Hi Mike, these are the results I obtained a little while ago when testing a batch of 6CA7's. All four were very lively compared to an EL34 and this seems to be evident in use, one has to apply a bit more negative bias than usual. But the graphs show a pretty good test at high Anode current even though the diode is in place, maybe it has gone open circuit
.
Looking at the graph though, something does not look right at maximum current and something I failed to see in the first place.
I am wondering if the valves tested behaved themselves at the AVO test settings but then went astray with more current, so maybe the diode was playing foul after all?
The four valves tested gave the attached results when tested with the AVO settings. These were taken as a spot measurement whereas the graphs were taken in steps of bias voltage over a period of time.
I am going to look into the viability of fitting some kind of transformer in the Anode circuit and see how that goes.