Some time ago I showed another engineer RCA’s data book for receiving tubes from 1934. She was stunned that most of the active electronic devices available to designers of that time fit into this ¼ inch thick volume. Having grown up on component data manuals larger than New York’s phone book, it was hard to imagine such a relative poverty of devices.
There was a time even a simpler than that. During the early to mid 1920s many of the tubes driving the blossoming radio industry were similar enough that you could jerk the tubes out of your radio and put in a different type—different than the radio was designed to use, and it would continue to work—maybe even better. To make this happen a small industry developed around making the little devices pictured below.
Dig around in any junk box of 1920s radio parts and you will run across these. They are tube adapters—made to allow you to try out the newest tubes in your set—a concept that would be absurd just a few years later as radios became more sophisticated and their components more specialized. Before I describe these let’s step just a few more years back to the beginnings of the vacuum tube. When Lee De Forest began to market his little amplifying bulb, it looked like a variant of a candelabra light bulb. It was fitted with a candelabra-sized light bulb base. The extra wires for the grid and plate exited the opposite end and were just connected to binding posts (on the right).
When De Forest discovered that a tubular arrangement of the tube guts worked better (on the left), his product still had dangling wires that had to be fastened somewhere. That led to the development of perhaps the first tube adapter, seen in this 1916 ad.
Yep, just strap in the tube and screw the filament end of it into the light socket on your Deforest Audion control box.
As vacuum tubes began to be used in industry and the military, it was obvious that a more durable mount was needed. The system that became standard in the United States was the bayonet socket below.
A pin on the side of the tube fit into a slot on the socket. A quick turn to the right locked the tube in and made a firm connection to the four electrodes on the bottom of the tube. The orientation of the four connections to the tube elements became standard as well. If you plugged most anyone’s tube into the socket, the filament, grid, and plate connections would be correct. That’s pretty nice.
I’ve even seen an adapter that let you strap one of Mr. De Forest’s tubular tubes into this socket. Wish I had one of those!
Well now we come upon the time of our little tube adapters above. Yes, most tubes fit that standard socket, but some specialty tubes began to crop up. One was Westinghouse’s WD-11. Most tubes before the WD-11 were pretty hungry, requiring a large rechargeable battery to keep the filament hot. The WD-11 was designed to operate on a fraction of the current, in fact on the power of one dry cell. To keep users from zapping their new wonder tube by plugging it into the wrong voltage, Westinghouse created a special non-standard base for the tube, so it would only fit radios wired for 1.1 volt filaments.
Well, lots of folks wanted to run their existing radios with more economical tubes, so soon an adapter hit the market to make the WD-11 fit the standard socket. Once the owner made the appropriate adjustments in filament voltage, he was ready to go.
Not to be outdone, General Electric designed its own low current tube, the UV199. This tube not only sucked less current, but was miniaturized as well. Of course, it had yet another non-standard base to prevent damage to its non-standard filament. Here is the small UV199 next to the WD-11 and a tube with the standard base.
The UV199 began appearing in GE’s own line of low power radios such as the Radiola II I wrote about recently.
And soon the adapter appeared to let existing radio owners take advantage of the new, small tube. Here are the UV199 and the WD-11 with their adapters.
RCA marketed all these tubes, including the UV-201A amplifier and UV-200 detector, both with the standard base. The WD-12 was a WD-11 with the standard base—no need for an adapter.
Aside from different filament voltages and currents, these were all triode tubes with somewhat similar characteristics. Very little tweaking of a radio was required for it to function with any of these amplifier tubes.
Since the WD-11 and UV199 filled the exact same niche, one had to win out. The winner was the smaller, sturdier UV199. Production of the WD-11 soon ceased. But wait. If you owned an old radio that needed WD-11s, there was a new adapter for you! This one allowed a UV199 to replace its former rival.
Here is an elderly Westinghouse from 1921 and amplifier from 1922. The two were retrofitted with adapters for the UV199. Unfortunately the marriage of tubes and adapters made the tubes so tall that the original owner had to throw away the lids that came with the radio. Well, it tells a story.
The venerable standard socket began to become less standard. New versions of amplifier tubes showed up with long pins. The first of these still had a locater pin so they could fit the standard socket, but the long pins allowed them to slip into a new less expensive style of socket. The filament pins were a bit larger than the others, assuring that the tube only fit the correct way.
A long-pin version of the now-aging UV199 appeared (UX199). These long-pin tubes could fit those new-style sockets that were quickly becoming the standard for radios.
OK, someone decided that it would be nice to use the long pin UX199s in the old standard style sockets, so here we go. Slip this on, tighten the set screw, and it will fit your old style radio.
For our final adapter, let’s look inside this 1924 superheterodyne radio that GE made for RCA.
We see a neat row of UV199s, but what is going on over there on the left?
In addition to the long-pin UX199 tubes, RCA introduced a long-pinned power amplifier that gave more volume to the final audio stage of a radio. It was the UX120. RCA immediately recognized that this tube could help out existing radios that used the older short-pin UV199 tubes. RCA and several others began making a rather odd adapter to fit the power tube into existing radios. However this tube didn’t just fit in. To give its audio kick, it needed different biasing and plate voltages so this adapter became an extension of the radio circuit, with wires dangling off to the additional batteries needed.
By this point tubes were becoming specialized enough that it was too complicated to pop one in instead of another. Up until this time most tubes simply had a filament, grid, and plate; all connecting to four pins on the bottom. By the late 1920s tubes with a separate cathode were introduced. Tubes with additional grids to offset internal capacitance problems also started to be used. By the 1930s even more complex and specialized tubes appeared. Soon each tube in a radio was different than its brothers, performing very specialized tasks. Such tubes were not interchangeable with the other tubes, and may not be interchangeable with tubes in another radio with similar features. The age of exchanging last year’s tube for the next new thing was pretty much over, and these tube adapters were relegated to the junk box in which we find them some 80 to 90 years later.
There have been two revivals in tube adapters. The first was during World War II. Sugar and gasoline were not the only things rationed during that war. Radio tubes were vital to our increasingly technological military. Civilians temporarily did not have access to the broad range of tubes needed to keep the country’s radios running. Radio repairmen built adapters that allowed the tubes left over to work adequately in applications for which they were not designed. A few books were published explaining how to retrofit radios to use older or more common tubes. These fixes often involved tube adapters and minor rewires within the radio itself. Getting by can bring about its own creativity.
The final revival of tube adapters is among collectors like me. Working tubes to fit an 85 year old radio are increasingly hard to come by. Many times the value of those tubes alone exceed the value of the antique radio. We use old and new adapters to allow more commonly available tubes from the 30s through the 50s to quietly do the work of a tube that disappeared from production when flappers still ruled the fashion scene.
But tube adapters do tell us an interesting story. There was a time in the history of electronics before advanced specialization. Consumers were accustomed to plug-and-play in ways that were not possible or necessary soon thereafter.