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...
--
randy guttery

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