In article ,
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?

This circuit is very confusing to me, I don't understand how it is
supposed to work. There seem to be two opposing forces at work in the
saturable transformer. The first is what you point out, that the two
secondary windings are connected so they "buck", so that when the
transformer is saturated by the DC in the primary, the coupling
decreases and the light dims. But at the same time when the transformer
is saturated the inductance also goes down, so even though the two
secondary windings might be not coupled as tightly, their reactance is
also lower which would tend to cause the light to become brighter. If
this is all there is to it the question would be which one of the two
effects is stronger than the other?

But maybe the windings aren't arranged as on an ordinary transformer.
What if we had E-core style laminations with the primary wound on the
center leg and one of the two secondaries wound on each outer leg. The
presence of the center leg would act as a magnetic short and greatly
reduce the coupling between the two secondaries even when the
transformer isn't saturated. When the transformer isn't saturated the
light would be dim because of the high reactance of the two secondaries
in series with the light. When the transformer becomes saturated at low
signal levels the lights would become brighter because of the lowered
reactance of the two secondary halves.

Just another shot from the hip, it would be interesting to know what the
actual disposition of the primary and secondary windings on the the
transformer core is?


Regards,

John Byrns

--
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