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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