[Amps] cathode drive

[John T. M. Lyles]

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.