The CV1724 is an audio output pentode for telephone repeater amplifiers. The prototype was the 5A/102D and both designations are printed on the glass. The old GPO code is POVT183.
The ITT 5A/102D was made in the UK by STC.
The anode side view: all three wire grids can be seen above the lower mica. The copper control grid supports are topped by heat singing fins.
The section above the pince seen face on. The insulated heater wire has been coiled. Normally tungsten wire would be wound on a molybdenum mandrel that would subsequently be dissolved away.
When tilted the cathode tube is seen to be cylindrical.
The balloon envelope is 44 mm in diameter and, excluding the IO base pins, is 109 mm tall.
