For my next amp, I have been testing a 12AT7 U-follower to drive a 300B. The U-follower is cap coupled to the 300B. .33uf and 220K
grid resistor.
The U-Follower is set up for 4.3mA and has an amplification factor of about 60, which is just right for my needs.
Based on my calculations:
Taking the output from the upper valves cathode: Zout=212R
Taking the output from the lower valves plate: Zout=8.1K
My question is where is the best point to take the output from?
Upper cathode or lower plate?
grid resistor.
The U-Follower is set up for 4.3mA and has an amplification factor of about 60, which is just right for my needs.
Based on my calculations:
Taking the output from the upper valves cathode: Zout=212R
Taking the output from the lower valves plate: Zout=8.1K
My question is where is the best point to take the output from?
Upper cathode or lower plate?
Upper cathode. The primary purpose of that upper valve is to provide a high impedance load for the lower valve to maximise its gain and reduce its distortion. Hanging the input capacitance and grid-leak resistance of a 300B across the lower valve will completely destroy the point of the mu-follower.
Hanging the input capacitance and grid-leak resistance of a 300B across the lower valve will completely destroy the point of the mu-follower.
Thanks for the reply EC8010
I thought that if you take the output from the lower valves plate,
the upper valve acts like a CCS..... so you still have a flat load line,
a high PSRR, and the input has one less coupling cap to go through.
The load line depends on the following stage too (ie that grid R).
OK... I use the output from the upper cathode to take advantage of the high R load created by the u-follower.
Now my 640K load in parallel with the 220K gird leak resistor = 163K for the ac loadline.
According to the spec for the 12at7, I could use up to 1 meg for the grid leak …
But I have never seen anything larger than 330K used before a 300B.
Question: is 220K about the right value for the grid leak resistor?
How about direct coupling a U- follower to a 300B?
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