The CV1220 is an indirectly heated triode based on the STC 4033A. The old service code was NT37.
The valve was introduced in the mid 1930s but proved to be difficult to mass produce and was subsequently black-listed from use in military or essential commercial equipment during WWII.
It has an anode dissipation of 25 Watts and can produce 4 Watts of audio with low distortion in a single ended Class A amplifier. Use 400 Volts on the anode and a 7,000 Ω load. The grid should be biased at - 20 Volts.
The anode is a small diameter cylinder attached to a large heatsink. The control grid within has a small grid to anode distance and the cathode to grid distance is also small.
The geometry of the pinch and lower components. The anode looks to be a complete cylinder but is in fact open at the back.
Looking at the back of the anode the opening can be seen. The bright structure seen in the top picture behind the anode is the grid support and the grid wire is attached along the length of this support.
The top mica with the cathode tube (top centre) held in place.
The control grid within the anode.
The pinch end: the opening at the back of the anode is clearly visible.
From the back looking towards the anode the length of the grid can be seen.
The balloon envelope is 54 mm in diameter, and excluding the base pins is 104 mm tall.