[R-390] Gassy Tube?

[Scott, Barry (Clyde B)]

I have two of the same kind of tubes (6CW5) and I think one of them is gassy.  I can put them in the tester and observe the cathode-to-control-grid voltages.

With the bias set to about 2 volts, when I depress the "test" button (applying HV to the plate), the grid voltage goes up to about 4 volts for one tube, but the other one goes to about 7 volts.  Would gas be responsible for this difference between the two?  If so, which one would be the gassy one?  I *think* it would be the one with the lower voltage, but I'm not sure.

Is this a valid thing to try to check?  I know that "in circuit" is the best test of a tube, but when the problem starts, I'm not sure what's causing what.  Several voltage points begin to go haywire and I'm not able to isolate it.  I do know that if I swap the tubes, the phenomenon seems to "swap positions" so I'm pretty sure I've isolated it to a bad tube, but I'm just curious which one.

I'm going to order two NOS tubes, but I would like to know if the above tests are conclusive of anything.

Thanks,

Barry - N4BUQ