Tube tester

Is a capable tube tester, yet its use scenarios can be multiplied with only modest efforts invested in understanding its schematic, regimes, and ways to add new regimes. Eico 666 is capable tester, known to be able to supply voltages and currents adequate for most tubes. It performs emission/conducance tests as well as leakage tests. The
Tube tester - schematic

short summary, see [1] says: The manual [2] explains the regimes and elaborates on various procedures. Dynamic conductance is the "merit" test for which the simplified schemaic shown on the left is supplied. It is not clear how the volatges Fi1-Fi3 form, although it is apparent, after careful reading, that "Fi" stands for AC volatge. Is grid voltage in-phase with the plate voltage Not clear, yet the fact that tester seem to measure tube parameters in AC domain, where the AC is taken directly from wall transformer, is interesting enough. Note that the grid indeed receives positive (in-sync) AC signal. It would be pretty trivial to flip the polarity and let it test tubes such as 6bq5 with negative (out-of-phase) grid AC. This thought triggered my interest; I became curiuus about how to a DC mode to my eico666 without changing its schemaic too radically, and how to better utilize its AC regimes. The former seems obvious and hence desirable, the latter appeared odd but led to very interesting possibilities. When triode plate sees a fixed voltage (aka "DC voltage", term which is standard but a misnomer by itself) its plate current can respond to changes in grid voltage and the slope of the change is called transconductance. You may think that nothing like that can be seen in AC-plate AC-grid regime; but what about AC-plate, fixed-grid regime Plate voltage swings across negative, unusable range, when the tube is closed, then...

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