Split Load Plate Stage - Freq Response and Impedance

[ChrisM]

Scenario:
Split plate load common cathode gain stage. The resistor going to B+ is 56K the resistor going to plate is 47K. Lets say the gain (A) of the stage is 100. The output comes from either the plate or the node connecting both resistors. The output feeds a 0.01uF cap with a 1M resistor to ground.

1. If we take the output from the plate the gain is 100.
If we take the output from the resistor joining node
the gain (A) = [47K/(47K+56K)]*100 = 45.6 Correct?

2. Lets say ra = 50K, this makes the output impedance
(Zout) = 50K*(47K+56K)/(47K+56K+50K) = 33.7K Correct?

3. Regardless of where the output is taken (node or plate) the output
impedance stays the same correct?

4. Calculating the frequency response for the plate output is simple f = 1/(2*pi*R*C) where R is 1M + Zout. f = 15.2 Hz. Is the cutoff frequency the same for the split node output? I would think it is but I want to confirm.

I am trying to wrap my head around designing a split load gain stage in case you were wondering

Thanks!

[martin manning]

Looks good to me, and an LTspice sim confirms the result. -3dB frequencies are the same, and gain is 6dB less for the split-load.

[ChrisM]

Thanks Martin. I'm calculating 6.8dB drop with the split load output, should be quite noticeable.

A unrelated question while I have you here...
Merlin says center biasing is -2V in his book, where the grid curves only go to -4. The datasheets I look at show curves that go to -5V. In reality is center biasing more around -2.5V? Or maybe though -4.5V and -5V are shown on the curve they are not actually obtainable in practice...?

[martin manning]

I had a 50-50 split in the sim. I get just about 6.8dB with 47-56 as in your example.

Grid voltage at cut-off depends on what the supply voltage is, and what you want to call cut-off. At 300V, -4V Vg is down in the tens of uA in actual traces I have done on current production 12AX7 tubes (see below; solid lines are one triode and dashed are the other in the test article). I've also traced some old tubes in good condition that displayed very similar behavior.

That would say that -2V is "centered," but the plate current doesn't stop at 0V Vg either. For the usual 100k Ra and 1k5 Rk configurarion, the bias point for the tube shown would be right about at -1.5V Vg at 1 mA and 200V Va.

[jazbo8]

What's the advantage of using a "plate-divider" as oppose to a plain voltage divider after the coupling cap for example?

[martin manning]

Lots of discussion on that. See here, for example: http://music-electronics-forum.com/t29077/