[ARC5] ARC-5 TX output circuit modeling

[J Forster]

OK guys, I went and dug out the book for the SCR-274-N. It's Army Air
Forces Technical Order 08-10-50 Dated Feb. 15, 1943.

On p 31, there is a simplified Tx diagram. It shows:

Two VT-136 tubes in parallel, cathodes tied with parasitic suppresser
chokes/resistors in the plate leads.

A parallel tank composed of the T54 primary L and the C65 // C67
combination.

C66 is a B+ bypass cap and can be ignored.

_______________

One end of the T54 secondary is grounded. The coupling between the
primary and secondary is variable.

The other end of the T54 secondary goes to the moving contact
roll-a-ductor L52 "Antenna Inductance"

The end of L52 goes to the antenna via the Antenna Relay and it's RF
Ammeter.



So, the model for the secondary of T54 is a voltage source in series
with a small inductor. This inductance is added to that of L52 and the
Antenna Relay.  The net result is a voltage source in series with an
inductor, as I said a couple of hours ago.

So, the whole circuit is a voltage source; series (fixed + variable) L;
antenna C; and radiation resistance.

The way to maximize the transmitted power is to maximize the current
through the radiation resistance. This is done by making the circuit
series resonant. If you connect the set to a pure resistive load, there
will be NO RESONANCE. What you wind up with is a voltage source,
inductor, and resistive load. This is a power/voltage divider and is NOT
the way to run the set. In fact, the power to the load will decrease
with increasing frequency.

As a corollary, if you want to run into a 50 ohm resistive load, put in
a series C. I don't know but would guess 50 to 100 pF would be in the
right ball park.

FWIW,
-John