[Amps] Tetrodes in GG Configuration

[Ian White, G3SEK]

David Pruett wrote:
>I'd like to point out another tetrode configuration which has some 
>useful characteristics, and that is cathode driven,
[...]
> but not in a zero bias, strapped as a triode configuration.  The grid 
>has negative voltage, and the screen has medium high voltage, very 
>similar to that of a grid driven configuration.

That is almost exactly the 30-S1 configuration that Colin K7FM and I 
were talking about.

The correct DC operating conditions are *exactly* the same as for 
grid-driven, because the grid, screen and anode voltages are all 
measured from the cathode. The tube only knows about the G1-cathode, 
G2-cathode and anode-cathode voltages that it sees.

So, DC-wise, the  tube doesn't care which terminal of the power supply 
you choose to connect to chassis. DC grounding of the cathode, G1 or G2 
terminal have all been used successfully in tetrode amps. The other 
terminals of the power supply then have to be above or below chassis 
accordingly.

Pictures are much better than words to describe this. There are diagrams 
of the various configurations in the Tetrode Boards manual which is 
downloadable from my website:
www.ifwtech.co.uk/g3sek/boards

Another common fallacy is that you can't have cathode-driven class AB1. 
Sure you can, with a cathode-driven tetrode. It requires exactly the 
same DC grid and screen voltages - relative to the cathode, of course - 
as it does for grid-driven. The difference is that the cathode-driven 
amp requires significant RF drive power, even in class AB1.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek