Marshall JCM 900 Red Plating...

[Lee W]

So I got some bias probes from Eurotubes and decided to try them out on my JCM 900, 4100 Dual Reverb. It's got tubes that it had when I bought it used in July or so, they are Ruby 6L6GC tubes. The guy I bought it from changed them but never biased them.

From what I read they should be 36-42mA. I measured at 19mA...so I definitely decided to get in there and change it. Anyways, I biased it to 38mA at 460V plate voltage...so 70% of max current.

I had bias probes on two of the four tubes, and they agreed to under halve a milliamp. However, one of the tubes that didn't have a probe on it got a glow like this:

[ATTACH]4167[/ATTACH]

So what's the deal here? Danger mode, or is it alright? The angle that I can view this from is very hidden when the amp is in its head case, so I can't say for certain if it was happening before. I didn't have a probe on this tube so I don't know what the current is running into it. I put the amp all back together afterwards and played it for about 15 minutes, and it continued to glow like this (now that I know where to look I could find the right angle to see it happening). However, the outside tube beside it also started glowing orange, but not quite as brightly. This one did have a probe on it so I know this one had a 38mA current.

Is this no good? Should I reduce current? Sign of old tubes or something in the amp wacky?

[Enzo]

That is a bad tube most likely. Move it to another socket. If it gets red hot like that in a new socket home, then it is definitely bad. If some other tube now gets red hot in the socket that one is in now, then the socket has isssues, like maybe an open resistor on pin 5.

That is what we call "red plating."

[Lee W]

That is a bad tube most likely. Move it to another socket. If it gets red hot like that in a new socket home, then it is definitely bad. If some other tube now gets red hot in the socket that one is in now, then the socket has isssues, like maybe an open resistor on pin 5.

That is what we call "red plating."

Thanks so much!

Very simple tube swap test that I see all the time recommended on here.

Part of me hopes it goes with the tube so that new tubes will fix it...but then again a resistor is dirt cheap to repair (if indeed that is all it is). Would it be advisable to not play the amp any more until the problem is fixed? Or if the tube goes would it just take out a fuse and that's all?

I really need to spend some time learning what each of the pins typically do on certain tubes so that I can troubleshoot these problems better.

Oh I have another question too. It is ok to power up an amp without any power tubes in it (ie. to test possible problems for blowing fuses). Then you can power down and put in a single tube, power up and check, repeat with a second tube, etc. This doesn't make any tubes act up? Because I've had tubes freak out on me before when they aren't making a great connection, but is this what happens when only a couple pins are in connection?

[Enzo]

Absolutely do not run the amp this way. That tube will probably never blow a fuse until it melts and shorts out completely. ANd when it does that is can burn resistors, kill bias to other tubes causing them to burn up, conceivably damage the transformer.

A tube manual of the data sheet for any tube will tell you what each pin is connected to inside.

Of course you can power up an amp without tubes in it. And you can fire it up with only some installed, or even just one missing. Obviously it won;t function right that way, but it is common for testing purposes.

[Lee W]

Thanks again Enzo.

I moved stuff around and it indeed is the tube that was shot, the working tube I moved into that socket was fine. Visually you can tell it was done when I pulled it out of the amp anyways (red logo turned brown, some black marks along top of tube as well).

I'll order up a new set and all should be well.

[gcb34]

What if the it stayed with the socket and not the tube? That is the boat I am in now.

[Chuck H]

What you are doing is called "hijacking a thread". It has all the charm of someone introducing themselves as a doctor and you fire right back with something like "Does this look like herpes to you?" It's best to start your own thread for your problem.

That said... Did you read the entire post? The subject of possible causes if the problem remains with the socket is in there. It's also been covered ad nauseum on this forum and a little time searching could be very helpful for you.

So, welcome to the forum gcb34. Standard etiquette is to search the forum and it's archives first, then start a thread if you still need info or explaination. Good reasons to post on someone elses thread would be to add helpful information, offer subject related humor, dispute percieved misinformation and basically just participate in the subject at hand.

Chuck