Calibrating Valve Testers

The comments made on this page refer specifically to the AVO VCM163, but some aspects will be applicable to other machines.

Most electronic instruments are calibrated against some form of a reliable standard. To this end, AVO used to sell a “standardised” valve (an E55L) to use to assess the performance of their testers. AVO no longer support their vintage valve testers, but other business’s do offer various valves for such use.

The E55L valve is now quite hard to find, and it was also a quite a strange choice for use with the VCM163 because it has a gm which is appreciably higher than most of the valves that are in common use. We have spent  some time, over the years, looking at different valves trying identify any which make a particularly good standard valve. In the case of the VCM163, we feel that a good choice would is;

  1. one which has a gm which is just less than 6.0mA/V – this is the FSD of the gm meter on the lowest range
  2. one which shows an Ia value of 10mA – this is the FSD of the Ia meter on the second lowest range.

Such a valve will not only give a good assessment of those ranges, but will provide sufficient meter deflection on the higher ranges to give a good indication of accuracy.  There is one quite common valve which offers this combination of characteristics: the ubiquitous ECC81.


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