[Milsurplus] 24V B+

[Ray Fantini]

All the reasons for operation at lower B+ are good and valid reasons but my point is that by operating the radio at or around 80 to 90 volts still provides most of the benefits and allows the tubes to act as amplifiers. Look at the plate voltage vs. plate current curves.

http://www.nj7p.org/Tube4.php?tube=6sk7

According to the published curve you can see the tube will be an amplifier at maybe 40 or 50 volts but at 20 volts the tube may just be passive at best.  Did not look at the 12K7 or the 12SR7 but the 12SK7 just not a tube designed for low voltage operation.
Myself I am not concerned with the ARC-5 as much as trying to develop guidelines for operating many of the tube items I have and use. Good examples are my BC-348 or HRO-RAS receivers because I use these radios on air at the home QTH for long range QSO, so I would prefer that they work along the lines of original design. If you have the pristine example of a radio that you feel is historically significant that's going to limit what you're willing to risk with it but for me the radios I have are going to be operated. It's like the vintage or antique car thing, many people own cars that are so expensive or unique that they don't dare drive them, but if I own a car you can be cretin that it will be out there on the road. Maybe that's why I prefer to get the radios that have been hacked or modified? But that is the reason for looking into how to get the maximum life from tubes that in many cases are over fifty years old. 
A interesting side note to all this is that I have found of the primary tube radios that I am using in the shack that include the BC-348, HRO/RAS, ARC-38 and although it has no military significance my RCA-BTA1 160 meter transmitter that the military radios are very good on operating the tubes well within their envelope and with my limited use the tubes last forever. The broadcast transmitter if ran at full power will eat 807 and 833 tubes at least once a year and most Ham transmitters with tubes appear to always be in need of output tubes but the military stuff almost never needs new tubes. But for reasons that I cannot explain I become obsessed with obtaining spare tubes, and in many cases spare radios to support the things that I am using. Think I have enough spare tubes to last for at least the next fifty years.


RF