[Amps] 3CX-3000A7 at 144

[Jim Thomson]

Date: Thu, 6 Feb 2020 11:55:07 -0600
From: Joe <nss at mwt.net>
To: peter.voelpel at t-online.de, amps at contesting.com
Subject: Re: [Amps] 3xx3000 at For tubes like this, BIG currents on the filaments.

<Has anyone made a switcher supply for it?

<Joe WB9SBD

##  Dont  waste  your  time.  7.5  vac  at 50A  fil  xfmrs  are a dime a dozen.  (center  tapped)..with a myriad of  primary  voltage  taps.  My  dahl  is  tapped  at  198-208-218-230-240-250  vac. 

##  Buddy  tried  using a  switcher  supply for  his  hb  2 x  GS35B  6M  amp...  4.7  kw  out.  Separate  switcher  for  each  tube  fil.  Problem  was,  even  though  each
switcher  has...soft  start  built in,  the  cold  resistance of  a  directly  heated cathode  is  typ only .1  to  .083  of  the  hot resistance.   So  even  though each  switcher was  rated  for  DOUBLE  the  ccs  rating of  the  tube,
the  switcher  would  still go  into   instant  shut  down  mode.   So  in  went an  even  bigger pair  of switchers,  now  rated  for  QUADRUPLE  the  CCS  tube  rating.  Ok,  works,  no  more  shut  down.   New  problem. The
B-  return on  a GG  triode is  via  the  CT  of  the  fil xfmr. Switchers  dont  have a CT !   A  DC  to  DC  converter  complicated  setup  was  engineered.   Worked  fine  until  one  day  we  lost  the  B-   connection....
problem  with the BLACK  Millen  HV  connector.   All  hell  breaks  loose  when  the  B-  is  opened  off  on a GG  Triode.  The  cathode  will  try  and  assume  full  B+  potential.  Of  course,  both  HD  switchers and  the
elaborate  DC  to DC  isolator  setup....all  blew  their  brains  out. 

##  Plan  C....  installed  2 x  new  hammond fil  xfmrs, one  per  tube,  each  with  its  own  CT.   Metering rewired,  such  that  one  plate  current meter  can  be  switched  to read   individual plate  current,  or  combined. 
1,2,  1+2.   Idividual W4ZT  adjustable bias  boards  used,  one  per  tube.

##  On  my  own  160-15 M   3CX-3000A7  amp,   A  sola  constant  voltage  xfmr....  feeds a regulated  240 vac  into  the  input  of a small 0-240 vac  at  5A  varaic.  Output  of  variac  feeds  the  input  of  the  dahl  fil  xfmr.
I  use  the  variac to  bring  up  the  fil  voltage  slowly. RF  chokes on each leg of  the 3x3s cathode.  Wired  to  both a 0-10  vac panel meter...and  also a pair of pin  outs.  Fluke 87A  plugged into pinouts for precise reading..
and  also to  calibrate the  panel  meter.  Measure fil  V..at the  socket,  with a 87-A.  Make sure  there are NO Vdrops  across  the chokes....  due  to loop current from  any  panel  meter.     

##  On  buddys  3x6  HF  amp,  no  sola  used,  but a  step  start  resistor  +  timer  used  to  shunt the  resistor....which  precedes the  variac.  His  variac  tweaked,  then  never touched  again.  

##  RF  parts  sells an  excellent  socket  for the  3x3.    The  F7  version  does not  require  a socket.  RF  parts  now  sells  a superb  HD  chimney,  made  from  .25inch  thick  acrylic.  Top  lined  with  .125  inch  thick
solid  teflon  doughnut.....whose ID  just  fits  the  OD  of the  3x3. 

Jim   VE7RF