On Mon, 15 Aug 2005, 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.

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,

Often there is no need for neutralization, but depending on frequency and
wiring layout, yes there is a need for neutralization. Usually its worse
at higher frequencies. And, it can get very very complicated, too.

I have an Ameritron 811-H (four 811s in grounded grid) and it comes from
the factory with one kind of neutralization!!!! It would not be there if
it were not needed.

I had a thunderbolt, too. And it also had neutralization built in.

I also had a Heathkit DX-40 back in my novice days and it would go into
self-oscilation (big time) on ten meters if I removed the crystal or shut
off the VFO. I made my own neutralization wire and fixed that. Its funny that
Rangers had no built in neutralization, either. Same tube, similar (but
not identical) circuit in final (6146). You don't know if you need
neutralization until you do the tests (read the manuals on this).

Art, W4PON

homebrewing a GG amp
is quite appealing.

I've built several (including a single 4-400) and I'm happy when I don't
need neutralization.

Roger K6XQ