[MrFixr55]

Hi Arran, 

Yes, you are correct, the "nominal" voltage for these Loctal tubes was the same 6.3 or 12.6 V as the other tubes whose numbers began with either 6 or 12.

My guess is that the Loctal tubes were meant for auto radios.  Since a car electrical system could run as high as 7.2V or so for 6V systems and 14.5V or so for 12V systems, this may be the reason for the nomenclature used for the 7 and 14  Loctal tubes.  Note that the first number, denting heater voltage was not changed for the 35V and 50V tubes between Octal and Loctal tubes.  Granted that the stated voltages for these "7" and "14" tubes are the same 6.3 and 12.6V as the tubes that they replaced, but I believe that when the original tubes were released for car radio use, they had wider filament specs than other tubes.  I don't know if these "7" and "14" numbers came into existence because type numbers in the "6" and "12" range were already reserved for the plethora of tubes to be introduced in the late 1940s and early 1950s.

Since Philco became a major supplier of automobile radios, Philco was an early advocate of using these 6V tubes, originally designed for DC filament or heater operation in car radios, in home radios, supplying these heaters with 6.3VAC, while others, especially RCA, advocated for the series of tubes using 2.5V AC heaters.  this allowed Philco to keep a trimmer tube inventory than RCA and others who were making both home radios using 2.5V tubes and car radios using 6V tubes.   Philco was also an advocate of the Loctal tube types that Sylvania was introducing, even using them in their TV sets where locking in the tube to prevent it from being dislodged by vibration was not a particularly big issue.

I would guess that some of the "7" tubes had 150 mA filaments for both economy (less power draw) and for compatibility with the 150 mA heater strings that the AA5 sets started using with the development of the 50L6 / 50A5 and the 35Z5 / 35Y5 to eliminate ballast resistors and "curtain burner" power cords.