Tim Shoppa wrote:
I'm not exactly saying that RCA lied in their graphs, but somehow what
is published there is not particularly useful for ham amp use.

I know that 833A's work in diathermy machines, and I think that those
were circa 27MHz at least in the 30's and 40's. So obviously in the
right kind of circuit I think they are useful (if maybe not top
efficiency) at those frequencies.

But my experiments with them in ham amps showed that I was unable to
get any usable output above 20M with a single tube. Meaning, I was
putting more power into the tube than was coming out. I think in push
pull some of the strays could cancel but bandswitching all the
inductor sections (as opposed to plug-in coilsets) for push pull is
onerous.

I once built a push-pull class C amplifier using 833 tubes, with three
continuously tuned inductors that could handle 80M through 20M. There
was some definite power loss at 20M, but it wasn't to the point of
having negative gain. Plate modulation using some kind of big
lamp ballast transformer that I found.

I was also running the 833 well below the maximum rated B+, in part
because I had a junkbox transformer and in part because I was using
833 tubes that were pulls from a local AM station which I was getting
for free.

that 60's article showing the 833A in a 80-40-20M amp is kinda clever
in sidestepping the neutralization issue by just swamping the grid
with a honking huge 50 ohm noninductive power resistor, but my
estimation is that more than half the drive power gets turned into
heat and never hits the grid. And note that they don't claim it'll
work well above 20M

These days, gain is cheaper than it was back then, too. But it looks
like an interesting design.

There's good reasons why from the 50's onward, the beam power tubes
and grounded grid triodes have ruled: neutralization is a zillion
times easier, bandswitching from 80M to 10M is not so hard, linearity
is useful for SSB, etc.

Sure, but can you get them free out of someone's trash? All the big
planar grid tubes I found being thrown out are destroyed...
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."