Hi Jim,
My understanding is that the cathode needs a low impedance to ground in
order for the high frequency plate current harmonics to develop properly.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of MU 4CX250B
> Sent: Saturday, February 11, 2017 3:01 PM
> To: Jim Thomson
> Cc: amps@contesting.com
> Subject: Re: [Amps] PowerGenius XL Video
>
> >
> > ## 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
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|