Sunday, Mar 29, 2015
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Tube Socket: Loctal


The loctal tube's structure was supported directly by the connecting pins passing through the glass "button" base. Octal tube structures were supported on a glass "pinch", formed by heating the bottom of the envelope to fusing temperature, then squeezing the pinch closed. Sealing the pinch embedded the connecting wires in the pinch's glass and gave a vacuum-tight seal. The connecting wires then passed through the hollow base pins, where they were soldered to make permanent connections.

Loctal tubes had shorter connecting lengths between the socket pins and the internal elements than did their octal counterparts. This allowed them to operate at higher frequencies than octal tubes. The advent of miniature "all-glass" 7- and 9-pin tubes overtook both octals and loctals, so the loctal's higher-frequency potential was never fully exploited.

Loctal tube type numbers in the USA typically begin with "7" for 6.3 volt types or "14" for 12.6 volt types. This was fudged by specifying the heater voltage as nominally 7 or 14 volts so that the tube nomenclature fitted. Battery types mostly 1.4 volt are coded "1Lxn", where x is a letter and "n" a number, such as "1LA4". Russian loctals end in L, e.g. 6J1L. European designations are ambiguous; all B8G loctals have numbers either in the range:
  • 20-29, such as EF22 except for early tubes in the series: DAC21, DBC21, DCH21, DF21, DF22, DL21, DLL21, DM21 which have either B9G or octal bases, the change to Sylvania's locktal standard coming in 1942
  • or 50-59 special bases, including the European 9-pin lock-in base, but other types are in the same range e.g. while EF51 is B8G loctal, the EF55 is 9-pin loctal, B9G, and the EL51 has a side-contact P8A base.

Other Loctals

  • 9-pin loctal bases, B9G, include the 1938 Philips EF50, EL60 and some type numbers in the European 20-29 and 50-59 range;
  • There is a different "loctal Lorenz" in the Mullard–Philips tube designation .

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