[Boatanchors] Acceptable filament voltage range

[Rick Poole WA1RKT]

Anybody know what the average acceptable filament voltage range is for most small receiving tubes as in, for example, a KWM-2?

I've checked some of the data sheets and they only give a nominal 6.3V range, they don't specify the minimums (or the maximums).

Each of my three Collins 516F-2 power supplies gives from about 6.0 down to occasionally as low as 5.9 volts RMS to the filaments at the radio, which is around 5% under spec (this, by the way, is with the power supply plugged into the wall without a variac or bucking transformer).  I can't find any specifications that tell me if this is too low, or not.

Thanks...

Rick WA1RKT 
Londonderry, NH 
Catch me on 14.300 MHz Intercon / MMSN most days