On Oct 18, 1:02*am, "Richard Knoppow" wrote:
* * *I have an actual TT-3 in front of me.

I've got one too. Best tube pr0n ever!

On page 91 there
is a chart showing the effect of frequency on output power
and efficiency and also allowable plate voltage up to about
150 mhz. Its obvious the tube falls off rapidly beyond its
full rating frequency of 30 mhz but most of the other tubes
available at the time would do no better although
plate-to-filament capacitance of the contemporary Eimac
triodes was a lot lower. Their maximum rating frequency
limit was 40 mhz.

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 will not really believe a 80M-10M grid-driven 833A actually working
usefully on the upper bands. I'm sure the OP has a very nicely
constructed box. I just won't believe that it's useful on 15M or 10M
until I see it working there. And even on the lower bands the problem
of neutralizing the circuit across band changes looks hairy. With plug-
in coilsets that changed the neutralization out with each band change,
I can see it. Bandswitching all that seems completely ungainly.

All just IMHO. I fully share everyone's fascination with grid-driven
big triodes and love playing with my 833A's and their link-coupled
output and neutralization networks in that spirit. But grid-driven
triodes just do not fit into the "twist the knob on the front panel to
change bands all the way from 80M to 10M" picture that modern hams
have of a linear amp: neutralization across just a single band can get
hairy, never mind across several octaves of RF.

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

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.

Tim N3QE

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."

On Oct 18, 10:04*am, (Scott Dorsey) wrote:
I once built a push-pull class C amplifier using 833 tubes, with three
continuously tuned inductors that could handle 80M through 20M.

Kewl. Neutralization in push-pull is in my experience much more
tractable because you just tap off the signal from each phase, feed it
through the neutralization cap which is nominally identical to the
plate-grid capacitance, and into the grid of the opposing tube.
Mathematically the plate-grid capacitance is constant as you change
bands so to zeroth order neutralization may not have to be adjusted
for band changes. Did the same neutralization setting work across the
bands?

I once tried to imagine a scheme where all 4 inductor taps in a push-
pull front-panel-bandswitching scheme were switched with ceramic
switch wafers actually inside each of the coils, and all the wafers
are ganged with plastic cog belts. In the end I went with a much
simpler plug-in coilset scheme.

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...

It just occurred to me why ham amps with driven grids are so hard to
neutralize compared to diathermy machines that donate their old tubes
to the projects: You don't need to neutralize a diathermy machine at
all!

Tim N3QE

Tim Shoppa wrote:
On Oct 18, 10:04=A0am, (Scott Dorsey) wrote:
I once built a push-pull class C amplifier using 833 tubes, with three
continuously tuned inductors that could handle 80M through 20M.

Kewl. Neutralization in push-pull is in my experience much more
tractable because you just tap off the signal from each phase, feed it
through the neutralization cap which is nominally identical to the
plate-grid capacitance, and into the grid of the opposing tube.
Mathematically the plate-grid capacitance is constant as you change
bands so to zeroth order neutralization may not have to be adjusted
for band changes. Did the same neutralization setting work across the
bands?

No, it didn't. Part of that may have been the RF transformer on the
plates not being completely symmetric and that symmetry changing with
frequency.

i once tried to imagine a scheme where all 4 inductor taps in a push-
pull front-panel-bandswitching scheme were switched with ceramic
switch wafers actually inside each of the coils, and all the wafers
are ganged with plastic cog belts. In the end I went with a much
simpler plug-in coilset scheme.

The plug-in coilset is a good one, unless you suddenly find you want to
talk on a band you don't have a coil set for. I like the whole idea of
being able to continuously tune across all the bands and between all of
them. The problem with this is that you now have expensive big variable
caps or variable inductors, and they are usually sensitive as hell because
of the required tuning range... even a tiny tap and you're out of tune
again.

Sure, but can you get them free out of someone's trash? =A0All the big
planar grid tubes I found being thrown out are destroyed...

It just occurred to me why ham amps with driven grids are so hard to
neutralize compared to diathermy machines that donate their old tubes
to the projects: You don't need to neutralize a diathermy machine at
all!

You know, I haven't heard anyone even mention a diathermy machine in
ages. These days the major ISM band polluters are plywood manufacturers
and WiFi users....
--scott


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