We are often asked what are the most meaningful test results when it comes to Vacuum Tubes or Valves?
That’s a good question as there seem to be as many different ways to report “results” as there are people selling valves. While it’s common enough practice to indicate the general state of a valve by using terms like “new, near new old stock (near NOS) or new old stock (NOS)” it is certainly not best practice to report valve test results in the same manner. Heres why –
- The first test on any valve is to identify any shorts and significant internal element leakage. We do this prior to testing the quality of the device because if these conditions exist the tube is essentially unserviceable and it is safely disposed of.
- The next test will give us the best indication of the valves quality. Its the measure of Mutual Conductance (Gm) on all tubes other than voltage rectifiers and diodes. In addition a gas test is carried out (When testing rectifiers and diodes there is no such thing as a Gm test).
- All valves (new or used) will exhibit variations in the observed Gm reading for a number of reasons. The brand, type and calibration reliability of the Valve Tester will also have a very significant influence and for this reason it is very important that the measurement is done using a recently calibrated machine. Some Valve Testers will only measure emission – this is a fairly unreliable test, it does not give the complete picture and it can also vary considerably between Testers.
- Many years ago standards were established and documented to determine the acceptable range of measurements for each tube. So if a particular tube has a rated minimum Gm of say 650 and we test it at say 870 that’s good. Its a quantitative measure having factual basis and is not a vague representation such as simply saying the quality of the valve is new, near NOS or NOS.
When we buy a car we care about and usually take note of the mileage it has done – that’s quantitative data because it has been measured and recorded. The same applies to the measurements we make when testing valves.
We use the latest model Amplitrex 1000 Vacuum Tube Tester to check many of our valves, give us assurance of an accurate result and to provide a complete set of quantitative data in electronic and/or printed form.