[Ron Ramirez]

All of this talk about volume controls in the Model 81 made me think of the Philco Model 600 - second to the last of the cigar box pee-wee Philcos to use only four tubes and a regenerative 2nd detector (the last was the 37-600).

The 600 uses an interesting volume control circuit that I've long though would be ideal for adapting to the 80, 81, 84 or 37-84.

http://www.nostalgiaair.org/PagesByModel...013794.pdf

Notice how the 15K volume control is not connected between antenna and ground, but between antenna and the 6A7 cathode (through a 200 ohm resistor). And the center tap of the control is connected to ground. When the volume is at maximum, the 15K of the volume control is across the antenna coil primary, the 200 ohm cathode resistor goes direct to ground, maximum volume from the set. When the volume control is at minimum, antenna is shunted to ground plus there is an additional 15K of resistance in the 6A7 cathode circuit, reducing its gain. So as the volume control is turned up, the 6A7 gain is also increased. This feature, on paper anyway, looks like it would overcome the lack of a good ground. A good ground is necessary for proper operation of the 81, 81, 84 and 37-84 for without a ground, the volume control circuit in these sets simply won't work as designed.

I've never owned a 600 nor have I (yet) tried this volume control circuit, but I may do so in the near future.