"...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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