Will be interesting to see what you get. If you're using a simple plate follower with an Rk to ground, I don't see how you'll get the swing you need. However, a negative supply, as in my 6c45pi design may do the trick (as I noted earlier, the output of this guy is a cascoded plate follower).
Progress:
Haven't acquired the target tube just yet, but did some experiments with a 6SL7 t-barred to 2.8mA with B+ of 340V and around 200V on the plates and 50K load.
With a 56K from output coupler to grid, and another 56K input to grid, gain is 0.95, Zout around 820R for 10Vpp input, and as far as I can tell it will swing around 180Vpp without difficulty.
It's instructive to measure the input on the grid with a CRO. With 23Vpp input, output is around 21Vpp, and voltage at the grid is only 0.52Vpp; feedback is nulling the input by resistive mixing at the grid BEFORE it enters the tube. Obviously the high feedback factor corrects distortions, but it looks pretty good and gives DC to light response.
So, with judicious choice of R1/R2, this will work extremely well. I would expect a Zout around 150R. It's looking good........ Now to get some 6GK5s!
Incidentally Stuart, I believe the microphonics would be highly attentuated in a unity gain plate follower, about the same as a CF. Should be fine.
Cheers,
Hugh
Haven't acquired the target tube just yet, but did some experiments with a 6SL7 t-barred to 2.8mA with B+ of 340V and around 200V on the plates and 50K load.
With a 56K from output coupler to grid, and another 56K input to grid, gain is 0.95, Zout around 820R for 10Vpp input, and as far as I can tell it will swing around 180Vpp without difficulty.
It's instructive to measure the input on the grid with a CRO. With 23Vpp input, output is around 21Vpp, and voltage at the grid is only 0.52Vpp; feedback is nulling the input by resistive mixing at the grid BEFORE it enters the tube. Obviously the high feedback factor corrects distortions, but it looks pretty good and gives DC to light response.
So, with judicious choice of R1/R2, this will work extremely well. I would expect a Zout around 150R. It's looking good........ Now to get some 6GK5s!
Incidentally Stuart, I believe the microphonics would be highly attentuated in a unity gain plate follower, about the same as a CF. Should be fine.
Cheers,
Hugh
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