There are a lot of broken variac transformers around. Often a few turns
burnt on end of winding or broken brushes.
I have used them to make low voltage high current transformers by removing the
hardware and wrapping heavy wire
thru the core. If I recall it works out for most higher power ones about 2 to 3
turns per volt. De-rate the transformer by
1/2 its VA spec.
By putting in spacers between the primary and secondary you can make a low
capacitance transformer and avoid having filament choke.
You may want to put a Fariday shield around the primary to ground to keep RF
from getting onto the power line.
73
Bill wa4lav
________________________________
From: Amps <amps-bounces@contesting.com> on behalf of Martin Sole
<hs0zed@gmail.com>
Sent: Saturday, February 8, 2020 8:01 PM
To: amps@contesting.com <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://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fludens.cl%2FElectron%2Ftrafos%2Ftrafos.html&data=02%7C01%7Cwlfuqu00%40uky.edu%7C1e81c1a8f79b4182e98708d7acfb91d7%7C2b30530b69b64457b818481cb53d42ae%7C0%7C0%7C637168068793334101&sdata=9cnJwT569oMq032ZR%2BVOguO74BLhfI9Q%2FUx9wYdJvXE%3D&reserved=0
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
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Flists.contesting.com%2Fmailman%2Flistinfo%2Famps&data=02%7C01%7Cwlfuqu00%40uky.edu%7C1e81c1a8f79b4182e98708d7acfb91d7%7C2b30530b69b64457b818481cb53d42ae%7C0%7C0%7C637168068793334101&sdata=iM7Bo7JScAlcexGjXPzwjLvhxAe05Fix%2F6tyiTXecZY%3D&reserved=0
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Flists.contesting.com%2Fmailman%2Flistinfo%2Famps&data=02%7C01%7Cwlfuqu00%40uky.edu%7C1e81c1a8f79b4182e98708d7acfb91d7%7C2b30530b69b64457b818481cb53d42ae%7C0%7C0%7C637168068793334101&sdata=iM7Bo7JScAlcexGjXPzwjLvhxAe05Fix%2F6tyiTXecZY%3D&reserved=0
_______________________________________________
Amps mailing list
Amps@contesting.com
https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Flists.contesting.com%2Fmailman%2Flistinfo%2Famps&data=02%7C01%7Cwlfuqu00%40uky.edu%7C1e81c1a8f79b4182e98708d7acfb91d7%7C2b30530b69b64457b818481cb53d42ae%7C0%7C0%7C637168068793334101&sdata=iM7Bo7JScAlcexGjXPzwjLvhxAe05Fix%2F6tyiTXecZY%3D&reserved=0
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|