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Re: [Amps] 3CX3000A7 on 144 MHz

To: "'Martin Sole'" <>, <>
Subject: Re: [Amps] 3CX3000A7 on 144 MHz
From: <>
Date: Sun, 9 Feb 2020 13:00:09 +0100
List-post: <>
I bought 500VA and 750VA  toroid core transformers from a German factory
without secondary windings, just the primaries.
I used them to wind my heater transformers in the past, if I remember right
they were about 0.35-0.5V/winding.


-----Original Message-----
From: Amps [] On Behalf Of Martin Sole
Sent: Sonntag, 9. Februar 2020 02:01
Subject: Re: [Amps] 3CX3000A7 on 144 MHz

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.
> Regards
> Conrad PA5Y
> -----Original Message-----
> From: Amps <> On Behalf Of Jim Thomson
> Sent: 08 February 2020 19:35
> To:
> Subject: [Amps] 3CX3000A7 on 144 MHz
> Date: Fri, 7 Feb 2020 16:05:05 -0800
> From: Jim Brown <>
> To:
> 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 quiet.
> <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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