The HF red ring was introduced by Mullard in 1924 as an RF amplifier or detector. The red ring around the top of the envelope. Mullard also made an LF for audio use and externally the difference is a green ring around the top of the LF.
The balloon envelope is 38 mm in diameter and, excluding the B4 base pins, is 85 mm tall.
Reference: 1003. Type HF was first introduced in 1924.
Absolute Maximum Operating Conditions¶
Updated January 23, 2013. shape:balloon construction:pinch type:triode age:1920.1930 base:b4 heater:4.0v pins:4 pin:1.a pin:2.g1 pin:3.f pin:4.f