What would cause tubes to start glowing?

[SSSnail]

Not normal glowing, but spot glowing and it gets hot that it darkens the glass.

This happens to my amp, I didn't do anything to it, was just listening to some music then all of a sudden I heard some pops and cracks. I looked over and seeing two of the tubes glowing bright, much brighter than normal. A closer inspection shows that there are burn spots on the inner glass of the tubes.

Don't ask me what those tubes do, because I have no idea and they're the red ones in my awesome illustration. See attached for the configuration of my amp, and please give me your opinions. Much appreciated.

 

[ericlp]

don't know much about tubes. But sounds like a short some place. Do you dust often? Maybe a dirty amp! I'm sure someone that owns tubes could help you out more.

 

[pcgeek11]

Not normal glowing, but spot glowing and it gets hot that it darkens the glass.

This happens to my amp, I didn't do anything to it, was just listening to some music then all of a sudden I heard some pops and cracks. I looked over and seeing two of the tubes glowing bright, much brighter than normal. A closer inspection shows that there are burn spots on the inner glass of the tubes.

Don't ask me what those tubes do, because I have no idea and they're the red ones in my awesome illustration. See attached for the configuration of my amp, and please give me your opinions. Much appreciated.

They are supposed to glow. The filiment heating is what allows the electrons to flow across the huge gap between the anode and cathode.

Try this open the amplifier and then thump each tube firmly if the filiment glow goes out or goes off and on several times; then you need to replace that tube.

It sounds like a filiment with a high resistance ( this would be causing it to generate too much heat ). Or you could locate a tube tester and find the defective tube that way. I used the thump method years ago. ( OK I'm Old ). Not real accurate but it will locate most of the troublesome tubes.

Those tubes do the same thing as the transistors do in a modern amplifier. They just do a better / cleaner job of it.

 

[Rubycon]

Is this the normal orange-yellow glow of the filament heaters or is it a purple (coronal like) glow?

Have the tubes been physically jarred in any way?

 

[RPB]

Tubes will often lose some of the vacuum over time causing them to over heat and fail.

 

[SSSnail]

Sorry guys, I'll see if I can take some pics, too lazy.

To answer some more specific question, the glow is a much brighter than normal orange, so much that the inner glass near the glow turns black. But, it turns on and despite the glowing, still work fine. I guess I'll just wait until they burn and replace them.

 

[Deleted member 4644]

If you go to head-fi forums they will know the answer. Tons of tube users over there.

 

[KGB]

What amplifier is this? Make/model?

From your description, it sounds like you have a few tubes which are 'red-plating'.
This is caused by excessive current draw which may be the result of many things (incorrect voltages, incorrect bias, bad resistors or caps).

 

[KGB]

Also, your 'E62' and '417' tube designations are incorrect.

AND, one of those transformers is most probably an inductor (choke).

 

[cheesehead]

It sounds like you've got an air leak. You likely need new tubes.

It could just be a biasing problem, though I doubt it.

 

[AlienCraft]

What amplifier is this? Make/model?

From your description, it sounds like you have a few tubes which are 'red-plating'.
This is caused by excessive current draw which may be the result of many things (incorrect voltages, incorrect bias, bad resistors or caps).

The last time I was poking around in an amp for this it turned out the bias resistor had degraded due to heat and was on it's way out of the building.
We replaced with some higher wattage metal film resistors with good results,iirc.

 

[KGB]

It sounds like you've got an air leak. You likely need new tubes.

It could just be a biasing problem, though I doubt it.

This is incorrect. If there was an air leak the tube would've burned up immediately like an incandescent filament.

If the OP continues using this with the "red plate" condition, it will likely fry the tubes.

To the OP: check your plate and screen voltages and the bias resistors.

 

[cheesehead]

This is incorrect. If there was an air leak the tube would've burned up immediately like an incandescent filament.

If the OP continues using this with the "red plate" condition, it will likely fry the tubes.

To the OP: check your plate and screen voltages and the bias resistors.

Not necessarily. I'm hardly a tube guru, but I've seen several tubes that started to glow shortly before failing. The air leak in question is generally very slow, but it doesn't take much.

That said, I misread the original thread - an air leak would not cause "spot glowing." That sounds like an overvoltage somewhere.

Is there any way for you to measure the bias current?

 

[bobdole369]

purple (coronal like) glow?

They call that the ole purple crystal.

Spot glowing (looks like a cigarette) and marks on the tube indicate that its overdriven, or depending on the amp config, out of balance.

In the RF world, where I have all my tube knowledge, you need to optimize the load for the signal.

Is there anything that could have happened at the load? Such as a short or maybe dust or moisture did something inside?

Tubes generally don't just die.