WD-11

The design of the WD-11 is somewhat flawed. When the filament burns out, it has a tendency to contact the plate. This feeds high voltages back through the heater circuitry, subsequently burning out the filaments on the remaining tubes.

The WD-11 has a unique 4-pin base layout that was unlike any subsequent UV and UX style tube bases. It had 3 "small" pins and one "large" pin. Later UV based tubes relied on an index pin on the side of the tube base and UX tubes had 2 large and 2 small pins to ensure proper indexing.

It was replaced a year after its introduction by higher performance tubes which were less likely to encounter the filament shorting problem, Westinghouse Electric's WD-12 and General Electric's UX-199. No radios using the WD-11 tube were designed after 1924. RCA ceased production and issued a service bulletin describing how to retrofit existing sets to use the newer UX-199 triodes.

Because of the rarity of the WD-11, it has become one of the most valuable vacuum tubes in the world. New-old-stock tubes have sold for as much as US$180 and used tubes have sold for over $100, more than the original price of the radios that use them. Collectors rarely use these tubes for fear of burning them out.^{[citation needed]}

Sets that use the costly WD-11 and UV-199 tubes can be modified to use the inexpensive 1A5/GT octal power pentode by wiring a 5.1 ohm resistor between the pins of the filament and fabricating an octal-to-four pin adaptor. The pin for the 1A5's suppressor is left unconnected and the screen connected to the plate.

The type 12 (also known as RCA-12) is electrically identical to the type 11, but with a more common UX4 base.

• Data sheet for WD-11

Here is a link on how to modify other tubes to use in place of a wd11:

• https://www.6v6.co.uk/vcomp/tech_tips/wd11.htm