Re: [Amps] Tetrodes in GG Configuration

[David Pruett <k8cc@comcast.net>]

I'd like to point out another tetrode configuration which has some useful 
characteristics, and that is cathode driven, but not zero bias.

Lew Gordon, K4VX is a well known amateur, contester, and former ARRL 
Director.  One time when I was visiting him (in Hannibal, MO - Tom Sawyer's 
fictional home town!) Lew showed me an amplifier project he has just 
completed.  The 28 MHz amp at his station was a 4-1000A strapped as a GG 
triode that had served long and faithful duty, but was worn out.  He had 
been given a 4CX3000A tetrode which he was determined to put into 
service.  Now those of us who know Lew know that he's a legal limit kind of 
guy - 1500.0 watts output is his target.  How's a guy like this going to 
utilize a 3000W dissipation tetrode?

Cathode driven is how, 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.  However, cathode drive produces less gain than grid driven 
which in this application was not a problem.  If I recall correctly, Lew 
said that 75W drive from the IC-765 produced 1500W output on 28 MHz.  I 
think the anode voltage was 4700VDC no-load.  The only real bummer is the 
filament requirements - 7.5V @ 51A or so.

The beauty of such an amp is that it will last forever with a 3000W tube 
producing 1500W.

73,

Dave/K8CC


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