Wierd problem with blues Jr redplating

[daz]

I''ve got a blues junior that had a dead el84 and when i replaced it with a known good one, the new one started redplating. what is so bizzare is that i have tried 4 or 5 more tubes and known good tubes that worked in that socket with the problem will then redplate in the OTHER socket ! It's like musical chairs ! I have had so many tubes in it at this point and the socket that has the redplating tube can be either one ! I can even have a tube redplate then use it in the other socket and it doesn't. then put another tube in that same socket and it does. then next time it a tune in the OTHER socket that redplates. At this point i am so confused i'm spinning. It makes zero sense. And i checked all the components like the 220k's and all that except the bias components in the PSU because i figured if that was an issue both tubes would redplate at the same time.

any ideas?

[52 Bill]

Have you checked for cold solder joints on the sockets? Have you checked the tube sockets themselves?

How old is this amp? Fender has had a real problem with tube sockets on a recent run of amps.

[daz]

yeah, in fact i know about Bj socket isssues and i removed the tube board and hit al joints that were remotly suspect and even stripped a bit of trace near each one to give the joint a larger area and improve reliability. But i think It's working now. i can't explain whats going on, but numerous el84's redpalted in both sockets and ones that didn't redplate in one socket would in the other. Yet other tubes would NOT redplate in that socket ! Just so bizzare. In any case i have a couple tubes now that are working and not redplating. so whatever it is, i'm just gonna let it burn for a while and play it and wiggle the tubes around now and again and see if it stays stable.

[Mark Black]

If that is an early revision Blues Jr (possibly confined to Rev-A) check out C34, a 220/25V cap (final filter) in the -15V supply. The polarity markings on the board (and even in the schematic) are exactly backward!

I have seen at least one amp where that cap was installed according to the markings (which actually makes it electrically wrong) and the resulting leakage would drag down the bias supply unpredictably. I was just lucky the cap was puffed up on that one, drawing my attention to the matter.

I think the problem was probably corrected soon after the earliest versions.

[daz]

It's actually a fairly new one, but i checked anyways and the polarity is correct according to the board markings which i assume was changed to reflect the correct polarity. But i can't find anything on the board or anywhere on the amp showing the revision.

[hasserl]

This may not be helpful, but I'll throw it out there anyway just in case. I've run across this a couple of times now with EL84's. One tube develops a short circuit that draws all of the bias voltage away from the other tube, so the other tube redplates. The "bad" tube, the one with the internal short circuit, does not; it looks just fine. First reaction is to assume the tube that is redplating is at fault, when in reality it is the other tube that is at fault.

Check your bias voltage on the tube that is redplating, check it on both sockets, check it with no tubes installed and check it with both tubes installed. The problem you have is you're losing bias voltage somewhere. Why you're losing it you have to determine.

[Enzo]

And one way around that is to test just one tube in there at a time.

[daz]

That scenario sounds very logical to the issue i'm having. And the fact that i now have 2 tubes in there that work bears that it i think. In any case it's stable now and i'm done. And i did check the bias and it was fine once i got tube in there that worked.

[Enzo]

And don't leave the bad tubes lying around "just in case they might work in something else." Throw them away.

[rockon1]

This may not be helpful, but I'll throw it out there anyway just in case. I've run across this a couple of times now with EL84's. One tube develops a short circuit that draws all of the bias voltage away from the other tube, so the other tube redplates. The "bad" tube, the one with the internal short circuit, does not; it looks just fine. First reaction is to assume the tube that is redplating is at fault, when in reality it is the other tube that is at fault.

.

+1 This has happened to me more than once. Might be coincidental but its always been with EL84's. I wasnt sure from the read if both tubes had been replaced or just the alleged "bad" redplating tube so this immediately came to mind. Bob