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Re: [Amps] Super Cathode Driven Tetrodes

To: AMPS <>
Subject: Re: [Amps] Super Cathode Driven Tetrodes
From: Alek Petkovic <>
Date: Fri, 02 Nov 2018 10:39:48 +0800
List-post: <>
Just as an aside, I have just finished refurbishing a Dentron GLA-1000B and I note that it is a Super Cathode design.

The Amp Supply LA-1000 and Ameritron AL-84 both use the same 6LQ6 family sweep tubes. The LA-1000 is also a Super Gathode and the AL-84 is straight Grounded Grid.

Cheers, Alek.

On 1/11/2018 9:59 AM, Jim Thomson wrote:
##  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 ;
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.


Bill wa4lav


From: Amps<>  on behalf of Jim 
Sent: Tuesday, October 30, 2018 12:27 PM
Subject: [Amps] Super Cathode Driven Tetrodes

Date: Sun, 28 Oct 2018 23:46:21 +0000
From: "Fuqua, William"<>
To: Carl<>, Jim Thomson<>,
Subject: Re: [Amps] Super Cathode Driven Tetrodes


   <   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.


<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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