>
> ## 8877 is rated for 4 kv at 1 amp CCS..provided you blow enough air through
> it.
> Even Dick Ehorn said you wont get more than 50% eff on any of these
> bandswitched amps,
> on 10 and 12 m bands. 68% on 160-80 40m. 65% on 20m. 60% on 15M. Those
> are his
> words not mine. But I would agree with that general statement.
I built my first bandswitched 8877 amp back in the seventies for
publication in the Bill Orr Handbook (which paid for the parts). When
bench-testing the amp, I was surprised to see the efficiency on 10m
was only 50%, even though the tube was rated to 250 MHz.
Bill put me in touch with his colleague Bob Sutherland (w6UOU and
later W6PO), who suggested rewiring the input pi-net so that its
output capacitor was soldered permanently to the cathode pin of the
tube socket, with no more than 1/4 inch lead length to ground. I was
dubious that this change could possibly have any effect at all, but
when I followed Bob's suggestion, the efficiency jumped up about 10%.
I never did understand exactly why this cap placement had such a huge
effect on the amplifier's efficiency, except that it had something to
do with the parasitic inductance of the capacitor leads and the
importance of having the stored charge in the capacitor readily
accessible during a large fraction of the 8877's conduction cycle.
Maybe somebody can explain it to me. Bob really knew the ins and outs
of vacuum tube amplifiers.
I miss both those guys.
73,
Jim W8ZR
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