After some conflab with others the caps are probably metalised polypropylene. Even better.
A nearby friend in audio DIY crime did some testing for me on Lagoon. This is a 4S 12AU7 single stage tube preamp. The results are good.
Distortion is quite low at 0.05% with the second harmonic predominant as expected. There was 50hz and 100hz hum there but low down and not audible in practice.
Frequency response was from less then 30hz to over 20K which is all the test gear goes to. BUT this was into a crippling 15K resistive load.
This leads me to question that these single stage preamps will only work into high impedance load, typically 250K!. There is an online impedance calculator in which you enter tube type, plate load resistor Rp and cathode resistor Rk and in bypassed or not. The calculator claims the output impedance should be about 13K! A lot lower than I have ever thought.
To test this out we maxed the output of the preamp just prior to clipping (30V) then loaded it down until we got half the output voltage - about 13V. This occurred at a load of 15K!!! I maybe be wrong but this indicates the output impedance is 15K. Right or wrong.
Very pleasing results all round and now I know this 4S preamp will drive SS power amps which have a input impedance of better than 15K (most will) and need less than a 13V swing - ALL WILL. Not only that the preamp sounds excellent.
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