Brenda Ann wrote:

Perhaps the fact the secondarys being wired in bucking fashion has not as
much to do with the function of the circuit itself as to assist in the
prevention of the AC filament voltage from being introduced into the plate
supply for the 1st. detector and 1st.

No, the phasing of the two winding to be bucking IS the purpose of the
circuit, and how it works.


IF (otherwise it seems to me it would
make a rather effective modulation transformer).

Still would - that's what those capacitors across the primary are for.


On the other hand, when DC saturates the secondary, wouldn't that tend to
make the primary look like a direct short?

There is no dc in the secondary - only the primary (the primary is to
the right in this circuit). Remember TRANSFORMERS by nature are designed
to operate on AC; DC only "messes them up". In this case - this circuit
is intentionally designed to take advantage of that.

Let's walk through it one more time - but this time secondaries first -
then primary.

First - let's think about the two (secondary) windings as a primary and
secondary - after all - what windings are "called" has to due with their
use, nothing more. If you hooked AC directly to one winding and a bulb
directly to the other - the AC would couple from one winding to the
other and light the bulb (leaving aside current density, etc. for the
moment). If the two windings are 1:1 ratio - 6.3V applied to one would
show up as 6.3V on the other. You could wire the bulb either way (i.e.
"turn it around") and the current would flow through either the same
phase as the primary (ignoring simple inductance) - or 180 degrees "out
of phase". Point being - the two windings are the same - both oriented
on the same core - and form a 1:1 ratio between them. As long as the
core works as a transformer - the voltage couple between them will be
(ignoring losses) 1:1 - the only "variable" would be the phasing - as
determined by how the windings are hooked up.

OK - now lets wire the two windings as shown in the schematic: Both
windings on the same core; both having "equal effect" (1:1). Now when
current (attempts) to pass through one winding - it "couples" to the
other winding - which then generates an equal (but because of phasing)
but opposite voltage - which tends to cancel (buck) the voltage applied
to the first winding. Think of it as two batteries. If two batteries
are wired "nose to nose" with a bulb in series with them - what happens
to the bulb? Nothing. The two voltage "buck" each other - and (provided
the batteries have an equal charge) equilibrium is reached - no current
flows. Same thing with our two windings - WHEN the transformer's
ability to couple is un-imparied.

This "wild card" then - is what makes the circuit useful. This
particular "transformer" is a special kind which unlike the "usual"
transformer (which has modifications to help it "ignore" DC in the
windings) - but rather is designed to indeed easily saturate the core
when (sufficient) DC passes through one of it's windings. As the core of
a transformer approaches saturation - it's ability to couple AC between
the windings starts to fail; to the point that a fully saturated core
couples virtually nothing.

So - in this circuit - when the AGC has the RF / IF stages biased way
down (on station) the B+ current draw is low - which passing through the
primary (or control winding if you prefer) has little effect on the
transformer's ability to couple AC between the other windings - in this
case causing one winding to "buck" the other - and the bulb is dim.
When the AGC falls - biasing up the gain (current) of the RF & IF stages
- the current in the primary (or control) winding increases - pushing
the core towards saturation - and the two windings in series with the
bulb loose their coupling, reducing the induced bucking emf - and the
bulb brightens.

As you noted - when there is significant coupling between the two bulb
windings - that will also couple to the primary (or control) winding.
That's where the two capacitors come into play - they bypass any ripple
impressed on that winding back down to the B+ rail - which of course has
it's own filtering to ground.

best regards...
--
randy guttery

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