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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