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."