[Amps] Tips For Winding a Plate Choke

[Jim Thomson]

Date: Thu, 10 Oct 2019 11:38:25 -0400
From: Roger D Johnson <n1rj at roadrunner.com>
To: amps at contesting.com
Subject: Re: [Amps] Tips For Winding a Plate Choke


<A simpler alternative is to switch another choke in series for the lower
<frequencies. This is quite simple to do. Think of two chokes in series
<with a capacitor connected to the junction of the chokes. A relay grounds
<the other side of the cap on the high frequencies thereby effectively
<removing the lower choke from the circuit.

<73, Roger

##  That trick  was  used  by  asi,,and  its  forerunner  company. 
50uh  +  135uh.  Both chokes  used  on  160m.    50uh  choke  used
on   80-10m. 

##  Both  chokes  in  series cant  be  used on  80m......  u  will  get
fireworks  and  huge  arcs  across the  junction point....and  also
fried  relay  contacts.

##  Buddy  tried   135uh  +  50  uh...and  also  170uh  +  50  uh.  
Same  deal,  both  in  series  can  only  be  used on 160m  only. 
Since  the  HB amp   was  160-15,   final config  was  135uh  +  70 uh. 

##  One  other  important  note,  3 x  1 megohm  3 watt  mofs  in  series
are  wired  between  relay  contact  and  chassis. ...  IE:  cold  end  of cap.  THEN  the  junction cap
is  in  DC  block  mode.    With the  spst-NO  mech relay  energized,  the 
Cap  is  now  RF  grounded...as  well  as  DC  grounded     Without  the  3 x megs   in  place,  u  will get
arcs  across  the  open contacts when  relay  de-energized.   Reason is  cuz
of  the  leakage R across the  cap....  even  HEC   HT-57   15  kv  types.  

##  we  tore  our  hair  out trying to  figure it  out.    In  desperation,  we  noticed 
Alpha used  the  same  2 x choke  config  +  junction cap......  but  used spare contacts on  bandswitch 
instead  of a relay.  Alpha  used  3 x 1 meg,  1/2  watt  Carbon  comp  resistors  in  series.....wired in parallel  with
contacts.   Cold  side of cap  is  dc  grounded on  low  band....and  RF +  dc  grounded  on  high  bands.   So  we ripped
off  the alpha resistor concept.....  which  works. 

##  another  method Is  2 x chokes, big and  small....  with  bottom of each choke fed  with  B+.  Bypass  caps
at base of  each  choke.   A  spst  vac relay  with  COM  wired to anode.   NC wired to  top  of  1st choke. 
NO  wired to  top  of  2nd  choke.   Be  careful with this config !    Vac relay will  have  B+    plus  peak  RF  voltage
on  its  contacts.   When  tube  is driven  full  bore, the  RF  voltage  developed in the  tank  circuit is AC.....and  will
backfeed  through the  plate  block  caps.....and  superimpose  itself onto  the anode.   Total peak  V  will  be a little
bit  less  than double the  loaded  B+  voltage.   The  advantage is.... each  choke  can  be  optimized.   IE:  200-300  uh
for   160+80m  for  choke one......... and   50uh  for  choke  two.   The  drawback  is... the  spst vac relay  has to be  able to
withstand the  B+   and the  rf  peak voltage.   RF  voltage rating of vac relays  drops  like a rock  as  freq  increases. 

##  The  ameritron  choke....if  u  cant make  it  fit....  is the cheap and easy way  out....even if a few turns  have to  be
removed  from the bottom...as  in  martins 30m  band issue. 

##  2 x ameritron  chokes... WELL  spaced  apart..with separate  B+  and  bypass  caps  at  base of each  choke....  and  each  top separately
wired  to anode....  will  also  work.  aluminum  bar  acrosss  the  tops  of a pair  of 15 kv  HEC  bypass  caps.  Choke  mounted on  top  side 
of  al  bar...dead  center.   Used  for  amps  that  require   up  to  3A   of plate  current.   Then  u can  use 8  kv  for  B+.       

Jim   VE7RF