[Amps] 160-6m amp designs?

[EP Swynar]

On 9th January, Marv wrote...

"...If you do the math, you'll discover that the tube
output capacitance is all that's needed for the input
capacitor in a Pi-L output network on 6 meters. The
regular input capacitor, which would still likely have
to be a vacuum capacitor for the remaining 160-10m
range, could switched out by means of a vacuum relay on 6 meters.  6 meters
could then be tuned with a variable coil -- a small roller coil, a variable
geometry assembly or some kind of variable reluctance thingy. Alternatively,
a small (.5uH or so) coil may be inserted between the plate and the regular
input cap creating another L stage in the output network. .."

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Hi Steve & Marv,

MOST interesting discourse, Marv...! It's obvious that you're a student of
Bill Orr (W6SAI) by espousing these ideas...! Hi Hi

I did the very same things here, but with a HB amp using a pair of parallel
grounded-grid 813's: the ONLY band that utilizes the "stand alone" 100-pfd.
air variable plate tuning capacitor is 40-meters...

On 15-meters, I use a HD ceramic rotary switch & remove the plate tuning cap
entirely out of the circuit: the high output "C" of the 813's (some
28-pfd.!) is used to tune the tank, by virtue of a 0-26-uh. 1/4" edge-wound
vatriable inductor. On 10-meters, this same trick applies, but I switch a
0.3-uh. coil in series with the plates, as an "L" network to "fool" the
813's into seeing an output impedance that is manageable.

It's all detailed in a 70's issue of QST --- warning: there is a LOT of math
involved!

On 160-, 80-, and 40-meters, I "pad" both the plate tuning AND the plate
loading capacitors with extra "C" in the form of a fixed vacuum variable,
combined with pre-set air variable capacitors (I learned the hard way that
air & vacuum are relatively unaffected by RF, if you will, whereas ceramic
doorknobs & mica blocks CAN heat-up internally, with consequent changes in
the tuning parameters of the amplifier).

The upshot is that the amplifier is just as efficient on 28.8-MHz, as it is
on 1.8-MHz --- with the added bonus that it will cover the whole HF
spectrum, too, by virtue of that variable tank coil...and NO, I have not yet
discovered the resonant frequency of the plate B+ RF choke, as it remains
intact here! Hi Hi

~73~ Eddy VE3CUI - VE3XZ