@atmasphere We get pretty high figures and we do it with a tube.... As a hint, look into 2-stage CCS circuits. You aren't going to get good numbers without a decent CCS, a resistor or a single-stage CCS won't hack it.
Of course i use good CCS, what concerns me is the variations in the gain of the two sections of the 6SN7. Since I have the M-60 on the bench I will measure that. What kind of numbers should I expect and at what frequency do you measure it?
I guess this really depends on what is meant by 'current' (since the word has become a charged term in audio)!
Current is well defined by science. I didnt know there was a special audio definition.???
And a lot depends on the 6AS7 in question too- the GAs don't hold up; most American tubes have problems in our circuit as well since they really aren't intended for fixed bias operation. We prefer the Russian variant; they hold up the best of any we've seen.
The 6AS7 is a pass tube in a DC power supply. Heres a link, one has to scroll down a bit and read the application paragraph at the beginning. I dont think brand is going to make a lot of difference as they are all made for the same application as stated clearly here. .
https://frank.pocnet.net/sheets/191/6/6AR11.pdf
Speaking of bias I note the M-60 offset, which is entirely based in bias stability or lack of. The offset wanders +/- 100 mV at 0.5 to 5 second rate. Perhaps its all power line related, but nevertheless its quite easy to see on a scope. The DCR of a quad 57 is 0.5 ohms so there is considerable current if the offset gets large.
Having my hands on an M-60 for several weeks now has given me a better opinion of it than I had before. My customer does not want to be adjusting the offset all the time or at all. He has requested I design a servo. I now have the offset down to about 1/10 of the original.
Although we disagree on many things I do appreciate your Gentlemanly approach, unlike that other fellow who left us.