Cathode driven tetrodes is often used for VHF/UHF applications, as
the input impedance of the grid is difficult to match with low enough
Q. As Rich said, the driving Z is easier to match driving into the
cathode where the cathode current and driving voltage help determine
the input impedance. I wrote a paper years ago at Broadcast
Electronics (still have it scanned I think, if anyone wants a copy
let me know) in which I built the 3500 watt FM amplifier (class C)
using the new 4CX3500A, and drove it with cathode grounded and with
grid grounded, separately. There is a comparison of the resulting
phase and magnitude of the gain response through the power amplifier.
It was fun work building a socket for both versions before embarking
on the preferred approach, which for 100 MHz was grid driven,
grounded cathode.
I am presently building 500 kW and 3 MW pulsed VHF cavity amplifiers
using cathode driven tetrode (and Diacrode) devices. Building
suitable bypass capacitors for the control and screen grids on these
beasts have really been a chore.
John
K5PRO
If one grid is directly grounded, and the other is bypassed to ground,
and the g1 and g2 voltages - measured relative to the cathode voltage -
are the same as for grounded cathode, then the tube's DC operating
conditions will still be OK. Many VHF/UHF tetrode amplifiers use this
configuration, because at those frequencies the cathode driving
impedance is much easier to match than the grid driving impedance.
** Especially above 1GHz.
In other words, cathode drive is strictly an RF thing. It doesn't *have*
to involve incorrect DC operating conditions.
However, for this particular 4CX350 amp, it makes no sense to convert it
to cathode drive, because the amp will then require a band-switched
input network. As many have said, resistor-swamped 'passive grid' is the
>way to go.
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