Date: Wed, 13 Oct 2010 01:10:36 -0400
From: "Fuqua, Bill L" <wlfuqu00@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
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