[old_tv_nut]

The 12AX7 cathode follower gets its input from the phase detector through an RC highpass (0.1 uF and 220k). Cutoff would be 7 Hz (probably modified by other things hanging on the phase detector output).

The phase detector lowpass filter (reading from the 15 inch receiver schematic) appears to have a very low first break point with a 4 uF and 104.7 K = 0.4 Hz, then attenuation shelf of 20x out to a final rolloff of 318 Hz. (The corresponding rolloff points on a CT100 apear to be 29 Hz and 1592 Hz.)

In the modified circuit, the 4 uF seems to be missing, and I'd guess its rolloff is replaced by the coupling capacitors from the 12AX7, but I can't read their values, nor the resistor that goes from the common point of the two electolytics to ground.

Summary thought so far: the 12AX7 serves to increase AC coupling above 7 Hz. Still, I'm puzzled as I don't see any provision to roll off its effect at higher frequencies - it seems like this circuit would swamp the effect of the 100k and 5000 pF regular filter. I would have thought therre should be a resistor in series with the connection from the 12AX7 and the test point, just like the resistor in series with the 4 uF in the unmodified circuit.

Maybe someone else can work their way through this to see what it's actually doing.