[Amps] Super Cathode Driven Tetrodes

[Jim Thomson]

##  That was back in the 70s and early 80s that I know was last done in a high mu config.
I  mentioned the high Mu   config to Reid Brandon  at  Eimac, 10-12  years ago, and he told me it was very feasible,
and done quite often. Gain is down a bit from a high Mu  triode..and also imd is not as good, but the
4x5  had acceptable imd up to  7.5 kw pep out..into a 50 ohm load.   What I dont remember, is what the input Z  is. 
High mu  config , of course is  when the screen grid... and control grid  are both bonded directly to the chassis.   Tube
is operated like a high Mu  triode. 

##  One method of determining input Z,  is to 1st guestimate  the input Z... and build a low Q   PI input  for one band, say 40m. 
With drive applied, both input caps are tweaked for 1:1  swr.  Then everything is shut off.   100 ohm pot temp wired between cathode
and chassis..and tube left in socket.   MFJ-259 on input.. and pot tweaked for  1:1  swr.   Pot  then measured with a DVM...and that
value measure is the actual input Z.     At that point,  knowing the real input Z,  correct valued, and optimized  low Q  PI  tuned inputs
can quickly be designed for each band.    I use the superb  GM3SEK  PI  spread sheet  for  designing both the  PI tuned inputs.
and also the  PI /  PI-L  high powered  tank circuits.   One caveat  /  limitation  when using the spreadsheet  for a tuned input is....
it will handle any Z  to Z transformation combo you want.. EXCEPT  when the  2 x Z’s  are identical !     IE:  IF  tube Z is 50 ohms,  you can
not enter  50 to  50 ohms ....  or  say  70-70 ohms.   The software bombs and displays a bunch of  ####s.    The best you can do is to enter
50  to 49 ohms...... or  49 to 50 ohms.... then it works fine. 

Jim   VE7RF      





From: Fuqua, William 
Sent: Wednesday, October 31, 2018 4:22 PM
To: Jim Thomson ; amps at contesting.com 
Subject: Re: [Amps] Super Cathode Driven Tetrodes

  Do you know anyone that has had first hand experience in running 4CX5000 tubes in conventional grounded grid configuration? I have some that I would like to try perhaps. They have 75Watt control grid dissipation and 250 Watt screen grid dissipation. 


  73

Bill wa4lav






--------------------------------------------------------------------------------

From: Amps <amps-bounces at contesting.com> on behalf of Jim Thomson <jim.thom at telus.net>
Sent: Tuesday, October 30, 2018 12:27 PM
To: amps at contesting.com
Subject: [Amps] Super Cathode Driven Tetrodes 

Date: Sun, 28 Oct 2018 23:46:21 +0000
From: "Fuqua, William" <wlfuqu00 at uky.edu>
To: Carl <km1h at jeremy.qozzy.com>, Jim Thomson <jim.thom at telus.net>,
"amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] Super Cathode Driven Tetrodes

<Ok,

  <  Now that this discussion has gone on for some time, and everyone has had a opportunity to read various articles, here is the question.

<Can anyone give me a reason why a cathode driven tetrode with the control grid connected directly to

<the cathode be unstable. The screen grid is connected directly to ground. Low mu grounded grid configuration.


<73

<Bill wa4lav

##  Its  rock stable, based on data  from the 70s, from nearby hams who used that config.    But that was a 4CX-1000 
in both cases.   160 w of drive for  1200 w pep out on ssb / cw..... =  power gain of just  7.5..... =  8.75 db. 
The  super cathode driven config  was only used on tubes that had virtually no control grid dissipation, like 4CX-1000, and  4CX-1500B  etc.
Tubes like  4CX-5000, and bigger, can have both their control grid and screen  grid bonded to chassis.. and run like a high mu triode.... just like a 4-400,
or  4-1000. 

Jim   VE7RF

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