Should I Return this Tube to Seller?

[Username1]

Originally Posted by earlyfilm

Yep!

Simply stated, think of it as the gas allows the control grid to go too positive and this makes the tube conduct too much current and shift the grid cut-off-point.


I was always taught that once a tube gets gassy, it will only get worse.

True then, but not always true now, when one tests an old tube that has not been used for 50 years. The heat from the filament can, over several hours or days running, actually improve emission and reduce gas.

James

Tubes generally do not have current flow at the control grid. It is a control
point of a near perfect infinite impedance amplifier. Typically -7 volts DC bias
at G1 to the cathode. It's always one of the first, and easiest checks to see
if a tube circuit has a problem. Plate, Cathode, and G1, should typically have
a reasonable range to them. I never looked into what the "gas" was, and
what it did. Since 'early film' has a description of how the tube testers check
for a leakage current at the G1, it would seem that as the tube ages some
gas is developed inside the tube and behaves like neon or some other non
inert gas would at voltage. It begins to conduct. So it would allow an
unexplainable shift in G1 bias. You may see it there...

I was also always taught that gas will always get worse, and never get better.

If something is turned off 50 years and it gets better with use, I would assume
it is because the cathode gets waken up and it's emissions get better with
time because it's getting back into the heat - reheat cycle - the environment
it was designed to be operating in....

I would imagine the gas condition to be an out-gas of materials used inside the
tube that time and heat, and high current have produced, and once this occurs
it generally can't be undone....

.