JJ glowing twice as bright

[Rockmore]

Read quite a few threads of over bright red (orange) plate glow. Can't quite know what to make of this.

Have 4 JJ EL84's on my Yaqin MC-84L. Bought new and matched very well with about 200 hours on them now. Amp on about 4 hours a day with no short on/off's.

All of a sudden with amp on for about 15 minutes, front left tube is glowing twice as bright. Jumped up and shut it down, let it cool for about five minutes and switched two front tubes. Everything normal with an hour of use so far. No audio distortion or anything else. Clean white tube socket scorched a little bit. Amp has no manual bias.

[ChrisA]

Originally Posted by Rockmore

Read quite a few threads of over bright red (orange) plate glow. Can't quite know what to make of this....

Are you saying that in normal use the plates glow orange? That certainly would explain why one failed after only a 200 hours. I surprised they lasted that long.

[Rockmore]

Pardon? Nothing failed (yet). As stated, the front left tube all of a sudden is glowing 2 to 3 times brighter (plate is all bright orange). Again, shut it down quick and let them cool and switched front two.

Everything normal with an hour of use so far. No audio distortion or anything else. Clean white tube socket scorched a little bit. Amp has no manual bias.

[DF96]

Are you saying that the anode was glowing brighter than normal? EL84 anodes should not glow at all. Only the heater and cathode should glow, and most of that is hidden.

[Alastair E]

Originally Posted by Rockmore

Pardon? Nothing failed (yet). As stated, the front left tube all of a sudden is glowing 2 to 3 times brighter (plate is all bright orange). Again, shut it down quick and let them cool and switched front two.

Everything normal with an hour of use so far. No audio distortion or anything else. Clean white tube socket scorched a little bit. Amp has no manual bias.

Hmmm....

There should be NO Plate Glow At All.....
(Although a VERY SLIGHT redness could be seen in a fully dark room on the plate--the outside grey/black bit-- of a tube running at its absolute limit....)

ONLY the Heaters and Cathodes, (The bit in the middle...) should glow orange....

The Plates of an EL84--no matter What Make-- should Never heat to an Orange-colour, unless there's some severe overload or faults!

[Sheldon]

I think this amp has a shared cathode resistor for the output stage. If one tube loses its cathode connection (poor pin connection?), the other tube will conduct much more to make up the loss.

BTW, it's considerate to post a schematic if you are asking for free help.

Sheldon

[Rockmore]

Here's the schematic: Transistor phase splitter (Yaqin MC-84L)


I realize there should be no plate glow...... Never had a problem or this happen before.

Have not been able to duplicate this problem after switching the two front's. The obvious problem (to start) was possible "poor pin connection" even though the tubes and sockets on the amp itself are all fairly new.

Let me repeat again that the entire plate in the said left front tube was 2 to 3 times brighter than normal and glowing bright orange. If I hadn't caught it quickly, I'm sure it would of burned.

[Sheldon]

I mentioned pin connection, because the problem was at least temporarily resolved by taking the tubes in and out. You might get some wire the same diameter as the tube pins, and check each connection for tightness. Another possibility along the same lines, is a loose connection at the grid pin. In that case the grid can drift positive and cause the tube to conduct more.

Were you listening at the time. If so, did you hear any change?

Sheldon

[Rundmaus]

Originally Posted by Rockmore

Let me repeat again that the entire plate in the said left front tube was 2 to 3 times brighter than normal

Hey there,

let me repeat again that there is no "2 times brighter than normal" if we're talking about EL84 plates. The plate should *not* glow at all, not red, not orange, simply *no* light.

As long as you're talking about "brighter than normal" you imply that it has always glowed in what you consider a "normal" operating mode. This surely isn't a normal operating mode for an EL84.

Greetings,
Andreas

[ChrisA]

Originally Posted by Rundmaus

Hey there,

let me repeat again that there is no "2 times brighter than normal" if we're talking about EL84 plates. The plate should *not* glow at all, not red, not orange, simply *no* light.

As long as you're talking about "brighter than normal" you imply that it has always glowed in what you consider a "normal" operating mode. This surely isn't a normal operating mode for an EL84.

Yes. I could not understand the "2 times brighter than normal" either because "normal" is dead-on black. Only the heaters should glow.

If these plates had even a HINT of "glow" it means something is wrong and that alone could explain the root cause of the problem.

It's like if I said, "I was driving to work and noticed the flames and smoke coming out the engine was twice as much as normal."