[Amps] PowerGenius XL Video

[Jim Thomson]

From: MU 4CX250B <4cx250b at miamioh.edu>
To: Jim Thomson <jim.thom at telus.net>
Cc: "amps at contesting.com" <amps at 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

##  W4ETO suggested that the PI  input is a low pass filter, and the idea was
to provide a low Z  for odd harmonics.   W8JI  said the same thing, except the
PI had to provide for a low Z  for even harmonics.

## On the drake L4B amps, they installed the coupling cap  directly between the
output of the bandswitched tuned input assy..and the cathode of the 2 x 3-500Z. 

##  another trick is to solder thin cu strap to the leads of the coupling cap, if u think
excess  xl in the small gauge wire leads is an issue.  Another method that works to increase
eff  on 10m is to install a 100-200 pf  NPO doorknob cap between  chassis  and cathode. 
Strap from top of doorknob to cathode. Doorknob mounted right at the tube socket.

##  W8JI came up with a similar concept, except no doorknob cap used.  Instead, he
paralleled 2 x RG-174U cables..to make a 25 ohm coax cable.  Aprx  12 inches  from
tuned input assy  on the ameritron amps to the cathode of the 8877, etc.  The 25 ohm
coax makes it look like there is XC  at the tube socket.   Paralleled coax is cheaper than
a NPO doorknob.  

##  I dont use 10 and 12m on my hb amp, so still installed a NPO  doorknob cap anyway,
except an even larger value.  You cant make it too big, since the C2 cap of the tuned input is
at the other end of the 10 inch piece of 213 U  coax used to connect tuned input to cathode. 
Input swr is still flat,  1:1    But the C2 value, and possibly the C1 value will have to be tweaked a bit
if any of the above methods are used.   Same concepts also apply to a 6m monoband amp.  But
on any monoband amp, the tuned input can be placed right at the tube socket and cathode. 

Jim   VE7RF