Retrotech(NVCF) - not long now

[pamphonica]

I did the testing with my bench PSUs and supplied photos This shows you can do valve testing without a valve tester. In fact this sort of test is the ultimate measure of a valve, it's how you can check the calibration of a valve tester.

The only suggestion re this testing method is that the Delta Vg should be kept small (often 5V will be too much to stay on the linear portion of the curve). Avo uses 1V variation in Vg for this very reason.