Re: [Amps] 3CX3000A7 on 144 MHz

[GEORGE WALLNER <aa7jv@atlanticbb.net>]

Conrad,
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,
George,
AA7JV



On Sun, 9 Feb 2020 00:46:39 +0000
 Conrad PA5Y <g0ruz@g0ruz.com> 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 
>
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