[retailer]

After 15 yrs of sitting on the shelf I've just got my MKI working - I've been running a few valves through to gauge how it is working and some of these included some 2A3's that I have - I tried various grid volt settings from 35 up to 50 - going lower than 45 will increase the anode current which may possibly go over 60mA, 2A3 data gives 60mA as typical operating figure, the absolute maximum is probably higher, as long as you don't linger too long with the anode current much over 60 then you should be able to get a Gm reading. I tried up to 70mA and down to 50mA and while the Gm did vary with anode current it was not a huge difference. One of them was slow coming up to anode current and increasing it much over 50mA reduced the Gm I suspect that valve was well used. I would try it at 40volts and keep an eye on the anode current.

The MkI and MKII have grid volts up to 100 - not sure about the MKIII and MKIV, I think that the CT160 was intended for the military and can only think that the military did not have a use for valves that required a bias voltage higher than 40volts