[ARC5] power supply hum

[Leslie Smith]

  Hello Robert,
  I got another e-mail suggesting I consider common current in the audio
  & power lines.
  The heaters and AF share a common ground wire, so I'm putting 450mA
  down the same earth line that provides the signal to my phones.
  The hum appears the moment I turn the power on - well before the 12A6
  filament is hot.
  This is a good clue.

  If I pull the 12A6 it will cut the current thru the earth line by 1/3.
   If the hum drops in level, but doesn't disappear, this will be a good
  indication.
  (Observation)  A drop of 1/2 in signal will be hard to hear - because
  ears are logarithmic.
  I'm going to separate the power and AF lines.
  Thanks for your observation.
  Will report the result.

  Meanwhile - I'm still taking measurements around the 12K8 local
  oscillator.
  A new 12K8 hasn't solved the problem.
  DS suggested I look at the tuning gang, and I intend to return to this
  area.
  The LO works when the tuning is set to around 200kHz.
  The LO stops working when I tune the set above 270 .. 280 kHz.
  I put the CRO on the oscillator plate lead.  ( see 15 to 20 volts RMS)
  
I put the CRO on the oscillator grid (triode section, pin #5).
I see nothing.

  I'm trying to get the osc to work around 500kHz, so I can hear it on a
  BC receiver.

  Thanks for you help,
  

  73 de Les Smith
  vk2bcu at operamail.com


On Fri, Jan 23, 2015, at 03:07, WA5CAB--- via ARC5 wrote:
> Les,
> 
> Pull the 12A6 and see if you still get the hum.  If you do, its source 
> should be pretty simple to find.
> 
> Robert Downs - Houston
> wa5cab dot com (Web Store)
> MVPA 9480
> 
> In a message dated 01/22/2015 07:24:02 AM Central Standard Time, 
> arc5 at mailman.qth.net writes: 
> > Les,
> > 
> > Since you notice hum before the tubes have warmed up,
> > the 12A6 should not yet be drawing plate current. In that
> > case there are other possible sources  of AC getting
> > into the audio.
> > 
> > Depending on their relative locations there can be magnetic
> > coupling between the power and audio transformers. I
> > learned about this the hard way :)
> > 
> > Also there can be voltage drop in a wire or ground connection
> > common to power and audio circuits.
> > 
> > Elden
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