humbled by the vote of confidence Ring..
truth is, those stickers are the wrong ones to use for 12AX7s since Gm pertains to pentodes and not triodes (12AX7s are twin triodes hence the dual set of values) whose gain is specified in
Amplification Factor or u (MU). this value specifies how many volts the output will change as a function of the variance of the input voltage (dV/dV). in a 12AX7 triode it’s usually about 100 meaning a 1 volt fluctuation at the input will result in a 100 volt swing at the output.
Transconductance (Gm) on the other hand is used in amplifying tubes (usually power) with 4 or 5 (instead of 3 as in triodes) electrodes denoting a swing in
current (instead of voltage) at the output as a result of an input voltage fluctuation (dI/dV). the folks at TC may have run out of stickers, who knows….
iP just means the plate current in each tube when the gain measurements were made by the tester as
u and
Gm are very dependent on this current (mA in this case) value.
this said, I would start with one of the mullards at the phase splitter location of the tube amp (since they are identical, hence well balanced for a splitter) and the Amperex at the gain stage. don’t really know the topology of the class D amp but these placement choices aren’t as critical in them since they don’t operate in a push-pull configuration…..
hope this is not too verbose, PM (or call) me if you want….alternatively, you can come over and we can verify those values with my Orange tester and do some tube rolling…
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