[Amps] 3CX3000A7 on 144 MHz

[Alek Petkovic]

Simplest way to get a transformer is to get an old arc welder and use 
the transformer out of it. There is more than enough heavy gauge wire on 
it to strip it down and rewind it for 7.5V or whatever you wish.

Old arc welders should be a dime a dozen. They are here. We just go to 
the local rubbish tip where they are generally for sale for around $5.00.

Good luck.

73, Alek. VK6APK


On 9/02/2020 8:46 am, 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 <amps-bounces at contesting.com> On Behalf Of Jim Thomson
> Sent: 08 February 2020 19:35
> To: amps at contesting.com
> Subject: [Amps] 3CX3000A7 on 144 MHz
>
> Date: Fri, 7 Feb 2020 16:05:05 -0800
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: amps at contesting.com
> 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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