[David Simpson]

Karsten, You've certainly done some excellent & extensive circuitry to protect the meter & give a stable mA/V readings. Anything that helps protect the CT160's delicate, old & expensive meter is always welcome.
How accurate is the Ia readings from the coarse & fine Ia pots when carrying out AVO's recommended "Standardised Valve" calibration test? With AVO's range of VCM's & Testers, for some time now, I've recommended use of a much higher source of DC Standardised Ia from a valve. As the CV455/491's are just small doubled-up triodes. Something like a DC Standardised 6AQ5 or 6AU5 pentode, for example, drawing 40 to 50mA.
With a CT160, its easy enough to work out mA/V by just altering Vg by a fixed small amount either side of the recommended Vg - then reading off the resultant coarse & fine Ia pot readings for zero meter resolution. Simple maths will give you mA/V. If calibration is fine, this result should be jolly close to the dialled mA/V procedure. I.e. Delta Ia by Delta Vg(RV2) should be the same9near enough) as delta Ia by delta vg(RV1) in that recommended Vg region of the mA/V graphed slope.

Regards, David