[Amps] Super Cathode Driven Tetrodes

[Fuqua, Bill L]

Mistype 4cx300a not 3cx300a

73

Bill wa4lav


________________________________
From: Amps <amps-bounces at contesting.com> on behalf of Fuqua, Bill L <wlfuqu00 at uky.edu>
Sent: Saturday, October 20, 2018 4:46 PM
To: Gary Schafer; amps at contesting.com
Subject: Re: [Amps] Super Cathode Driven Tetrodes


Some of you guys are reading completely different articles than I am.

The supercathode driven 3cx300a provided 375 watts out with 75 watts drive.

IMD 3rd order -46dB, 5th order -49dB.

   Screen grid connected directly to ground and control grid connected to cathode.


73

Bill wa4lav













v

________________________________
From: Gary Schafer <garyschafer at largeriver.net>
Sent: Saturday, October 20, 2018 2:44 PM
To: Fuqua, Bill L; amps at contesting.com
Subject: RE: [Amps] Super Cathode Driven Tetrodes

I remember the article by Bill Orr some years ago. Back then I don't think
that there was too much thought about linearity though.
Stability was good as I remember.

Some how I seem to remember that Bill Orr in later years said that the Super
Cathode driven system was not a good idea for tubes like the 4CX types.

Somewhere around here I have an amp with 4CX250B's in that configuration. I
acquired it several years ago but have not played with it yet.

Bill Orr also described a screen grid driven amplifier with control grid and
cathode grounded. I seem to remember that it needed to be neutralized with a
push pull neutralization circuit as bridge neutralization would not get the
job done. The tubes were operated single ended. It required lots of drive.

I don't think that distortion would be too good driving the screens as the
screen on most tubes is rather non linear compared to the control grid.

73
Gary  K4FMX

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Fuqua, Bill
> L
> Sent: Saturday, October 20, 2018 4:04 AM
> To: amps at contesting.com
> Subject: Re: [Amps] Super Cathode Driven Tetrodes
>
>
>
>
>  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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