[ARC5] R-10 in the receivers.

[Mike Morrow]

>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