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 to prevent flash-over at the base 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. In both cases the anode dissipation remains the same and so it would be wrong to think that the TT21/2 could be used at audio for greater power than the KT88.
The main picture shows the GEC logo - the professional brand of M-OV - as well as the cracked glass envelope. The milky appearance of the getter patch indicates the absence of the vacuum.
The top cap connection to the anode is on the right. There is a screen from the beam plates to above the control grid heat sink.
Another view of the top screens.
The wide glass tube envelope is 51 mm in diameter, and excluding the IO base pins is 114 mm tall.
References: Data-sheet. Type TT21 was first introduced in 1961.