LobalWarming
1000+ Head-Fier
Should be.... in theory... and in theory there's no difference between theory and practice.Taking a popular RCA 6AS7G as an example, it has a transconductance of 7000 µmhos, plate resistance of 280 ohm and amplification factor 2. Output impedance will then roughly be 1/0.007 + 280/(1+2) = 236 ohm. You will also see calculations without the plate resistance which brings it down to 143 ohm, but I believe that’s the case for grid-loaded topologies and I don’t think the H5 is. So 236 ohm it should be I think.
Without a curve tracer to plot the exact gm and other parameters of a specific tube, the initial 'datasheet' inputs for the calculation will be... ummm... suspect. However, we can be assured 99% of the time measured values will be worse. Probably only by 5-10% on a good tube, but still...
And for the great unknown... feedback is used in the output stage in most OTLs which reduces output impedance dramatically. I can't recall atm what W told me about the output impedance value of the H5 with a 6AS7, but around 100 ohms wouldn't be far off. I'll ask him again, once he's answered my latest round of 20 inane questions.