6CA7 Current Draw Increases

[Daver]

I'm testing a pair of Sylvania 6CA7's for a friend of mine. On my Eico 667 they both test fine. When I put them in an amp and set the bias one of them slowly, like over 5 minutes, starts to increase in current draw. It will go from say 30mA to 60mA at idle. The problem follows the tube not the amp or socket. I realize the tube is bad but I'm curious as to what causes that to happen. Thanks.

Dave

[Jazz P Bass]

I've seen that before.

Not too sure what is going on in the tube but that is one of the items on my check list when testing a tube in circuit.
If the idle current creeps up like that I consider the tube junk.

[bsco]

I'm testing a pair of Sylvania 6CA7's for a friend of mine. On my Eico 667 they both test fine. When I put them in an amp and set the bias one of them slowly, like over 5 minutes, starts to increase in current draw. It will go from say 30mA to 60mA at idle. The problem follows the tube not the amp or socket. I realize the tube is bad but I'm curious as to what causes that to happen. Thanks.

Dave

Actually, so am I....I am curious as well....that is a big difference...

[Tom Phillips]

Actually, so am I....I am curious as well....that is a big difference...

There is a discussion in the deep archives on the subject at Thermal Runaway - AMPAGE Forum

[km6xz]

Gas increases with heat and time, and increases plate current in fixed bias circuits. Does your tester have a Gas function?

[Daver]

Thanks, Tom, that does explain a lot. Maybe not how this particular tube is affected, but a good discussion all around.

km6xz, my Eico doesn't have a gas test. I have an old Mercury drug store style tester that does. The tube doesn't test positive for gas on it, but I'm not sure how accurate that one is.

Dave