[Amps] Super Cathode Driven Tetrodes

[Carl]

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Monday, October 22, 2018 10:27 PM
Subject: [Amps] Super Cathode Driven Tetrodes


> 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.

That seems very low drive for passive grid. The NCL-2000 could use up to 
100W in the high drive positon at 50 Ohms and 20-25W into the low drive 100 
Ohm configuration.

Carl 


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