U-Follower: Output from Cathode or Plate?

[sgerus]

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?

 

[EC8010]

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.

 

[sgerus]

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.

 

[Gluca]

If you mind that cap just wire the tube as SRPP. OH! The load line depends on the following stage too (ie that grid R).

Ciao
Gianluca

 

[EC8010]

The upper valve might still act as a CCS, but if you put the grid-leak of the 300B across it, it is no longer a horizontal loadline. And as for that 300B input capacitance...

 

[sgerus]

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?