to answer a question i was asked from the phillipines, i am posting this…

it is a direct coupled feedback scheme (important for me) and can be DC coupled further up the line using techniques outlined previously. yet another interesting thing about pentodes is that the plate voltage can go very very low and still be useful, providing the screen environment is optimal. i have gone as low as 10 vdc at the plate and 160 at the screen with certain tube types. i have built and  tested this circuit as the reverse of another attempt with a very low screen voltage. the trick of having a low plate voltage on the first stage and a more “normal” plate voltage on the second means they can share the same screen supply. a fabulous simplification. if the screen is low impedance, there is a beneficial dc feedback there. because of the cascaded pentodes, open loop gain is around 82dB. closed loop is 25dB. the performance is excellent out to 2 MHz. i am working on an all tube VCF (voltage to frequency converter) and need an “operational” amp with a particular stability. this gets very close. not there yet though.

it can be used for more mundane audio related things, or even as a regulator, with a change of buffer tube.

i can only measure down to -80dB from the 10 volt RMS range with the gear i have… i am making some things to help out. but 3rd harmonic is at least -80dB down with 80 volts peak to peak out (2 volts pp input).

i hope this shows that you can go the “other” way, too! (low plate voltage/high screen voltage)…

enjoy!