[ARC5] R-10 in the receivers.

[Brian Clarke]

Be very careful about accepting explanations in military technical
manuals.  During war conditions, able-bodied and intelligent people
were occupied in designing, making and using this stuff. Very few had
time to write explanations. The writing of explanations was often
given to prisoners, for whom high-level education in physics and
electronics would have been unlikely. I have found many explanations
that read well, but in terms of demonstrating electronics knowledge,
were pure, arrant nonsense. This explanation falls very near that
category. The problem is that a small part of it is probably correct -
and then the well-known psychological principle of the 'halo effect'
takes over.
Also, use of carbon pots in place of the original wire-wound pots will
result in noise that the 3 uF capacitor will not remove. That 3 uF
would be good for the noise in a wire-wound pot as the wiper moves
from one wire to the next. But with a carbon pot, sections will
develop that will be open circuit. You know the effect that will have.
73 de Brian, VK2GCE.
Brian Clarke
BE, MBA, PhD, CPEng, FIEAust
MD, Clarke & Associates P/L

OnTue, 23 Sep 2014 21:35:09 -0500 (GMT-05:00), Mike quoted:.

 >A friend who is working on is first BC-453, which was murderfied
before he 
 >got it, and which he is trying to get back in operation, has called
my attention 
 >to R-10, a 360K 1/2 watt resistor which is connected between the 
 >gain-control line and B+.
 >
 >The table of replaceable parts describes that resistor as, "H.V.
Bleeder to 
 >Gain Control."

 It's all in the readily-available SCR-274-N manual (all versions).

 From T.O. 12R2-3SCR274-2 pages 23, 25
 ( http://bama.edebris.com/manuals/military/scr274/ ):

 "Gain or volume is manually controlled by a 0-50,000 ohm variable
resistor
 located in the Radio Control Box BC-450-A. (This may be R-25, R-26,
or R-27
 depending on which control section is being considered) The cathode
circuits
 of the RF amplifier and first IF amplifier are completed to ground
through R-25.
 As this resistor is increased from 0 to 50,000 ohms, the voltage
between ground
 and either cathode increases, and since the grids of those tubes are
at ground
 potential for dc, the grids become increasingly negative with respect
to the
 cathodes. This results in a reduction of amplification in each of the
controlled
 tubes, and in an overall reduction of the "gain" of the receiver by a
factor
 exceeding 50,000 to 1 as R-25 reaches its maximum resistance. About
.6 of a
 milliampere of dc flows from the +250-volt dynamotor line though R-10
and R-25
 to ground. The voltage across R-25 is thus greater than it would be
if only
 the cathode current flowed though it, and this voltage is also less
dependent
 on the cathode current of the controlled tubes. In the minimum gain
position
 of R-25 (R = 50,000 ohms) there is a difference of potential of about
30 volts
 even though the cathode current is negligible. From this it may be
seen that
 R-10 acts to make the control voltage developed across R-25 less
dependent
 upon the cathode current of the tubes being controlled."

 Mike / KK5F

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