[MrFixr55]

I would agree that the voltage drop across R4 should be 0V with both tubes out if C15 is good.  Since the DC ripple at that point should be very low, the cap should not draw current, like the filter caps would.

This is a stab in the dark.  since you get a strong signal out of your AM transmitter, tune that in (with an audio program) and measure the voltage at pin 4 of either the Convertor or IF tube.  If you have a strong signal, the grid bias should be much more negative than with no signal received.  This may change the current draw of the tube and may raise the voltage.  It would be interesting to see the results of this test.  The problem is that the published conditions in the Service Notes (from Rider) are no signal and volume control at minimum.  However, I would still do the testing.

My calculations in the previous post are based on specs in the RCA Receiving Tube Manual, version RC13, published in 1940.  Attached below are the associated pages:
   

6A8 fr RCA Rcv Tube Man RC14 1940.pdf Size: 349.23 KB  Downloads: 151

My theory for the supply voltage to R4 was based on the total resistance from B+ (junction of speaker voice coil) to chassis ground (16585 Ohm), the ratio of the resistance from B+ to the supply side of R4 (3500 Ohm) to the total voltage divider resistance from B+ to Chassis Ground (16585 Ohm) being about 21% and if the supply voltage to the divider is 250V, than a 20% drop would be 50V, therefore R4 is supplied by 200V (I did not take current draw for these 2 tubes into the equation as I expected the current through the divider due to its own resistance to be rather high.)