WD-11 Tube

WD-11 tubes were made in the early 1920's for a very limited number of radios such as the Radiola III and Aeriola Sr or Radiola Sr. No radios using the WD-11 were made after 1924, so the tubes rapidly went out of production and are very hard to find or very expensive nowadays. This page shows a project that I worked on while housebound during the coronavirus pandemic in March 2020.

Aeriola Sr. radio showing the WD-11 tube

WD-11 Tube characteristics
    The WD-11 tube had a unique 4 pin tube base with one big 3/16 inch diameter pin for the plate, adjacent 1/8 inch diameter pins for the filament, and opposite 1/8 inch pin for the grid. It was a simple directly heated triode, with the filament being the cathode and operated with 1.1 volts at 0.25 amps for the filament, and maximum plate voltage of 100 volts. When used in the Aeriola Sr. the B voltage was only 22.5 volts for detection and regenerative amplification. A detailed blueprint of the tube base can be found at Radiomuseum.org if you want to see the measurements of the base layout. They use larger size tubes and fabricate a base for them. They do look more like real radio tubes than mine and use tubes like 1A5.. Other tubes such at the 864 can be used as direct substitutes with a base adapter or by just swapping out that tube's 4 pin base for a real WD-11 base. Radiomuseum link

Finding a subminiature substitute
    There are several candidates that could be used in place of the WD-11 and the 5676 submini tube is the closest match that I would find. It is a triode with a filament voltage of 1.25 volts at 0.120 amps. It can take up to 135 plate volts, and in circuit gives good results.
    Other tubes are the 6418 submini pentode. This tube used 1.25 filament volts at 0.01 amps, but can only take 22.5 volts max on the plate. I have tested this tube in an Aeriola Sr. with good results. It would not work for the Radiola III which has higher B voltage on the amplifier tube. To use this tube you would tie together leads 1 and 2 (plate and screen grid) to use as the plate lead. Another possibility is the 5672 pentode, which has 1.25 filament volts at 0.05 amps, and can take 90 volts on the plate. Again, you would connect leads 1 and 2 for the plate. I have made up tubes with all 3 of the above and all gave good listening on my Aeriola Sr.

Filament control
    Since all the above tubes have lower filament current than the WD-11, they need some extra resistance in the filament control to adjust the output. When using either the 6418 or 5672 tubes, I added a 1/2 watt resistor in series with the filament leads, ( 27 ohms for the 6418 and 15 ohms for the 5672) to limit the voltage across the filament to 1.25 volts when using a 1.5 volt battery, and added a 100 ohm rheostat in the A battery supply, in order to give you some volume control, so the tube doesn't come on at full volume and power. (Actually you could use a 10 ohm resistor for the 6418 and a 2.7 ohm resistor for the 5672 if you followed the precise math for the current, but the slightly higher resistance would cover for slightly higher battery voltage and work just fine.)

Above shows 100 ohm potentiometer to use as the filament control rheostat, it is a 5 watt wirewound pot for use with 6418 or 5672 tube.

For the 5676 tube, I added an 8 ohm 1/2 watt resistor in series with the filament, and that seemed to give a reasonable volume control just using the stock filament rheostat on the Aeriola Sr. (The math showed that just a 2 ohm resistor should limit the filament voltage to 1.25 volts  when using a 1.5 volt battery, but that gave too little volume control using the radio's rheostat, experimentation showed an 8 ohm resistor gave good results with the stock Aeriola filament control rheostat and no external rheostat was needed as it was when using the 6418 or 5672 tubes.)

Construction details
    The tube base was made from 2 pieces of wood dowel, the bottom piece 1 1/8 inch diameter, and the upper piece 7/8 inch diameter. Each piece was about 3/4 inches tall but 1/2 inch would probably have been enough. The 2 pieces were glued together with wood glue.

    I had ordered some WD-11 tube base wafers from an Ebay supplier, but found that the pins were too small to fit snugly in the socket. I unsoldered the pins from one of them and had a nice template to use to drill the holes for the pins into the wood base.

The placing of the pins has to be quite exact in order to fit correctly into the sockets. I screwed the template onto the bottom of the wood tube base and then used a drill press to drill the three 1/8 inch and one 3/16 inch holes all the way through the tube base. The drill press kept the holes straight and square with the base.

Then I painted the wood black with some spray lacquer.

The pins were made from solid brass rods, 18 inch and 3/16 inch diameter from a hardware store. They ended up being about 2 inches long and I rounded off the ends with a file.

I put a little wood glue into the holes and then tapped them in place with a little hammer, careful to not split the wood. I used a real WD-11 tube as a guide to gauge them to the right length out the bottom of the base. There was about 1/4 inch of pin at the top to solder the submini tube in place.

    My 5676 tubes came with pretty short leads, so I soldered on some extensions using enamel covered 24 GA wire. Be careful not to bend the tiny tube leads as they may break off. I have seen some for sale with longer leads, might be easier to work with.

The 6418 and 5672 tubes came with longer more flexible leads and were easier to work with, I just slipped on a short bit of shrink tubing for insulation, see below.

WD-11 pinout viewed from bottom of tube

    The diagram shows the pin configuration for the tubes, and the photo shows how the little resistor was soldered from the brass post up to one of the filament leads. Ignore the resistor value, it was later changed.

The above was before the little filament resistor was added.

    I found some lucite tube that was the right size to slip over the top of the tube to cover everything up.

    The final product doesn't look great, but keeps you from having to light up a real WD-11 in order to play your Aeriola Sr. radio. Since the 5676 tube can handle up to 135 volts on the plate this substitute should be able to be used in a Radiola III, or IIIA, or any other radio using WD-11 tubes. I will put up some results here when I manage to do a test in the Radiola III.