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Re: [Amps] 3CX3000A7 on 144 MHz

Subject: Re: [Amps] 3CX3000A7 on 144 MHz
From: Martin Sole <>
Date: Sun, 9 Feb 2020 08:01:06 +0700
List-post: <>
The third option of course is homebrew.

If you are in a position to be home building a 3CX3000A7 amplifier I would not think this to be a major hurdle and of course there is much info available on the web and re-purposing an old transformer core and primary should be eminently doable so all you need to wind is the secondary.

Not many turns for 7.5v though you might need a few in parallel for the current of course.

Might be worth spending a few hours with some old books, I recall the Radiotron books have a lot of transformer winding formation.

Martin, HS0ZED

On 09/02/2020 07:46, Conrad PA5Y wrote:
Thanks everybody for the info. The reason that I mentioned a switcher was that 
in the Netherlands suitable heater transformers are emphatically not a dime a 
dozen, in fact they are as rare as rocking horse s**t! However I hate SMPSUs 
because getting rid of noise is very hard work. I am mainly interested in EME 
and I have a rural location so I hear things that many cannot. Therefore I will 
bin the SMPSU idea, there are many good reasons to do so. I suspect that I will 
have to have a custom wound transformer made, either with a good number of 
primary taps or I will use a variac.

If anyone in Europe knows where I might find a suitable transformer please let 
me know. Alternatively any recommendations where I can get a custom transformer 
made without breaking the bank?

Thanks all of you for the advice.


Conrad PA5Y

-----Original Message-----
From: Amps <> On Behalf Of Jim Thomson
Sent: 08 February 2020 19:35
Subject: [Amps] 3CX3000A7 on 144 MHz

Date: Fri, 7 Feb 2020 16:05:05 -0800
From: Jim Brown <>
Subject: Re: [Amps] 3CX3000A7 on 144 MHz

<On 2/7/2020 3:54 PM, Jim Garland wrote:
One might mention that the switcher needs to have a floating 7.5V output, so it 
doesn't provide a ground reference for the filaments.
<Yes. BUT -- given that switchers are notorious for RF noise, I'd be <seriously 
cautious about selection of one that's quiet. We all know it's <POSSIBLE to build quiet 
ones -- the ones that Elecraft and Expert are <using with their amps seem to be pretty 

<73, Jim K9YC

##  Good  luck  trying  to  source a 7.5  volt  at  51 A  CCS  switcher...sans  
noise..with  adjustable output  V...and  HD  soft  start.
They  dont  exist. The  3x3  is  typ  operated with  6.64 Vac  on  the  
cathode...after  200  hrs  with  7.5  vac.  Emission  increases
during  the  1st  200  hrs.   Then  the  fil  V  can  be  reduced  til  PO  
just  drops  off a few  watts,  then  increased  by  .2  vac. It  cant
be  reduced beyond  15%.  Tube  is  rated  for  2.5A  plate  current.  Typ  is  
1.5A  plate  current...with  200w  of  drive.

## Cold  resistance  of a 3x3  is .01225  ohms.  Thats  a whopping 612 A   
start  up  current.
A  new  7.5  vac  at  51A  fil  xfmr  is  easy  to  source....complete  with  
CT...and  loads  of  pri  taps.
My  dahl  came  with  pri  taps  at  198-208-218-230-240-250 vac.   To  
complete  the  setup  however, required
a small  4A, 0-250 vac  variac...+ a 500VA  sola  constant  voltage  xfmr.

##  RE:  DC  for a directly  heated cathode.   There  was  some  trick  to  
this  I  believe.
I  think  the  polarity  had to be  periodically reversed.  Harris  reversed 
the polarity on   DC  fed tube filaments
on  aircraft...  but  those  were indirectly  heated  cathodes. The auto 
reversal  occurred  every time  the small power  amp was  turned on.

##  I  will  ask  Eimac.

Jim  VE7RF

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