[BrendaAnnD]

I've a question. It has probably been answered before, but here goes:

Why do loctal tubes have designations like 7B7 or 14B7, when in actuality the filament voltages are 6.3 and 12.6, respectively? Were they running out of letter designations for 6 and 12 volt tubes by that time? I note that ones like 35Y4 use the standard filament voltages.

[Mondial]

The way I understand it, Sylvania originally designed and marketed the Loktal tubes for mobile applications like car and aircraft radios. They rated the heater voltages at the nominal voltage of lead acid batteries under charge, 7 and 14 volts. If you look at early Sylvania tube manuals, the heaters are rated at 7.0 V and .32A or 14 V and .16 A

Of course it was also a way to differentiate this new line of tubes from the competition, so there was probably some marketing considerations involved.

In later manuals, Sylvania just rated the tubes at 6.3 V, .3 A like everybody else. It is doubtful that the heaters were any different than those used originally, they just previously rated them at the max voltage of 7 V instead of the usual design center average 6.3 V

[BrendaAnnD]

Thank you.

[codefox1]

Yup, heard that one before about the filament voltage. Good old Loctals should fare well with increased line voltage we see today, and other than corrosion and improper removal from sockets may outlast many of us. So a Loctal should be good up to 122 volts supply if fed from a supply that was designed to run on 110 volts. Been a while since I had to toss one, other than glass types dropped on the cement floor.

[Arran]

Actually if you read the spec sheets on most loctal tubes, like a 7C6, 7A8, 7B7, etc. the filament current is only 150 ma rather then 300 ma, so they were not just another 6.3 volt tube with a different base, this was why Philco used them in many AC/DC radios during the 1940s. I think that the main reason for denoting loctals with a 7 or 14 rather then 6 or 12 was to avoid confusion with octals or older glass tubes, a 6B7 is an RF pentode, double diode, a 7B7 is just an RF pentode. The curious part is that they didn't do this with battery operated loctals, they added the letter "L" to differentiate them from the octal types.
My understanding is that Sylvania supposedly developed the loctal types for use in car radios and other mobile uses, and there were car radios that used them, but most seem to have ended up in AC sets. The idea being that the tubes would not get rattled out of their sockets, but I think this was the case of a solution that was seeking a problem. I have had several car radios, some used octals, many used seven and nine pin tubes, and I have never found one where the tubes become unseated and was rattling around inside the metal case.
Regards
Arran