red plate 6L6 tube - next steps?

[tweedydum]

I haven't had a chance to double check on this yet -
but last night my allen encore was making a crackling sound at the loud range of my volume pedal. For some reason I whacked it on the top, and crackle heard again. For some reason I looked behind, and noticed the 6L6 furthest from the rectifier tube was red-plating.

Now, I intend to go back and check the bias and open chassis up and see if anything looks awry.

This amp has workded for probably 20+ hours with no problems.

P.S. I suspect the crackling I mentioned may have been loose reverb pan connection.

Any thoughts what I should do next?

thanks,
Dave

[Alexo]

Before you even crack it open, swap the redplating 6L6 with the good one, power it up and see if the issue follows the tube. If so, you've got a bad tube and you haven't wasted any time turning screws.

If it follows the socket, check the bias setting, check the bias voltage, the screen voltage, etc., check the solder joints on the plate, screen and cathode pins. Etc etc.

[tweedydum]

thanks -
I tried swapping tube and I can't recreate red plateing - not even in same socket. Once I got some sizzle coinciding with notes I played, but reseating the tube seemed to do it. Wiggling the tube in socket seems to casue a bit of crackle, and once chopsticking some wires on the socket caused some crackle - and seeminlgly one wire in particular - but even this seems to have gone away with tube re-seating.

I guess I'm intending to leave it alone for now - or maybe reflow some of the conncetions on the socket.

Any suggestions?

Dave

[TheGimp]

sounds like a bad connection at the tube socket. Moving wires can change the stress on socket pins causing them to shift which will cause the pint o wipe against the mating socket and clean it. Wiggling the tube wipes the pin against the mating socket cleaning it.

I'd watch closely for it to occur again and if it does I'd retention the socket pins. If it occurs again after that I'd replace the socket.

[Structo]

Reflow those joints on the socket.

There could have been a bad joint that caused a poor connection, which in turn caused the tube to redplate.

Was this a kit?

[tweedydum]

thanks, I will try that too.
yes, I should have mentioned this is a kit.
Dave

[tweedydum]

ok, after resoldering tube and testing in the basement, all seemed to be fine. take it to gig and crackle, then fuse blow on first song. I can only assume it was redplating again, but don't know for certain.

Is it foolish to hope that maybe one of the power tubes had become bad? Aside from re-checking the socket, I'm not sure where to proceed.

I'm tempted to open it back up, see if it looks like the recent fuse blow took anything with it, pop in new 6L6s and pretend like nothing happened. What SHOULD I do?

thanks,
Dave

[Firestorm]

Did you replace the tube? Or just swap the two of them? Usually a shorted tube stays shorted, but I suppose it could be intermittant.

Take a look at the blown fuse. Sometimes a small overcurrent condition will just melt the fuse open; a good solid short will often vaporize part of fuse and coat the inside of the fuse glass with something that looks like a getter flash.

You could try replacing the output tubes, but watch them carefully for 10 or 15 minutes to be sure there's no redplating; you should probably be putting signal through the amp, too, to replicate actual playing conditions.

If a tube redplates, it's likely a bias failure: bias pot or coupling cap to the output tube in question.

[iknowjohnny]

Also you might want to do what i do when i suspect a socket issue. This is a good tip generally whether or not it's your issue. I always tighten them, but i also clean them by taking an old tube and nipping away at the pins with a wire cutter to rough them up. Then i put it in the socket and ram it in and out. (no jokes please:)) It acts as a file and cleans the oxidation and whatever contamination there is out of there. I also shoot some deoxit on the pins first. Then tighten them and the sockets can then at least be removed from suspicion.

[tweedydum]

thanks all,
I have done what everyone has suggested. In the end, I can not attribute it to anything except a bad tube. I'm now running - about 3 hours on a new pair. Hopefully this will not recur.

thanks,
Dave

[Cliff Schecht]

Yup sounds like it was a bad tube and I bet reflowing the joints on the sockets helped as well.