Marv, I save very few messages but this one is a keeper. I have heard
mention of that circuit but this is the first real info....tnx for
sharing it.
73 Carl KM1H
On Thu, 29 Oct 1998 20:59:54 EST wc6w@juno.com (Radio WC6W) writes:
>
>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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