View Full Version : Difference between octal and loctal tubes?

08-13-2010, 10:52 AM
Can anyone enlighten me on the main differences between octal and loctal tubes? Are the sockets different? Sorry for the dumb question of the day. Robert

08-13-2010, 10:56 AM
The sockets are different, Octals used 6 volts where sometimes their Loctal equivalents used 7 volts. Although I have not had much experience with Loctals I have read that some of them sounded better than their Octal brothers.

08-13-2010, 11:00 AM
Different sockets, different pinouts. Sometimes had the same or similar internals to some octal tubes. They didn't run on 7 volts, the '7' designation was simply to differentiate them from the "normal" octal tubes.

08-13-2010, 11:52 AM
Nearly all the 7 (and 14) series tubes were identical to 6 (and 12) series tubes. They were offered in locktal base for mobile use, as they were more secure in their sockets. Occasionally, home units used them but for the most part they were in mobile radios. Of course exceptions include sets that used 50A5 outputs, 35Z3 rectifiers, and the 14 series.

Then came miniature tubes, Compactrons, and almost any variation the tube people could invent in a futile attempt to stave off the onset of solid state.

08-13-2010, 01:40 PM
I believe Philco was the innovator behind the Locktals. A lot of their radios pre-war around 1939-40 and up until miniture tubes had 'em. Most other manufacturers stayed away from them except for maybe when a certain application/circuit utilized them. Zenith used them too but not to the extent that Philco did.

08-13-2010, 01:45 PM
Loctal tubes can often be real ($$$) bargains compared to their Octal counterparts.


08-13-2010, 02:24 PM
Loctal tubes can often be real ($$$) bargains compared to their Octal counterparts.

More threads like this and that won't be true at all! ;)

08-13-2010, 03:01 PM
I heard they were developed for mobile use too, as in car radios and such.

08-13-2010, 03:02 PM
More threads like this and that won't be true at all! ;)

I learned this lesson the hardway more than once,:thumbsdn:

08-13-2010, 05:42 PM
What is the difference in the socket? Pin sizes, center post? Does loctal stand for large octal? Thanks for enlightening me further! Robert

08-13-2010, 06:01 PM

A loctal-base tube.A variant of the octal base, the loctal base (sometimes spelled "loktal"), was developed by Sylvania for ruggedized applications such as automobile radios. The pin geometry was the same as for octal, but the pins were thinner, the base shell was made of aluminium, and the center hole had an electrical contact that also clamped the tube in place. Loctal tubes were never that popular, and were only adopted widely by a handful of equipment manufacturers, most notably Philco, which used the tubes in many ordinary table radios. Loctal tubes are often difficult to remove from their sockets (look for a small indexing mark on the side of the base skirt and push the tube from that side to unlock). Because the pins are actually the Fernico or Cunife lead out wires from the tube itself, they are prone to intermittent connections due to the build up of electrolytic corrosion products because the pin is of a different metallic composition to the socket contact.

08-13-2010, 06:01 PM
loctal is a sort of locking socket. The pin diameters are much smaller, but the center post has a groove in it where "fingers" in the socket lock it in place. has a picture of one, you can see the base design and how it differs.

08-13-2010, 07:15 PM
Great info! Thanks. Robert

08-13-2010, 07:18 PM
As a data point my Sparton console radio phonograph is all loctal except for the reto and output tubes.