[ARC5] ARA heater configuration

[Kenneth G. Gordon]

On 4 Jul 2016 at 11:09, Jack Antonio wrote:

> There is a "gotcha" here.
> 
> ...the AN/ARC-5 receivers use a 12SF7 as the second IF amp, which
> has a different basing diagram from the 12SK7 used in the ARA and SCR-274N.

The 12SF7 has an additional diode internally to provide AGC voltage for the 
up-dated AGC circuit in the AN/ARC-5 receivers. Furthermore, the base 
diagram of the 12SF7 is enough different from the 12SK7 that they cannot be 
interchanged by simply plugging one into the socket wired for the other.

The reason the AGC voltage is taken from the 2nd IF tube rather than from 
the detector which was most commonly done at that time-period is so that the 
AGC can be on continuously and it will not be effected by the BFO. 

Therefore, the AGC works whether or not the BFO is on. In fact, the AGC 
cannot be turned off: it is on continuously whenever the receiver is powered 
up.

In almost all other receivers of the period and for some time afterwards, the 
AGC had to be turned off in order to use the BFO, and vice versa. Otherwise, 
the BFO signal-voltage would raise the AGC voltage so high that it would 
reduce the gain of the receiver.

This is yet another example of the genius of the designers of these receivers.

Ken W7EKB