[Amps] 3CX3000A7 on 144 MHz

[Jim Thomson]

Date: Fri, 7 Feb 2020 16:05:05 -0800
From: Jim Brown <jim at audiosystemsgroup.com>
To: amps at 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