Unlike the interesting but not so successful Motorola 41-S radio from 1939 (see, A Nifty Little Radio and a Vicious Cycle) the 1940 RCA BP-10, introduced at the 1939 World’s Fair, was very successful. It was arguably the most successful “pocket” (granted, a big pocket-but a coat pocket nonetheless) radio made prior to the advent of the transistor radios of the 1950’s. Sure, Belmont’s 5P113 “Belmont Boulevard” pocket radio of 1946 has got to be the tiniest consumer tube set made…but it was expensive and not made in overly large quantity…it also had to be listened to via an earphone, the cable of which made the antenna as well. Revolutionary…and very rare today, often commanding well over a thousand dollars at auction. The BP-10, however, is more readily available today and at a price most collectors can stomach. Unlike the prior year’s offering from Motorola, its batteries were of a more standard type that was used in other sets…this meant that they were available during the war years and remained in production long afterward…I’ve even restored 1950’s vintage Geiger counters that used the same batteries……therefore the BP-10’s that had been sold were still useful to their owners for quite a few years and subsequently you can still find them today. There are even some very detailed websites dedicated to the BP-10, so details and restoration tips can be found. In my opinion, the BP-10 is one of the sets that every collector should have or at least be aware of.
The success of the BP-10 was made possible by several factors. First of all, it really would fit in a coat pocket. The dimensions are about 3.75x9x3.125 inches and the construction is of metal and plastic. The antenna coil is inside the top lid, opening this lid exposes the speaker, volume and tuning knobs–and turns on the power switch! The tubes used have a very short warm-up time so music is almost instantly available after the lid is opened. It is a beautiful little packaging job that appeals to the lover of art-deco style. Being this portable, and being released just prior to the start of the war, meant that a large number of them ended up in the duffle bags of soldiers going overseas.
Another reason for the success is that RCA heavily promoted this product. It appeared in movies and numerous ads of the 1940-41 period, being endorsed by many celebrities (RCA had ties to RKO Pictures, so arranging such endorsements was easy) and RCA also promoted it in public demonstrations. Over a quarter of a million of these were produced! Even with such a staggering number, portable radios made up but a fraction of total radio sales in the prewar period, varying in estimates but generally less than 10%. No doubt, RCA’s heavy promoting effort helped sales….David Sarnoff, then head of RCA, had long dreamed of a personal radio. He had even promoted the concept of it prior to the twenties, basically “inventing” the concept of radio broadcasting by way of his dream of a “personal music box” that he had written about before radio was ever used for more than point-to-point messaging. His vision was a driving factor behind the level of support that the product got from the company. In addition, this was the very first consumer product to feature RCA’s newly developed line of miniature tubes.
Vacuum tube construction had its roots in the incandescent lamp industry and many of the glassblowing and sealing techniques were the same. It was common practice for a tube to consist of its internal filament, grid, and plate structures held within a glass envelope that was mounted into a base that held the contact pins to allow it to be plugged into a socket in the device. The new miniature tubes incorporated the envelope with a thick flat base glass that held the connecting pins. The result was a significant reduction in size. This smaller form factor enabled radios such as the BP-10 to be created. The BP-10 made an excellent test bed product to introduce these tubes to the general public. RCA, therefore, had added incentive to make sure the BP-10 was a success.
The miniature tubes introduced in 1940 rapidly lead to more miniaturization of electronics, including designs that allowed practical solutions for early computers, RADAR (arguably the most important invention of the WWII period) and, after the war, modern television. The BP-10 is therefore a very historically significant product….an example of which was even placed in the cornerstone of RCA’s new R&D building.
As can be seen, there is a very significant size difference from the standard tubes of the late 1930’s to the miniature variants that RCA released in 1940. The line of miniature tubes, initially in 1.4v filament versions, later expanded to fulfill needs in a wide variety of equipment and some, like the 12AU7 and related types are still found in professional audio and music applications. Not bad for a concept introduced almost 80 years ago!
The construction method introduced with these tubes lent itself well to adaptation into other variations of the base sealing technique and was widely employed in cathode ray tubes for television and RADAR applications, being produced in various forms even today.
While not the smallest tube types developed (that goes to the subminiatures of the 1943-1944 period that were used in proximity fuses and the postwar nuvistor types), the RCA 7 pin and later 9 pin miniature styles combined a small size and low filament current consumption with power handling capability that made them useful in a very wide range of products. While I do not have total production figures, I am confident in stating that more of this style of tube was produced than any other envelope type. These are nearly ubiquitous across the world, so much so that even those who are not tube savvy would likely recognize what they are.