[Milsurplus] ARC-5/SCR-274 Receiver HV

[kgordon2006 at frontier.com]

On 7 Sep 2025 at 10:23, Ken Kinderman wrote:

> 
> Has anyone had experince with receiver HV lower than the nominal 235 VDC dynamotor output? I like to 
> run everything at or slighly below ratings. Not interested in squeezing out every last watt from the 
> transmitters, etc.

Yes. As a matter of fact, there was quite a long discussion on this very subject on the ARC5 
forum some years ago.

In the case of the ARC-5 receivers, it was found that they worked just fine at voltages as low 
as 24 VDC, the only downside of this being that audio output was much reduced.

As I brought up at the time, a ham wrote an article on how he reduced the operating voltage 
for a Drake receiver in steps to as low as 12 VDC, finding that receiver noise was much 
reduced, and the radio worked as well as it did at the higher voltage, EXCEPT that audio 
power output was very much reduced. He finally raised the voltage back up to about 50 volts 
and left it there.

Our own Dave Stinson routinely operates ARC-5 receivers at 24 VDC.

> So except for the times I am using the dynamotors, I run the receivers
> in the test jig and out of the racks at about 175VDC, transmitters at
> about 540VDC. (By the way, even 175VDC is enough to blow the .05's, so
> I am on a long term quest to replace all, either with re-stuffed cans
> or just wiring them in. Otherwise, letting them fail is like playing
> condenser Whack-a-Mole) 

And FYI, I found that 170 VDC was about the "best" compromise for the ARC-5 receivers 
too, although one had to increase the screen voltage to the same as the plate voltage. For 
the ARC-5 receivers, all one has to do is to move ONE wire from the junction of the two 
vertically-mounted resistors in the back of the receiver to the hot end.

The RAX series of receivers normally operate at (as I remember it) 180 VDC. Someone will 
correct me on this, if I am off.

> 
> My question is about the receivers... aside from maybe the 12A6, are
> there any downsides to lower overall plate voltage? 

Screen voltage must be adjusted upwards. Otherwise, there is NO downside, other than, as I 
have said, audio power output. And there are certain definite advantages to using the lower 
voltage. For one thing, as I said, receiver noise is much reduced.

Ken W7EKB