The 2151 from Philco is an indirectly heated audio output pentode for use on series heater chains. The load impedance is given as 5,000Ω and the output power is 5 Watts in a single ended amplifier or up to 12 Watts when used in push-pull.
The control grid is wound on copper supports and has heat radiating fins at the top.
The UX6 base pins.
Thanks to Mark Mallison for sending the images.
The classic envelope is mm in diameter and, excluding the UX6 base pins, is mm tall.
Reference: 1043. Type 2151 was first introduced in 1940.