I've been working on a Stewart-Warner Model 305, a TRF from the mid-1920s: http://www.nostalgiaair.org/PagesByModel/796/M0029796.pdf

There are two rheostats. One just controls the filament voltage for a volume control. No problem there.

The other is labeled "Modifier." It's a center-tapped variable resistor that bridges between A+ and A-. By inspection of the schematic, it controls the grid bias of the second RF and possibly monkies with the antenna circuit, too.

Can somebody help be understand what's going on with that control? It certainly is effective in controlling the performance.

(By the way, this set has very nice ERLA-branded componentry. The tuner end-plates are chromium plated, and the tuner leaves are heavy gauge aluminum. The audio interstage xfmrs are also ERLA. Both S-W and ERLA were located in Chicago, so there must have been at least an informal relationship?)
Doug

Doug,
Sounds similar to the volume/gain control of my Edison 7R TRF. It was a uniquely packaged dual graphite rheostat in the cathode circuit of the second RF and an antenna shunt with a switch for phono. The 2 were counter rotating in such a fashion that as gain was raised, ant shunt R went up, and vice versa. Even stranger was a note stating that one of them (don't remember which)had no change/affect until half way thru its rotation. I repaired it with powdered graphite and copper tape to closely approximate the docs.

marv

:I've been working on a Stewart-Warner Model 305, a TRF from the mid-1920s: http://www.nostalgiaair.org/PagesByModel/796/M0029796.pdf
:
:There are two rheostats. One just controls the filament voltage for a volume control. No problem there.
:
:The other is labeled "Modifier." It's a center-tapped variable resistor that bridges between A+ and A-. By inspection of the schematic, it controls the grid bias of the second RF and possibly monkies with the antenna circuit, too.
:
:Can somebody help be understand what's going on with that control? It certainly is effective in controlling the performance.
:
:(By the way, this set has very nice ERLA-branded componentry. The tuner end-plates are chromium plated, and the tuner leaves are heavy gauge aluminum. The audio interstage xfmrs are also ERLA. Both S-W and ERLA were located in Chicago, so there must have been at least an informal relationship?)
:Doug
:
: