[Amps] Super Cathode Driven Tetrodes

[Jim Thomson]

Date: Sat, 20 Oct 2018 14:21:46 -0400
From: "R.Morris" <robrk at nidhog.net>
Cc: "amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] Super Cathode Driven Tetrodes
Message-ID: <7DA69145-8F85-497E-A73A-EA706B9BBDC0 at nidhog.net>
Content-Type: text/plain; charset=utf-8

<Whats the Collins 30S1 doing?

##  The  30S1  is  cathode  driven.   Requires  70-80  watts  of  drive.  It uses a PI  tuned input, one for each band,
bandswitched, just like a SB-220.  Added twist is... the  B+  supply, screen supply, and bias supply are all strung in series. 
Collins used this higher drive config, since  tube type xcvrs and  TXs  back then, had no easy way to vary the power output on SSB
except  via  alc.   Their  100 w pep  xcvrs and  TX  could then be reduced to  the 70-80 watts  required. 

##  If passive grid  config used on  the same 4CX-1000, or  4CX1500B,  only 10 watts of drive required.  Grid driven, with
a regulated  screen supply.   Buddy across town uses this config on his hb  4CX-1500B.    1.5 kw out with 10 watts of drive
on the low bands,  20 watts of drive required on the upper hf bands.   Cathode is RF grounded with a pair of .01 uf bypass caps. 
On bigger tetrodes, a 1:4  unun  or 1:9 unun is used, to step the input Z up  from 50 ohms to  either  200 ohms or  450 ohms.  Then less
drive is required to  develop the required peak grid voltage, when driving the grid positive.   A  50-200-450 ohm  small globar is wired between
control grid and chassis.  

##  RE  the W8JI  writeup on his website.  That is for the  G2DAF  configuration, and NOT   super cathode driven.  G2DAF is a grid driven
config,  BUT  rectifies the  incoming drive power, to  make screen voltage !   Keyed, but with no drive applied,   you have NO screen voltage..
and also NO idle current.    I have a 2 x 4-400A  G2DAF amp, works good, nobody bitches about imd,  but its a silly circuit really..and used
tubes to rectify the incoming drive RF.   Local buddy used  SS to rectify the RF, worked on the lower bands.   The screen
caps have to be very carefully selected to make it work right.   The G2DAF amp bascily   goes from class  C to  class B, to Class  AB and back
around in a circle.    I have seen everything from  4-1000s,   2 x 4-400, and also  4 x 4CX-250Bs  using the  G2DAF config..back in the  70s. 

##  Super cathode driven config    bonds the control grid to the cathode.    Screen  grid bonded to chassis.  Those were  popular back in the
70s.  A local ham, used a 4CX-1000  in super cathode driven config..and also another ham  buddy, when I was up north back in the 80s.   Both
amps operated  much the same.  160 watts  of drive  to get 1200 w pep out.  You cant just ground the control grid and screen grid, and operate the
4CX-1000  like a high mu triode...like a 4-1000.   Grid diss  is  ZERO  watts on a  4CX-1000..and only  1 watt on a 4CX1500.   The collins config is unique,
in that it gets around this problem, albeit with a semi complicated circuit, that requires higher drive levels. 

##  Bigger tubes , like the  4CX-5000A, can have their screen grid and also control grid bonded to chassis, and operated like a high mu triode, albeit with the
caveat that the rated control grid diss is only aprx  1/2  that of the eq  triode version of the tube.    Same deal on a 4-1000  or  4-400, when used as a high mu triode.
with both screen and  grid bonded to chassis.  Grid  diss is only 1/2 that of a  3-1000Z or   3-500Z.    Higher drive required ....and IMD is not as good.   

Jim   VE7RF.