms up all the way and the distortion creeps in and stays in. Keeping the volume turned very low lets it sound OK.

[iknowjohnny]

I've fixed a ton of these and i can tell you of two extremely common problems with them....the circuit traces where the output tube sockets are soldered separate due to heat all the time. Second, the sockets need to be retensioned. Some of the cheapest most unreliable sockets ever ! If it's not due to one of those 2 issues i'd be surprised.

[PCollen]

I've fixed a ton of these and i can tell you of two extremely common problems with them....the circuit traces where the output tube sockets are soldered separate due to heat all the time. Second, the sockets need to be retensioned. Some of the cheapest most unreliable sockets ever ! If it's not due to one of those 2 issues i'd be surprised.

I've had my green-board 97 BJ for going on 14 years and never experienced any problems at all. Maybe becasue it's Made in USA, vs. Mayheeco .

[xtian]

Tthe circuit traces where the output tube sockets are soldered separate due to heat all the time.

Good call. There are no lifted traces or discolored fiberglass, but there was one poor connection that I resoldered, and I put some Deoxit on the tube pins. Didn't help.

Then I noticed that just placing my finger NEAR the ribbon connector stopped the bad sound...oscillation? And that replacing the SHIELDED back cover solves the problem.

Is this a problem I can fix, or is the answer, "Don't play this amp with the back cover removed"?

[xtian]

Sorted! It was the ribbon connectors to the power tubes running too close to the OT wires. This is a known problem with some cream board Juniors:

http://billmaudio.com/wp/?page_id=115

[Dingleberry]

Just a while ago I nearly lost my mind with my friends blues junior when trying to bias it:
https://tubeamparchive.com/viewtopic.ph ... r+voltages

-T

[xtian]

Yes, sounds like exactly the same problem, cream board, too.