Randy or Sherry Guttery wrote:
Dave Burson wrote:

S-W model R-1822 has a "reactance dimmer" (item 2) with a 6.3 v lamp
that illuminates the band indicators. The bandswitch physically moves
the nameplates over the lamp. The transformer primary appears to be
open, and a 10K resistor across it does provide a little signal to the
speaker. C21 & C14 are listed as 10 mfd, 25 v and 0.1 mfd, 150 v, and
both are absent. I am suspicious that C21 of that value doesn't
belong, since the schematic has misidentified a C22 elsewhere.
Schematic is from Nostalgia Air.
This radio had been severely hacked, but the beautiful cabinet has
kept me picking away at it for a long time.
I've found a little about reactance dimmers but nothing about use in a
tube radio, mostly fluorescent dimmers. I'd really appreciate any
explanation of the function here and especially thoughts about the
caps that parallel the primary.

What this appears to be is a saturable reactor who's input is the B+
current to the RF/IF stages - which means the less signal strength - the
higher the B+ current - due to AGC bias action.

This is the same idea that "drives" the Philco shadow meter. Low signal
- high current - since the AGC is low - and biases the tube more "on".
Signal strength comes up (as a station is tuned in) AGC goes negative,
turning the RF & IF tubes "down" (less current). As the current through
the primary rises and falls - so does the saturation - effecting the
transformer's coupling.

Now notice the two secondary windings- If the transformer's "coupling"
is working well - the two windings "buck" - the lamp is dim. However -
if the transformer's coupling isn't - the two windings "interaction" is
reduced - and the lamp is brighter.

Oh, the two caps - well the last thing you want is for the AC signal on
the secondary to be "coupled" through to the B+ - so the two caps act as
bypasses to keep the 60 cycle out of the B+.

(shooting from the hip - again - (sigh) - OK guys - what'd I miss?

best regards...
So with high B+ current, the xfmr is saturated, less bucking, lamp is
bright? That means the bulb dims when on station? Ken