Hi all,
I've been following the choke discussion and I would just like to
share a technique which I observed in a 2-36 MHz Hughes Aircraft airborne
kilowatt amplifier that was designed circa 1958.
The fixed portion of the pi coil is fabricated from what appears to be
a section RG-402. The center conductor is connected directly to the
plate of the tube and the plate blocking capacitor is connected in
"conventional" fashion between the plate and the outer conductor of the
semi-rigid cable plus the plate tuning capacitor. The other end of the
center conductor that emerges where this coil is connected to the
bandswitch and the lower frequency coils is connected to the HV supply
through a rather ordinary looking 1mHy choke of sufficient current
capability.
When this arrangement is operating at it's highest frequency, the
effective impedance presented at the top of the choke is only 50 ohms!!!!
Of course, as more coils are switched in the impedance level rises but,
since the operating frequency is simultaneously decreasing, the applied
stress on the choke probably remains somewhat constant. An amplifier
employing a pi-L output network would raise the impedance level somewhat
though the stress on the choke would still be greatly lessened as
compared to the conventional connection.
RG-402 is rated for 5KV or thereabouts DC, so this technique would be
applicable to a legal limit amplifier. I suppose the method could be
extended to higher power levels by employing RG-401 (7.5KV) or if that
isn't available, just running some high voltage / high temperature wire
through a piece of tubing before fabricating the coil.
I've never seen this exact circuit employed in any amateur equipment
nor even described in the literature.
There was a similar concept presented in June '78 Ham Radio, p.98,
which suggested placing the plate choke after the pi coil and relocating
the RF coupling cap to that point. This would keep the impedance at the
top of the choke low and constant over the entire operating range in
exchange for increasing the current through the coupling cap and doubling
the required voltage rating for the plate tuning capacitor. These are
likely uneconomic tradeoffs!
73,
Marv WC6W
*
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