[Amps] 3CX-3000A7 at 144

[Steve Thompson]

GS35B, 8877 and the like have separate cathode - they don't need a 
CT for the B- return. DC switcher supplies work just fine as long 
as they go into constant current rather than tripping out on overload.

Steve G8GSQ

> ##  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.