[ARC5] Filaments - How Low Can You Go?

[Leslie Smith]

Hello Wayne,
Good question.  Speaking generally, it's hard to go lower than a snake's
belly, but that idea's got nothing to do with ARC-5 sets, has it? 
Speaking of the sets, I think there are useful "clues" to answer your
question in the tech manual. I refer here to the 250 odd page document
commonly available on the net, (AN 16-30ARC5-2) also called AN-ARC-5
Aircraft Radio Equipment.

Generally speaking the ARC-5 sets (I include ARA in this) are
transformer coupled.
The exception is the supply to the BFO and local oscillator circuits,
where a good sized dropping resistor is in series with the supply.  Now
check the diagram of the plate voltages.
This is figure 5-1.  Check pin 6 (anode) of the detector (12SR7).   The
plate voltage is 85.

Now check the circuit and find the plate dropping resistor - in the R-25
(and above) it's 100k.  It's higher in the R-23.  

Next, turn to the mixer circuit.  The plate dropping resistor, R6, to
the LO is in the order of 200k.  According to the diagram (5-1) the
plate voltage lies between 13 and 29 volts.  Yes!  I took the
opportunity to measure the plate voltage when I ran a set from 60v B+
supply.  It was not significantly "off" the spec - but the set suffered
from distortion because the injection power was down.  I replaced the
dropping resistor with a lower value (can remember the new value) and
the distortion was gone.

There has been some discussion here about audio output, and output
power.
Others on this list have noted a need to correctly match the speaker
impedance to the set.
I found a set would deliver listen-able levels to an 8 ohms speak from a
60V B+ supply, and this was without the lower value cathode resistor
said to increase power out.  

As for starving the filaments, I wouldn't under-run them by more than
5%.
There is little disadvantage in a lower B+ supply - see my notes on
ARC5_Radios at Yahoo.

Best wishes from Australia


  Les Smith
  vk2bcu at operamail.com


On Fri, Oct 19, 2012, at 23:45, Robert  Eleazer wrote:
> Here is a stupid question for y'all.
> 
> Considering that the overall recommendation is to operate the command
> receivers with reduced B+, just how low can you go on the voltage for the
> filaments before you have a problem?
> 
> With say, 125V B+ and 12V filament power with a command receiver that has
> 12V tubes and wired for 24V, how well will it work?
> 
> In experiments with my first command set, an R-11A, I was astonished that
> I could get the B+ down to around 28V and it was still working, if rather
> faint on the audio output. 
> 
> Thanks!
> 
> Wayne
> WB5WSV      
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