The TT21 and TT22 differ only in heater voltage. The TT21 is for 6.3 Volt use and the TT22 has a 12.6 Volt heater and was aimed at mobile transmitters. Both valves are electrically identical and aimed at the SSB transmitter market. Both are also rated for Class C use as well as for the linear amplifiers required for Single Sideband transmission.
Maximum power output of 146 Watts comes in the efficient Class C mode, SSB will produce 110 Watts and as a Class AB1 audio amplifier some 50 Watts are possible or 100 Watts with fixed bias. The data-sheet says that the TT21/22 may be used, as a pair, in place of the KT88 for audio use. This is not a revelation as the two Types have the same design history, the TT21/22 being modified for higher peak anode voltages and having a top cap anode connection but otherwise the same valve.
In an earlier generation of valves the landmark 6L6G had a transmitting version also - the 807 - again with a top cap for higher anode voltages.
The valve sits on a pressed glass foot rather than a pinch and is essentially an all glass valve on an octal base. Note the three getters to achieve maximum vacuum.
The anode box and to the right the copper control grid support and large heat radiating fin.
The glass foot base.
The wide glass tube envelope is 51 mm in diameter, and excluding the IO base pins is 114 mm tall.