Many questions about tube testing and matching have been asked. DIYHFS uses an Amplitron tube tester. This machine offers a number of advantages over traditional tube testers.

It is fully programmable which means any filament voltage, B+ and bias voltage can be dialed in by the accompanying software.

Rather than just a good or bad reading or a single point Gm it outputs a multiple operating point curve trace which reveals problems that may be missed by other tube testers. For example the first of an 11 point curve trace for a 300B tube could start with an applied bias of -20V and a B+ of 75V. The B+ is continuously increased up to 225V producing a curve showing the current flow at any voltage between 75 and 225V. This is repeated at 11 different bias V from -20 to -150V with each incremental increase in bias voltage accompanied by an incremental increase in B+ until the last trace which would be operating at -100V and 350 to 450V B+. A curve trace looks like this (taken with Sofia but similar with Amplitron):-

This method of testing reveals many anomalies not shown by ordinary testers. Those testers may match two tubes with the same matching Gm based on single point testing but an Amplitron trace curve reveals major differences at other operating points proving that the tubes are not really a good match overall. Also, a tube may test with a 'good' Gm but a curve trace of that tube may produce an output that is 'S' shaped, meaning that at higher B+ voltages the tube isn't passing proportionately higher levels of plate current. It can be expected that in operation that tube will perform in a distorted non-linear fashion. Sometimes even new tubes show this condition.

For example the second curve trace shows two tubes which match at a nominal B+ and bias voltage around 50ma. The curve trace shows some non-linearities at other voltages not revealed by single-point matching.