6L6GC plate glowing red (MC-30's)

Audible Nectar

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I powered up the amps this evening, and observed one of the RCA/McIntosh labeled 6L6's with a red plate glowing on one "quarter" of the plate after a short time. This is in contrast to the typical glow at the top and bottom of the 6L6. This seems unique, as I have not observed this before. It did not look healthy, so I powered down the system. Is this the sign of a failing 6L6?
 
I suggest you get ahold of Terry. :( Glowing plates is bias voltage type thing.

Carl
 
The exact thing happened to me in one of my MC30s a few months back, the plates in one of my GE 6L6GCs started glowing red. After a few days of tube rolling (and sweating bullets) I discovered it was a flaky tube (I believe it was one of the 12AX7s). I replaced the flaky tube and all is well.
 
A Fred Flintstone style "uhhhhhhh, boyeeeeee" goes up here.

I was hopeful that swapping in a different 6L6 to replace the glowing one would solve the issue (and I do have one on the way). Terry thought that it could be a bias resistor showing a problem in the amp, but he suggested that I swap the two 6L6's within that one amp and see what happens. After one hour, all was normal. Terry then thought that 6L6 could be going bad, so he has one on the way to me to assist in troubleshooting this.

Then I read efgjr's post.

Question/problem: How did you discover the flaky tube? Is this something that would show up on a tester? My tubes appear to be "normal", with exception to that one 10 minute glow episode on that one 6L6, so I am wondering how to go about troubleshooting this?

I do not have a tube tester, but could get access to someone who could test these. Would that help?
 
Question/problem: How did you discover the flaky tube?
Trial, error, and a lot of tube rolling. Simple as that.

I did roll 6L6s, and do recall that a Svetlana I put in place of the GE whose plate was glowing did the same thing, which led me to start rolling the other tubes.

I don't have a tube tester either, so it was sheer luck that I got the problem solved. I posted about the problem here, and Terry offered me quite a bit of very good advice, sent me a schematic, and told me about checking voltages (thanks, Terry!)
 
I am suspicious of tube testers now, even good ones. See one of my previous posts on my MR67 and its Teles 6U8 tube.

Tested 100% perfect on 2 different testing machines, but in reality, did not work right. Put in a new tube, and tickity boo, problem solved.
 
Well.........

Terry sent me a replacement 6L6, and after 2 sessions with 6 hours combined playtime, no issues whatsoever. Same old yummy sound that these have always delivered. I am watching them closely, and never leave the room when playing, just in case, but so far, all seems well at this point.

Terry suggested that it could have been a weak small tube or mismatched triode sections in that last 12AX7, or a poor connection in socket on that tube - so I reseated those.

Not complaining by any means, but I've always been less than comfortable with a fix where the ultimate cause is not determined.

This reinforces the fact that I need a tester, so I could rule out or include some of the above theories, at least where tube strength and section matching are concerned. I find this aspect of tube ownership to be the most frustrating.....no way to verify anything regarding purchased tubes - as I don't want to test tubes "in circuit" - and for troubleshooting in situations like these.

I have a LOT to learn.


Number 9:

Do I EVER hear you regarding those 6U8's. I've had 6 of these, supposedly tested, and 3 didn't cut the mustard. One worked good in my MX-110 for a week, then went bad, as I began getting flabby, distorted, loose sound from one channel. Thought it was an amp, but when reversing the channels, I discovered it was one of the 6U8's in the pre.

I am told that this is not unusual where the 6U8 is concerned. This is where a reputable, satisfaction guaranteed tube dealer would come in handy.
 
Originally posted by Audible Nectar
This is where a reputable, satisfaction guaranteed tube dealer would come in handy.

Well call me lucky, but I do have a good tube dealer here in town that is only a 10 minute drive away (its like going to a candy store when I see his stash).

Anyways, he tested my 6U8 and he was the one who told me not to necessarily trust the tube tester results. The issue, is that the testers test the tube cold, and they run hot in the circuit. I don't really understand what that means, but that is why he said that the tester may produce inaccurate results.

He gave me a Philips 6U8 and said I should give it a try, and I could bring it back if I got the same results. I hope to save up for one of those France Amperex Globe 6U8s at one point.
 
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A tube going redplate is a serious problem. It usually means a bad capacitor or bias circuit in the amp and that's not something to be taken lightly. Even if this goes away when you change tubes, unless you can determine definitely that the 12AX7 CF that biases the 6L6es was bad and caused the symptoms the amp should be looked at by a tech. Output tube runaway can cause major expensive damage.
Sorry to bring that bad news, but overcurrent conditions are the fast track to serious amp problems, so I'd have it checked out.
 
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