[ARC5] Loctal Tubes

[Mike Morrow]

Bill wrote:

> The published heater ratings I've seen are the same 6.3/12.6 volts
> as many other tubes. I haven't seen the tube manual that specs loctals
> at 7 or 14 volts. I haven't seen the Taj Mahal either but I believe
> it exists.

No need to travel even half so far!  I recently wrote:

> ...Sylvania claimed that loctal filaments were nominally rated for 7.0
> or 14.0 volts, rather than 6.3 or 12.6 volts.  That is reflected in the
> 1951 Sylvania tube data manual...
>
> See these documents at http://www.tubebooks.org/sylvania_tube_manuals_online.htm

The URL cited above leads to scans of the 1951 and 1959 Sylvania tube data
manuals.  For *any* loctal tube in the 1951 manual, the nominal heater voltage
is listed as 7.0 or 14.0 vdc/vac, but **no** heater current value is given.  The
"Typical Operation" tables report *only* the use of 6.3 or 12.6 volts, with
associated heater current values specified.

A note will often be found on those loctal data sheets that states "For a-c
service the 7-volt heater rating corresponds to a 130-volt line voltage
condition...It is also the nominal voltage for automotive receiver service."

The reference to 7- or 14-volt heater ratings in Sylvania tube data manuals
disappeared for all those loctals by the 1959 edition (see URL), in which
only 6.3 or 12.6 volts ratings are cited.  Sylvania thus acknowledged that
reference to "actual ratings" for loctals being 7.0 or 14.0 volts was total
nonsense.  That's particularly clear since heater current ratings were *never*
supplied except for 6.3 or 12.6 volt operation, even in the old data manuals.

It's all a minor point that never mattered anyway.

I have a 1940 Philco AM BCD/Police Band a-c table radio that uses "7-volt"
loctals.  Why?  I have no idea, unless it was for marketing propaganda. :-)

73,
Mike / KK5F