Quote:
Originally Posted by David Simpson
Please peterp1, any chance on some info on the circuitry in which your KT66's are being used? A Circuit Diagram would be great.
Also, perhaps you could ask your local electronics man about which Valve Tester he used, & obtain info on the Anode Currents (Ia) achieved, Anode & Screen Voltages used, and the Grid Voltages used ? Or the Valve Data Book he used ?
For example, if he used the AVO Valve Data Manual(which quite a few of us use), Va = 250V, Vs = 250V, Vg = -15V, and expected Anode Current (Ia) is 65mA. Gm = mA/V & should be near enough 6.3 mA/V. Assuming the valve is in good condition. Some Valve Testers(or Tube Testers) just give a coloured meter display. I.e. RED zone = Fail, WHITE zone(middle of the meter) = maybe just usable, and GREEN zone = Pass.
Regards, David
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That's roughly what I was going to say, the gm is normally quoted at the chosen point on the valves characteristic curve.
e.g. AVO use an Anode Current of 65mA for the KT66 and 57mA for the KT88.
For the KT66 they chose -15V to give that 65mA and expect a gm of 6.3.
The meter on an AVO tester will have a Red Fail zone for a gm of 4 or less.
The Green Pass zone is for a gm of 5.6 to 10.4.
So another tester type may choose a different Anode Current point on the graphs and the gm may be subsequently different.
I use my valve tester to do a plot with various grid voltages and I consider it to be a most excellent way of matching valves in addition to verifying their health
Les-G4CNH