[ARC5] Heater voltage split in "command" receivers

[Leslie Smith]

Hello Ken,
I'm certainly impressed that a set will run from 1/2 the supply voltage.
"Bragg apparatus" is a simple piece of lab equipment used to replicate
the work of William & Lawrence Bragg.
It consists of a "toy"  X-ray tube, X-ray detector and a rotating table
to measure deflection of the X-beam by a salt crystal.
It's interesting because it allows you and me (and a first year physics
student) to repeat the Nobel prize winning experiement of the two
Braggs, from 1910 or 1912 (around that period).  Amazing!  This is the
first (and only time) I reproduced such a significant experiement.  
Actually - not true.  I remember the Millikan oil-drop experiment ... 

After many years of use the gear died.   (sigh)  There was a problem
with the heater circuit in the X-ray generator.  (Heater circuits are
very complex - there is a voltage source, and a load resistor (the
filament) and these two must be wired correctly, or the heater remains
cold.  Much trickier than - for example - a light globe.)   A modern
current limiter cct was built - using one of the 78xx regulators to
provide EXACTLY the specified heater current to the X-ray generator. 
And it was "spot on".  I checked it many time over a period of some
hours.  But the gear wouldn't generate X-rays.  After a LOT of checking
I found that "bumping" the filament current up by a fraction (a few mA)
over the spec made all the difference.  The difference in filament
current between no X-rays and correct generation was very small.  I was
very surprised.

I think vacuum tube operation is less demanding than X-ray tube
operation, but a 50% reduction in filament voltage IS significant.

73 from Australia 

  Les Smith
  vk2bcu at operamail.com


On Sat, Jan 26, 2013, at 5:03, Kenneth G. Gordon wrote:
> On 25 Jan 2013 at 21:13, Leslie Smith wrote:
> 
> > Yes, Ken, I did the same, and made the same observation.
> > The heater resistances are unequal.
> > 
> >   Les
> >   vk2bcu at operamail.com
> 
> I suppose an high-enough quality-control to preclude such things was way
> at 
> the bottom of the priority list at the time. :-)
> 
> What prompted this investigation on my part was that the tubes were not 
> getting particularly hot. I was somewhat concerned that the filament
> voltage 
> was VERY low. It wasn't.
> 
> Also, I was reading in one of Gordon White's early articles that in one
> case, 
> the Army was testing an early version of the "K" model 28 Volt receiver
> in an 
> Army plane with a 14 volt system, and were very impressed with its 
> advanced capability despite the 1/2 voltage on the filaments.
> 
> Ken W7EKB
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