[Dekatron]

Thank you David (Radio Wrangler) for bringing up the repeatability as that is an issue with many valve testers that people often forget.

Even with computerized valve testers there is a problem with repeatability in the tests, component temperature affects the measurement circuits and even though the voltages and currents are measured and controlled with precision A/D & D/A ic's the components in the measurement circuits vary due to the temperature and that results in measurement errors. Modern computerized valve testers also needs warming up to get stable readings, and more so to get repeatable readings. Don't confuse this with the time you need to heat up some valves properly as that is something else, but it sure helps some testers if you heat up the valve in the tester as that also heats up some or all of the measurement circuits too.

One more very important thing is to make sure that the valve you test have clean pins and that the socket you put it in also is clean as you will otherwise get somewhat different readings every time you test a valve, even with the same valve. High heater currents and low grid currents are affected by dirty pins and sockets more than people realise and that spoils repeatability and measurements in general. High contact resistance (or poor contact) is not only a problem by the voltage drop it incurs but also by its changing nature, it usually varies with the current flowing through it.

So using an old grubby valve tester or a new one with grubby sockets or grubby valves will not result in good, and neither repeatable, readings.

/Martin