[Amps] cathode driven 4CX tubes.

[Jim Thomson]

Date: Wed, 13 Oct 2010 01:10:36 -0400
From: "Fuqua, Bill L" <wlfuqu00 at uky.edu>
Subject: [Amps] cathode driven 4CX tubes.


I was just looking at Bill Orr's article about super cathode driven tube amps in the July 67 QST.
He has a example of a 4CX300A (control grid connected to cathode) cathode driven amplifier.
It only has a power gain of 5.     375 watts out with 75 watts input. But the nice thing is that the 3rd order products
were 46db down and 5th were -49. This is quite an improvement over most amplifiers. 
    I have always wanted to do this using two stages one produce around 300watts output and the second 1500 watts.
A 4CX250b driving 4 parallel 4CX250B tubes. Should get similar results.  This naturally would require 50 to 70 watts drive. 


###  This is the real deal  ....  'super cathode driven amp'.     Not to be confused with semi floating the grids on 3-500Z's like W8JI  mentions.
Super cathode drive and semi floating grids are 2 x different things. 
VE7TB [sk]  built a 4CX-1000  using super cathode drive.   It took 160 w of drive to get 1200 w out.   Power gain of 7.5 = 8.75db. 
I never read the original article, still haven't.  I didn't realize the imd was that good.  The intent was to take a zero watt grid diss rated tube
like a 4CX-1000..and be able to use it in GG... and dispense with the  screen supply.  Does anybody know how much bias is used in
this config  ?   With the control grid connected to the cathode...the screen now acts like the control grid in a GG triode.   This config
is a cheap way to make a triode out of a tetrode. 

Jim  VE7RF