This months tube is a half wave rectifier, a rather unique one, the 217C.
The 217C is a directly heated rectifier with a thoriated tungsten filament. It is a half wave rectifier so only contains a single diode.
It is a transmitting tube on a Jumbo 4 pin base, the same size as that of the 211. It has a top cap for the plate connection. The pinout is shown on the left. At first look the 217C looks like a transmitting triode with a plate cap, for example the 805. Construction wise it is probably just like a 805 without grid. The filament has the same specs of 10V at 3.25A like a 805 or 211. It was advertised for applications which are sensitive to RF hash which can be present when the usual mercury vapour rectifiers are used. To my knowledge not many rectifiers with a thoriated tungsten filament existed, which makes this tube rather unique. There was also a 217A which did not use a plate cap. The plate connection was moved to the top in the C version
to increase the peak inverse voltage rating from 3.5kV to 7.5kV. Up to 150mA current can be drawn from a single 217C which doubles to 300mA in a full wave rectification scheme with 2 tubes. I never came across many of these tubes and it was probably not used very much. A 211 with its grid connected to the plate could serve the same purpose as was done in the Western Electric 43A amplifier. This had the advantage that 211 tubes which got too weak for the amplifier section could be rotated to the power supply and continue some service life as rectifiers. I have never seen any 217C used in an audio amplifier and have also not used it myself. Not only would the space and heater power requirements be prohibitive but the same function can be had from a tiny 6AX4 which in addition has a nice slow ramp up of the output voltage while such directly heated tubes, especially with thoriated tungsten filaments pretty much conduct instantly after turn on.
Here a 217C with the filament lit up in all it's glory:
My entire collection of 217C tubes:
Except for the 3rd from the left they are all identical and Amperex branded. The one which is a bit taller does not have any branding but looks very similar in terms of construction to the others except for the taller glass.
Lets have a closer look at the tubes.
Guaranteed for just 1000 hrs:
Some more pics of the tube lit up:
I hope you enjoyed the presentation of this rather obscure tube.