2. amps-preamps
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Tube glows cherry red...then it doesn't ?

The other morning I warmed up my power amp and sat back with a cup of coffee to listen. A couple minutes later I got up and on the way out of the room noticed that one of the eight GE 6550A power tubes was glowing cherry red. I immediately turned down the bias and check the other tubes. They were all normal.

Over the course of the next twenty minutes the current drifted downwards and I kept rebiasing to keep the current constant at the nominal value. The setting of the bias control went back pretty much to where it had been before this mysterious event occurred.

For the past few days since it has behaved normally.

What gives?
the_smokester
ratsrats
5 posts
 
I would bet you have a pin not making good contact.Go to your local auto supply and purchase a spray can of electric contact cleaner.Spray your tube pins and sockets.Should take care of your problem. Mike
kana813
956 posts
 
I would take the bottom cover off and take a look at the plate resistor and see if looks burnt or is cracked. I'll bet you had a high voltage condition in your neighborhood
and that tube or the associated resistor is weaker than the
other seven.
rdean
7 posts
 
Get rid of it before it takes out a bias circuit or causes more damage. if a new one does the same thing, investigate other problems.
herman
2,259 posts
 
It sounds like the tube lost bias and went wide open. That is very hard on the tube.

I agree that it could be a bad connection between the socket and grid pin of the tube or somewhere further back in the bias circuits. FatParrot's ideas are good. I'm not sure about spraying cleaner in it unless it leaves no residue. There are many electronic cleaners on the market. Some leave a residue and some don't.
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