[Lucien Nunes]

In the real world, 'goodness' could be a measure of either emission (anode current) or mutual conductance, or a function of both according to the particular demands made upon the valve in-circuit. Any given valve tester would show goodness typically as a ratio of one or other of these parameters relative to the figure for a new valve. With a basic tester the assumption is that if the valve is faulty, it will perform poorly whatever test is applied, so the tester only has to test one of these parameters. The need to make such an assumption stems from the high cost of building a comprehensive valve tester.

Measuring anode current at reduced voltage is easy and was the basis for lots of low-cost testers. However they would not generally measure the current under representative conditions, so could not be used for quantitative analysis of the valve performance, only comparison with published figures for a new valve. An arbitrary scale could be devised where a valve was considered OK if its emission was (say) at least 60% of the published nominal, although how this compared to that particular valve's new performance would be an unknown, as would the mutual conductance.

Measuring mutual conductance was a bit harder but not excessively so if one was again prepared to compromise and operate the valve under non-standard conditions. This is the basis of the original AVO valve tester, which uses zero-bias conditions to simplify the circuit. It does not display a useful figure for anode current but concentrates on measuring Gm. After zeroing the meter, you can read off the actual Gm in mA/V while the Set mA/V control remains at its initial 100 position, or by dialling-in the expected value on this control get a comparison with the valve under test using the red / green scale. The measurement is the same in each case, only the method of displaying it differs.

Measuring both mutual conductance and anode current under normal working conditions is the province of the characteristics meter, which can give a numerical answer to both tests. As an example, consider the case of the AVO VCM; when you have tested for sufficient emission, by comparing the meter reading with the published figure and drawing your own conclusion, you null out the standing current and switch to Gm. You can leave the dial set to the expected value and see where the pointer lands on the meter scale's coloured sectors (the goodness scale ranks +/- 33% as good and <50% as bad) or turn the knob until the pointer stands on the calibration mark and read the result off the dial.

So this is the crux of the coloured scale on a tester that measures Gm like the AVO - it is a ratio of the expected Gm that you have dialled in and that of the valve under test, by which the valve is ranked for goodness without specific regard to its emission performance.

Lucien