Re: The "Sussex" Homebrew Valve Tester.
Hi Les
Dont worry it looks like it is a random thing where this seemingly innocent diode upsets things. The scope traces that I have done since cutting the diode all seem similar to as they were before. So they are possibly fairly normal.
It seems that there is a 50Hz pulse that builds up at the point of maximum conductance and lower bias, at this point the trace goes haywire, but the valve is still conducting and the GM is still OK so long as you dont wind the bias down too much further.
It would still be interesting to me if I could compare the trace of my anode, screen and grid supply with some others at the correct grid setting, slightly more negative and also slightly negative.
What happens during a "flash over", surely it can only be high voltage DC that can appear at this point.?
If the meter is protected from DC by an on board capacitor it may only be a few 10's of volts and if high voltage DC is suddenly applied to the meter this in itself may flash over delivering HT DC to the meter electronics.
Would a 630v .1uf cap block these higher voltages safely and be sufficiently "invisible" to the AC voltages.
Mike
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