[ARC5] Command rx B+ supply - 60V vs 90V

[Leslie Smith]

 Hello list-readers,   

                                                   Regarding B+ supply
                                                   voltage.

  When I got my first "command" set I  I listened to the performance of
  the set, on air, during the Remembrance Day Contest, using a B+ supply
  of +60 and +90V DC.
  With an insignificant aerial, I heard ZL stations better than many
  other stations here in Australia.

  GENERAL PERFORMANCE.
  I compared the performance in operation of the sets.
  I found no difference in operation (that I could hear). 
  (I was using 'cans'.  Yes folks, I had genuine Department of Defense,
  WWII 'cans', the real thing.  No loud-speakers here!)
  I knew nothing else about these sets; somehow I DID find a circuit
  diagram that allowed me to figure how they might be made to work.
  I noticed the 'toobs' were cooler to touch when operating from a +60V
  supply.

  L.O. OPERATION.
  After I committed the 60V doubler to a PC card, I discovered that the
  local oscillator and BFO were starved for plate current when operation
  from the lower +60V.
  (Grrr!)    Yes, some operate from a low B+ line, as Dennis reported
  not long ago.
  My 'test' set was one of the 'lucky' units  where the BFO and LO start
  easily.
  Some don't, as Dennis wrote before.

  If you wish to operate a set completely unmodified, don't use a 60V B+
  line.
  If you measure the working voltage on the oscillator plate(s) it is
  close the the figure shown in the manual, but the 'toob' is starved
  for current.

  Consequently:    I solder a low value R across the plate dropping
  resistor in both oscillator circuits.  (18k to 33k)
  Doing this, the oscillators are reliable.  

  SOUND LEVEL.
  Using a 60V supply, I get 'soft' volume from the speaker.  It's
  suitable for listening in quiet conditions.
  A 90V supply WILL give more volume.

  If I re-designed the power supply, even now, I can't say whether I'd
  choose the B+ line to be +60V or +90V.
  At the lower voltage, the 'toobs' run wonderfully cool.  I figure my
  70 year old "command" set may still be working in another hundred
  years  - long after the AM broadcast band is gone.
  

  73 de Les Smith
  vk2bcu at operamail.com

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