6V6G bias current rising and rising... ad infinitum!

[slidincharlie (Carlo P)]

Hello fellow Ampagers.

I have four Westinghouse 6V6G tubes that I tried in my SFDR today.
I bought them as NOS unmatched tubes, but I saw that I can couple them in
various combinations in my amp so that I have two matched pairs at last.
I accomplished the matching procedure simply reading their bias current in
situ.
Then I tried one of them with a single 6V6G Marconi that I bought from
another vendor (Marconi=Westinghouse), and things went odd:
the bias current drawn by the Marconi tube started at around 25 mA after
flipping the standby switch, then grew steadily until about 40 mA in about
30 seconds! Then I got scared and switched the amp off.
Changing slot yielded the same result: the bias current in the Marconi
grows steadily while the Westinghouse decreases by about 2 mA in the same
time.

What can cause such bias rise?
Is the Marconi gone?
Is there any part in the circuit that I should check?
The Marconi is a NOS tube, used only 2 or 3 hours in a SF Vibro Champ.
BTW, the amp seems working quite well, no issues so far.

TIA,
Carlo

[Steve Conner]

The Marconi tube probably draws a little too much grid current, either due to gas, or contamination of the control grid with cathode material. (Given the tube is NOS, and has been lying around unused for years, I'd expect it to be gas.)

The grid current causes the tube to pull its bias voltage down, which makes it draw more plate current and get hotter, which makes it draw even more grid current etc. This process is called thermal runaway. If you left it, it would probably red-plate, and die.

The tube might still work fine in an amp with lower B+. The SFDR is pushing the limit (some would say well OVER the limit) of what 6V6s are designed to handle, so I'd expect it to show trouble with tubes that were even slightly weak.

I once played with an amp running EL34s that would go into thermal runaway when stood on its end, but not in the normal orientation where cooling is better.

[MWJB]

The current drawn by one tube (call it A) will affect the current drawn by it's partner (B). When matching tubes in a fixed bias amp just use one socket to try the potential matches (therefore keeping all operating factors equal, other than the tube you're trying), if you start swapping both sockets it'll drive you mad as a 20mA tube will start showing 25mA etc.

Does the single Marconi sound gone in the Champ?

Do all Marconi 6V6s = Westinghouse (which would therefore = GE, as the last Marconis I bought were made at the Canadian GE factory)? I thought that there was more than one factory making Marconi tubes, but I could be wrong?

[slidincharlie (Carlo P)]

Steve:
thanks for the possible explanation of what's going on with that tube.
For one I won't use it in this amp. It did ok in my Champ (which I haven't got anymore).

Does the single Marconi sound gone in the Champ?

Do all Marconi 6V6s = Westinghouse (which would therefore = GE, as the last Marconis I bought were made at the Canadian GE factory)? I thought that there was more than one factory making Marconi tubes, but I could be wrong?

The single Marconi worked fine in the Champ. I haven't that amp any longer, so I cannot try it again.

Re: 6V6G manufacturer, I don't know anything about it. I simply supposed (because I was told so) that Marconi and Westinghouse 6V6G's were the same tube.