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sgerus 22nd May 2007 04:05 PM

U-Follower: Output from Cathode or Plate?
 
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 22nd May 2007 06:57 PM

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 22nd May 2007 07:22 PM

Quote:

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 22nd May 2007 08:01 PM

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 22nd May 2007 08:12 PM

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 23rd May 2007 08:30 PM

Quote:

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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