The little tube shown in Fig 1 is the elusive 221-A. This tube went through an evolution unusual for Western Electric. Normally when W.E. released a new tube it was given a three digit number followed by a letter (generally A on the first version) and then changed the letter as new versions were released. New issues reflected design improvements or variations, but the basic form and function of the tube did not change. However, new versions were not necessarily plug-in compatible with the earlier types. This was not the case with the 221 series. As shown in Fig 2, the more familiar 221-D was a totally different tube than the 221-A, not similar either electrically or mechanically. The author is unaware of any other early W.E. tube that underwent such a drastic change under the same basic number.
Mechanically this tube is unique. The base (Fig. 3) somewhat resembles a UV199 but is not the same. The base diameter is about .87 inches, with the pins about .22 inches long and spaced about .36 inches center to center. The overall height is about 3.2 inches. This results in a base larger that a 215-A but smaller than a UV199. W.E. probably designed a matching socket for the 221-A but no information is available. See Fig 4 for a size comparison of the VT-3, the 221-A, and the 215-A. Also compare the internal structure of the 221-A to the VT-3. The 221-A looks like a scaled down version of its older brother.
No published electrical data for this tube has been seen, but fortunately Jerry Vanicek found in his files a Western Electric document dated 2-2-23 which provided some curves for the Type NN, Code 221-A. This sheet was labeled Engineering Dept. Sketch No. ES 355772. The filament was designed for .25 amps, and the filament curves show a corresponding Vf of 1.9 volts. Using the other curves the following parameters were obtained:
Ep = 60v Eg = 0v Ip = 1.5mA Gm = 265 Rp = 25K u = 6.625
The actual intended operating point is not shown in the curves. Other Western Electric information states that the design goal of this tube was to produce more output than the 215-A. The data above indicates that this goal was not achieved and may explain why this tube design was dropped.
The 221-A was manufactured in 1924, and was originally given the letter designation of type NN. The type letter N had previously been given to the 215-A. The part number is stamped into the metal shell along with the usual string of patent dates. The last patent date on this sample is 6-18-22. The bulb is etched with the serial number, 126498. Apparently the project was dropped early on and abandoned. According to Bernard Magers (75 Years of Western Electric Tube Manufacturing) only 50 of the 221-A's were made. The 221-D appeared in 1926 and was similar in size and function to the UV-201A.