I bought a 8V 80A transformer in the Netherlands:
http://www.elektrodump.nl/en/
They offer modulation transformers custom made, so probably can wind heater
transformers as well.
73
Peter
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Conrad PA5Y
Sent: Sonntag, 9. Februar 2020 01:47
To: amps@contesting.com
Subject: Re: [Amps] 3CX3000A7 on 144 MHz
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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