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Re: [Amps] 3CX3000A7 on 144 MHz

To: Conrad PA5Y <>, <>
Subject: Re: [Amps] 3CX3000A7 on 144 MHz
Date: Sun, 09 Feb 2020 16:54:36 -0500
List-post: <>
I don't think you should give up on the switching power supply yet.
I have recently built a 1.5 kW solid state PA for DXpeditions and because of the weight issues, I had to use a switch mode power supply (49 V, 40 A). I put the power supply into its own metal case and thoroughly filtered both the input and the output. I used an RF gasket under the lid and put a shielding mesh over the fan air intakes. The power supply is dead quiet. Using a test receiver with small magnetic loop I could barely detect switching nois directly at the output terminals. The noise drops below -140 dBm at 10 mm from the terminals. I have the PA in December on a small island in the Bahamas (where the ambient noise is very low) without any issues. It may be that this will be an easier (and cheaper) path than a custom transformer. There are many low cost surplus 5 or 6 V SMPS-s that can be tweaked to output 7.5 V. Alternatively, you could put two 5 V units in series, each adjusted to 3.5 V.

GL and 73,

On Sun, 9 Feb 2020 00:46:39 +0000
 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.


Conrad PA5Y

-----Original Message-----
From: Amps <> On Behalf Of Jim Thomson
Sent: 08 February 2020 19:35
Subject: [Amps] 3CX3000A7 on 144 MHz

Date: Fri, 7 Feb 2020 16:05:05 -0800
From: Jim Brown <>
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 

<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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