[ARC5] Re: Question about ARC-5 mods

[Neil Barnett]

  ----- Original Message ----- 
  From: Tom Bridgers 
  To: neilba at clear.net.nz 
  Sent: Sunday, September 12, 2004 1:25 AM
  Subject: Question about ARC-5 mods


  >Neil...
  >The analysis you gave of the ARC-5 mods seems to suggest that the 12A6 audio stage can be tweaked enough to provide good listening levels >through a loudspeaker.  I believe that also meant one needed to change the output transformer to one having a 4 ohm or 8 ohm speaker secondary.

  Tom, you're correct in that assumption.
  Under the biasing conditions for loudspeaker operation given in
  my earlier post, the 12A6 will be properly matched if it sees a
  reflected load impedance of 7.5 K ohm.

  Any single-ended (not push-pull) tube type of output transformer
  having a 5 K to 7 K primary and designed to operate into a 4 ohm
  or 8 ohm 'speaker will be satisfactory.


  >Now my question is this:  What about going on step further and changing the 12SQ7 triode section from being the BFO oscillator to one being the 1st >preamp of audio driving the 12A6?  How does the 12SQ7 audio mod improve the audio output and how good or worthwhile would that mod be?

  >I've been torn between doing the 12SQ7 audio 1st stage mod, and then adding a transistor circuit to do the work of the former BFO stage.  Because >you do need the BFO to receive SSB.

  >And of course, there is the reverse of the above scenario.  Keep the 12SQ7 triode section as a BFO, but add a one or two stage transistor preamp >audio stage to drive the 12A6.  I don't know if such is possible, but the thought does rattle around in my brain!

  >Thanks in advance for any help on this or reference material you might point me to.
  >-Tom

  Firstly, I think modifications should be kept to a minimum,
  unless they are essential for putting a piece of equipment into
  running condition.

  Getting rid of the BFO is a step you'll regret. As you've pointed
  out, the BFO is essential for receiving SSB (and CW) and it is
  also a useful zero-beater for tuning AM signals accurately.

  I'm not convinced that its necessary to turn the triode section
  of the 12SR7/12SQ7 into an audio pre-amp. Its been a long
  time since I owned an ARC-5 receiver, but as I recall, the 12A6
  was capable of delivering plenty of audio, and didn't need a
  pre-amp.

  I think you'll find the audio output to be more than adequate if you
  do the following:

  1. change the 12A6 cathode resistor to 390 ohm, 5W.
  2. disconnect the 12A6 cathode bypass capacitor, and solder in
      a 47uF,  25 volt electrolytic instead.
  3. replace the original output transformer with a 5 - 7 K to 4 ohm
     or 8 ohm 'speaker version.

  If there's still not enough audio, you probably have an unrelated
  problem in some other stage, or the 12A6 is bad.

  In my receiver years ago, I replaced the 12A6 grid resistor (2 Meg)
  with a 2 Meg volume control pot mounted in the front panel control
  box. An even better arrangement worth trying is a 1.5 Meg resistor
  in series with a 470 K pot, with the 12A6 grid connected to the pot
  slider.

  Those values retain the original grid loading of 2 Meg, and mean
  that the 12A6 grid will never be more than 470 K above ground, but
  there may not be enough audio appearing at the 12A6 grid in this
  setup. Its worth trying though. How sensitive is your sixty-five year
  old 12A6 ?

  73 de Neil, ZL1ANM
  in Auckland.



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