[Amps] Super Cathode Driven Tetrodes

[Fuqua, Bill L]

 Check out July 1967 QST.

Bill Orr has a super cathode driven 4cx300A where the control grid is  connected to the cathode and screen grid directly to ground. Very low distortion and should be completely stable.

Now I know where the notion comes from about Super Cathode Driven Amplifiers.
  Basically it is like the tube is operating class AB1 but instead of the screen grid being bias to several hundred volts and the control grid swinging to near zero volts on signal peaks, it is the opposite.
   The control grid remains zero in respect to the cathode and the cathode swings negative the same several hundred volts relative to ground and the screen grid bias.
   The best parts of this idea is that there are no bias voltages required, the idling current is quite low, practically no grid current due to the design of the 4CX300 tube design and very low distortion and all the drive power is added to the plate output as well.
   Down side, power gain is low, around 5. Compared to 15 or more with most grounded grid triodes.

73
Bill wa4lav



________________________________
From: Brian Hind <ve7cra at gmail.com>
Sent: Friday, October 19, 2018 8:21 PM
To: Fuqua, Bill L
Subject: Re: [Amps] Super Cathode Driven Tetrodes

Hello:  I believe W8JI  describes the "super Cathode Driven" circuitry, & why it does not work, or does not work "properly" on his web page. If not there, then on this reflector some years ago.  I am 99% certain that it was Mr. Rauch that brought the circuits alleged difficulties to the attention of Hams initially some years ago.

Good Luck!
Brian, VE6XX

On Fri, Oct 19, 2018 at 8:32 AM Fuqua, Bill L <wlfuqu00 at uky.edu<mailto:wlfuqu00 at uky.edu>> wrote:
  Someone had posted that Super Cathode Driven amplifiers are unstable.

Can someone elaborate on that notion?


It does not make since due to lower gain and that screen grid is directly connected to ground.

I hope in the future to do some experimentation on this design. Goal is one 4CX300Y driving 3ea

for legal limit. Neat thing is that just about all the tx, driver power finds its way to the output.

This tube has 400Watts plate dissipation.

73

Bill wa4lav
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