What about tapping the inductor down from the antenna side with a batch
of Really Husky relays?
I have some here I bought from Array Solutions that have held up to
between 10 and 3000 ohm antenna loads at QRO power levels. (3PDT 30A
contacts with huge spacings and the contacts wired in series)
Or - surplus SPST vacuum relays?
Diving under the truck,
Jim, N7CXI
Edward Swynar wrote:
> "...Can you imagine building an all band HF amp and what it'd take for a
> tank circuit and band switch?"
>
> *********************************
>
> Hi Don, et al,
>
> In truth, it's really not all that difficult IF one incorporates a rotary
> inductor as the final pi tank coil...
>
> I did that very thing here with my 2x813 amplifier. The inductor (WW2
> surplus, purchased NOS in 1975!) is wound with 1/4" edge-wound copper
> ribbon, & has enough "L" at its maximum to be effective on 160-meters...on
> 10-meters the turns are set at "0", & I can only assume that it's stray
> inductance from the circuit wiring that resonates the thing...
>
> As well, I have the ability to switch the plate tuning capacitor (a 100-pfd.
> Johnson air variable) competely out of the final pi section, by way of a HD
> Centralab ceramic rotary switch: on 10- and 15-meters the interelectrode
> capacitance of the 813s resonates the tank, aided & abetted, of course, by
> virtue of the adjustable tank coil.
>
> Resonance on the lower bands, i.e. 40-, 80-, and 160-meters, is achieved by
> switching fixed capacitance in parallel with either the plate tuning
> capacitor, or plate loading capacitor (separately, for more flexibility), as
> may be required.
>
> The only band where there is no extra"C" added, or where "C" is not removed,
> is 20-meters.
>
> I started out with a solenoid-type plate choke of 200-uh.: this worked well
> on 80- to 10-meters, including the WARC bands (no resonances anywhere,
> because the choke was still intact when I retired it!)---however, I didn't
> think this was big enough electrically for 1.8-MHz, so I replaced it with a
> multi-pi 2.5-mh. National choke just for that band...now I have to figure
> out a way to bypass this choke in favour of the original 200-uh. unit for
> the higher frequencies (I can just here those pie sections slamming together
> now in the bigger choke, having found self-resonance on one of the higher
> bands!).
>
> One of the beauties of homebrewing, certainly, is the ability to achieve a
> level of flexibility rarely afforded anywhere else commercially---and at
> affordable prices, too! I think one of my biggest inspirations in the ideas
> used with this amplifier were the write-ups by Bill Orr in his old "west
> coast" RADIO HANDBOOKs...
>
> ~73~ de Eddy VE3CUI - VE3XZ
>
>
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>
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