"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.

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.

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.

Roger K6XQ