Hi Jim,
.......
A separate and isolated power supply is driving the filament with 6.3V, and 300mA is drawn - which is the correct level. Again, this supply is completely isolated from everything else, e.g. the filament is not connected to the cathode.
I can and should introduce an additional current meter to check grid current. (At one time I looked at grid current for some 6SN7's, and it was as much as 10-20uA, but only when the grid was very close to 0V. I think that's to be expected.) But I don't think it's gas. The time constant for the anode current to stabilize after a cool-down period appears to be *minutes*, and also quite repeatable. It also drops, rather than rises, as it stabilizes. All of that strikes me as being due to thermal effects. But I'll see if the effect is present in other tubes of the same type once I get them. The one tube I've been experimenting with is a National Union military edition (i.e. JAM-CNU-7193). I'm getting a few more of the same, along with an RCA commercial edition 2C22. I'm not sure if there's much difference between the military and commercial construction.
The slight photosensitivity is baffling. I can cause it with a DC incandescent bulb in an old-school flashlight, so I shouldn't be generating any EMI. At a plate current of about 8mA, I get an additional ~40uA with the illumination. It is very repeatable. I'll see if the same happens with other tubes.
Jim
Tubes can get one puzzled, indeed. See for example the two E88CC/6922 i tested would be ideal tubes for an epay-sale when checked only on an emission-tester. I wonder how the heck it got through the Siemens-QC.......
Only the first tube has a defective system (low control-function, too much current), the second is really good.
Given that the 2C22 is a UHF-oscillator-tube it might be "hand-sensitive", have you checked for oscillation
(shorted scope-probe with the ground-wire as antena)?
Filament-supply should be connected to some potential (cathode or a bit positive), Rfc-max is stated in some data-sheets.
Also i meassure currents not directly but over a 1 Ohm (Caddoc MP900) resistor in the supplys minus-lead to ground.
So one can better isolate tube from instrument (and wiring!).
The rise in grid-current near 0V is normal, the grid is slowly becomeing an anode.
Photosensitifity: It's just 0.5%, can you modulate it with a squarewave and see what the maximum frequency is?
Maybe it's a hidden photo-multiplier..... ;-))
Maybe i'll ask around at
https://www.gfgf.org/Forum/ on my Tube-radio-forum where the wise geezers are.
The 15 pieces of E1148 (2C22, VT232, CV6, CV1135) from N.U., GenRad, GE and Marconi are in storrage at the other end of town, my curve-trace-system is in a upgrade-stage (Keithley 237 SMU for grid-supply) and i was building myself a CNC-mill/-Laser. So much to do, but sometimes i'll see if i can replicate your findings.
Best regards