Filament Voltage

[N2IXK]

The earliest 27's used a twin bored ceramic core to allow the filament heater wires. The ceramic was fit into the cathode sleeve. The earliest UY227 was a notoriously slow warming tube which took a couple of minutes.

Interesting. Some of the ultra-reliable Bendix "Red Bank" series of tubes went with a similar construction, with an alumina ceramic heater core with tungsten wire threaded through holes, then inserted into the cathode sleeve. This provided excellent vibration resistance and heater to cathode breakdown voltage capability, at the expense of warmup time.