Quote:
Originally Posted by Bazz4CQJ
I have to admit to not being at all familiar with the Hickok testers, but obviously, they have come up with what looks like an attractive technique. Among the Avo testers, the VCM163 also uses a system which does not need a standard valve, either for servicing or day to day use. The servicing set-up procedure does involve a final verification of the instrument using a standard valve (originally supplied by Avo, but now no longer available), but only after all other adjustments have already been made.
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Yes the advantage of the military Hickok designs (TV-3, TV7 TV 10 etc) is that no actual vacuum tube or "transconductance device" with a said to be "known transconductance" is needed to calibrate them. So the notion of a calibration valve is a redundant concept in the Hickok testing framework, in fact, using one to make any adjustments to the reading given on one of these testers would merely throw in a variable making the tester less accurate and less consistent with its fellow testers. (Which is also what these do on other testers too, if the problem is carefully considered). Needing a calibration valve for any valve tester is like saying a transistor hfe tester needs a "calibration transistor" when it simply requires calibration
signals. Hickok appear to have figured this out, while others have not.
I'm not in entire agreement with the the notion about emission testing being a useful parameter and that transconductance is a factor not significantly affected by emission. One of the main reasons why manufacturers of valve testers chose transconductance as the main parameter to test, is that it falls as the emission falls and the valve ages. In fact, this was the basis behind many early signal control circuits where, to control the stage again, the filament or heater temperature was manipulated. An early Theremin being just one example of transconductance being manipulated by heater temperature.
A good "life test" would be to reduce the heater voltage by 10 or 20% and see if the valve still retained a reasonable transconductance and that would give some very rough indicator of how much practical life the valve might have left. However, as everybody knows, the exact future for any component might only be predicable with a crystal ball, because until it has happened, the future always remains uncertain.
Also, emission testing would likely be more useful for valves in switching and pulse application, TV deflection stages etc, where the peak currents were an important factor. For example reduced emission in a horizontal or vertical scan output valve shows up much more readily than the same valve operating in a Class A audio output stage and the raster scan on the CRT face gives a very good graph of it.