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I have some 6V6GT's that the seller had tested and marked up with mV/a figures.

What do they mean?

What does a higher vs a lower reading mean in relation to anything? Does it affect tone? Power handling?

Please help tubeyonekenubi, you're my only hope.

There are 3 fundamental Vacuum Tube (Valve) constants. These are transconductance (gm), plate resistance (rp) and mu. For tetrode and/or pentode devices, mu is not usually significant, since the plate resistance is usually much higher than the load resistance. There is a simple relationship between these: mu = gm * rp. In a triode, the mu is substantially geometric factor, so it does not change much as the tube ages. Rather, the gm decreases with time and the rp increases.

The measure of the "goodness" of a tube is generally related to its measured Transconductance (gm). This is done in a "transconductance" tube tester.

Transconductance is defined as the incremental change in plate current for an incremental change in grid voltage, with all other parameters (plate voltage, for example) held constant. The way this is done is to place a small AC voltage (lets say 100 mV) on the grid and measure the output AC current A/mA on the plate.

In practice, this is done by measuring the voltage across a small resistor, (lets say 100 ohms) connected from plate to a constant DC voltage source. The current can be controlled by placing a constant current source in the cathode circuit of the tube under test, and bypass the cathode for AC purposes. For the example given (100 mV AC on the grid, and a 100 ohm plate "current sensing" resistor), a transconductance of 1 mS (1000 micro mhos) would be indicated as a 10 mV signal across the 100 ohm resistor.


TL:DR

don't worry too much about them if it sounds ok, the values are only relevant under known testing conditions which are unlikely to be anything like your amp.

sizzlingbadger wrote: don't worry too much about them if it sounds ok, the values are only relevant under known testing conditions which are unlikely to be anything like your amp.

So higher values vs lower values don't mean a lot?



And thanks!!

Basically the gain (gm) of the tubes drops over its lifetime so if the gm is low then it could be an older/worn tube. The thing is all tubes start off with different levels of gm to begin with. This means a part worn tube that started with a high gm may seem to be "just as new" as a new tube that had a lower gm to begin with. The 12AX7 triode for instance has a published gm of 100 but it can vary anywhere from 80 to 120 in practice on a new tube. I guess once it gets down to 70-ish you could argue that its out of spec (either new or used) but that doesn't mean it won't work.

What sizzlingbadger said. mA/V is another way of saying 'transconductance'. Apart from being an individual measure of each tuube's grid-voltage-swing against plate-current-swing, knowing the transconductance of each individual output tuube is handy when figuring out whether pairs of output tuubes will be matched for minimum hum in the (push-pull) output stage of an amp.

Just read this thread - thanks for the great explanations, those numbers make more sense to me now!