[MrFixr55]

Arran, that is really interesting information. Amazing that RCA negotiated with a small company as opposed to eating it up a la Standard Oil. (In an amazing coincidence, RCA's corporate headquarters and the NBC Radio and TV studios occupied a lot of Rockefeller Center, which has a sculpture of "Promethius stealing fire" in front of its 5th Avenue entrance.)

Varying screen voltages was a method of controlling volume by controlling RF gain in late 20s / early 30s radios that utilized tetrode RF amps. However, I believe that controlling RF (and IF if a Superhet set) bias and shunting the antenna coil was more popular, prior to the advent of AVC. By then, volume was controlled at the 1st AF amp.

A good example of this is the Philco Model 20 which used a ganged pot. One pot increased the cathode voltages of the 2 RF Amp 24A tetrodes, therefore increasing grid bias and lowering gain. The other pot shunted the antenna input.

Another method was to use the same pot to shunt the coil and increase bias at the same time. An example of this was the Gloritone 27. One leg was connected to the cathodes of the RF amps. The other leg was connected to the antenna terminal. The wiper went to Chassis ground. Turning the control CW lowered cathode resistance and increased shunt resistance, raising volume. Turning the control CW increased cathode resistance and increased shunting of the antenna input, lowering volume.

Gary, for my $0.02 worth, I think if you can't get a good replacement of the original pot but could get your hands on a 30KOhm linear pot and wiring as described above, it should work satisfactorily. The pot may need to be wire-wound as it will have to dissipate some heat. It would not have to be a dual pot; a single pot would work, though it is not the correct one per the schematic.

Alternatively, connect the screens as per the schematic and the 200 Ohm resistor to chassis ground. Short the antenna terminals and scan the entire tuning rang of the set. If the set is sufficiently quiet, you may get a way with using a 10K pot solely to shunt the antenna. The problem in doing this is if the signal is so strong that it saturates the RF amp, youmay have to turn the set off to reset it.