Philco Balloon 01A Vacuum tube.
The 01-A was an all-purpose low-mu triode, used as an RF
amplifier, detector, AF amplifier and power triode.
The most popular tube of the 1920s. "01-A" is the number
used for replacements manufactured after 1930 and in tube
manuals. Numbers for early versions include UX-201-A and
CX-301-A (long pins, push in socket) and UV-201-A
(stub pins, bayonet socket).
There were four tubes in the "01" series, each with
different current ratings for their filaments. Type 01-A
was the most commonly used.
Types UV 201 and UX 201 - 1.0 ampere
Type 01-A (UV 201-A, UX 201-A, etc.) - 250 milliampere
Type UX 201-B - 125 milliampere
Type UX 201-C - 60 milliampere
Preferred Substitutes ........... UV201-A, 201A, 301A