[Milsurplus] [ARC5] Hello Smart People: Output Network "L"

[J. Forster]

Does the transmitter have a variable coupling, like a rotating link?

If you don't have effectively a variable ratio transformer between the
tank and the load, you can have a mismatch.

Changing the choke inductance, changes the impedance at resonance, despite
changing the capacitor too.

In a resonant circuit, there are two things you can vary, L and C. These
control two other parameters, the resonant frequency and the impedance at
resonance.

What matters is the match between the tank at resonance and the load.

-John

=================



> I learned the lazy-man's way (tinkering) just
> how critical to PA stage efficiency and output
> is getting the right value of "L" in the output network.
> A single turn in a "Pi" net can make a large difference
> in how much you get out vs. how much you put in.
>
> While working on the RAAF AT5 transmitter,
> I was getting 12 AM watts out in R/T, just as specified.
> CW output was a little lower- about 35 watts.
> But in AM I was seeing 100 mills into those two 807s.
> That's 55 watts in for 12 out unmodded carrier... hmm.
>
> The output for the AT5 is designed for 100 ohms
> and normally feeds a tuner.  I was using, of course,
> a 50-ohm load, feedline and wattmeter
> (The original antenna connector is broken, so
> I've temporarily mounted an "N" connector there).
>
> I got to thinking of my "lessons learned" while
> building "Old Fireball (1935 RK20 rig)"
> I dug-out a very short screw with course threads
> and "screwed" it between two turns in the PA output inductor-
> the turns between the two first vertical taps from
> the front of the coil- effectively removing a turn.
> After re-tuning, I still have .100 mills on AM,
> but now the rig has 25 watts of clean, grid-modulated
> carrier out and on CW, 50+ watts.
>
> So, for the dozens of you smarter than me,
> here's the question:
>                               Why?
>
> 73 DE Dave AB5S
>
> P.S. Australian folks:
> If any of the AWA engineers still live, please tell them I'm
> going to have one to their health. These are neat radios.
>
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