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.
73
Peter
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Martin Sole
Sent: Sonntag, 9. Februar 2020 02:01
To: amps@contesting.com
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 https://ludens.cl/Electron/trafos/trafos.html
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 <amps-bounces@contesting.com> On Behalf Of Jim Thomson
> Sent: 08 February 2020 19:35
> To: amps@contesting.com
> Subject: [Amps] 3CX3000A7 on 144 MHz
>
> Date: Fri, 7 Feb 2020 16:05:05 -0800
> From: Jim Brown <jim@audiosystemsgroup.com>
> To: amps@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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