STC describe the 3A/167M as a wide band repeater triode. It has a very high mutual conductance as is common with all specialist wide bandwidth valves.
The 3A/167M has found additional uses in top quality audio applications and especially as a cathode follower stage for matching a condenser microphone to a low impedance mixing desk input. The application report referenced above contains circuit diagrams of suitable circuits including a transformer-less audio amplifier.
The maximum anode voltage is 350 Volts but typical operation would be with an anode voltage of 150 Volts and 40 mA anode current. The high anode current helps reduce the effects of random fluctuations - noise.
The valve also has a Common Valve specification the CV5112.
The STC logo.
The gap in the end of the anode. The white line is the cathode. The grid is wound with fine wire close spaced on copper rods.
The wide glass tube envelope is 28 mm in diameter, and excluding the B8B base pins is 40 mm tall.