Roger Leone wrote:
"Ken Scharf" wrote

One advantage of "cathode driven tetrode" (where normal grid bias and
screen voltage are applied) is that you get higher power gain and
usually don't need to neutralize the amplifier (technically it is
IMPOSSIBLE to neutralize a ground grid amplifier anyway).



Ken:

I think you meant to say that cathode driven tubes have LOWER power gain.
Grid driven amps can have as much as 10 dB or more gain than cathode driven
(GG) amps, all other factors being equal. My Johnson Thunderbolt running a
pair of grid driven 4-400's can be driven to full output with about 20 watts
of RF drive (representating close to 20 dB of power gain). The same tubes
in GG configuration would need close to 80 watts to drive them to full
output.

Actually I meant to say that the "cathode drive tetrode" has HIGHER gain
than a TRUE grounded grid amp. The only difference in gain between a
grid driven amp and a cathode driven tetrode amp would be due to the
driving power lost in the input circuit. But don't forget that in a
cathode driven circuit, some of the input power actually feeds through
and appears as part of the output, so it really isn't 'lost'.

Your statement about needing 80 watts to get full output in gg would
NOT be true if you ran the 4-400's in "cathode driven tetrode" service.
The BIG difference is applying normal bias and screen voltage rather
than grounding both grids and running in zero bias class B. I suspect
you'd need less than 25-30 watts for full output in "cathode driven
tetrode". BTW you'd need quite a bit LESS than 20 watts drive in
grid driven service, if you ran the tubes in class AB1 where only
voltage (no grid current) is required. (But it might take THREE tubes
to get the same output as TWO in class B or AB2).

A grid driven 4CX250 can produce over 200 watts of output with 2 watts of
drive (per the RCA manual). Because the tube isn't commonly used in GG
configuration, that manual doesn't provide power gain figures for cathode
driven service. My guess is you would need more like 8 to 10 watts of
cathode drive to get the same 200+ watts out.

Sounds about right. Depends on how good your input matching network is.
Those figures must be for class AB2 though. In class AB1 your IMD goes
way down and the driving power might be only 1-2 watts in cathode drive.
The widespread use of transceivers in the 100 watt class has made GG amps
popular since there is no need to attenuate the transceiver's output. With
my grid driven Thunderbolt, I use a 6 dB attenuator at the amp's input to
keep from overdriving it.

With the right choice of tube, a GG amp can have a much simpler power
supply. And since there is no need for neutralization, homebrewing a GG amp
is quite appealing.

A newer way is to run tetrodes in grid driven service with a 50 to 200
ohm high power non inductive swamping resistor from grid to ground. NO
matching network needed, same drive requirement as grounded grid, no
neutralization. Waste of power though. Many commerical amps using
Sevtlana tetrodes using this circuit.