Working as I do here for the global leader of high-performance signal processing solutions, I often speak or write about Analog Devices and how our semiconductor products represent the "state of the art." The challenge for ADI (in fact, for any company seeking a technology advantage) is to advance that state of the art and make it commercially viable. As I've learned in my hobby of antique radio restoration, the same was true in the early days of mass-market consumer radio. The technology might be 80 to 90 years old but, as I'll show you now, we can see great examples of how circumstance and ingenuity led some companies to find - or miss - that sweet spot.
From its founding in 1892 as the Helios Electric Company (manufacturing carbon-arc lamps) then, in 1906, as the Philadelphia Storage Battery Company (making batteries for electric vehicles) and finally, in 1919, adopting the name by which we know it today, the Philco Company (when it turned its attention to providing consumers with storage batteries for the burgeoning radio industry), the company was always looking for technology that could meet the needs of the market. For example, because those early batteries were expensive and messy (they required the monitoring and refilling of liquids) in 1925 Philco would introduce the first "Battery Eliminators." Today we call them power supplies. Back then it was a boon for those home owners who had electricity because now they could run their radios directly from a wall socket. (Just a quick side note: many in rural areas did not have power in their homes and still required batteries to run their radios. Philco continued to market their batteries to serve that market and maintain their brand so that when New Deal projects such as the TVA brought power to farms across the South, Philco was the preferred brand when home owners upgraded.)
In 1926 Philco decided to get on the broadcasting bandwagon, but it took almost three years before they released their first radio. That's because they saw how radio manufacturers, such as Atwater-Kent, were individually (and expensively) assembling their products and, as a result, having to charge high prices, limiting the market and, by extension, the radio audience. The planners at Philco saw how Henry Ford was dramatically reducing the cost of manufacturing with assembly lines, which they co-opted for the mass production of their radios. According to Wikipedia, only a couple of years after introducing their first mass-produced radio in 1928, Philco was already the leading maker in the country, grossing $34 million with the sale of over 600,000 radios. For the mass market these included the well-made, beautiful, yet inexpensive "Cathedral" model, and for customers who liked cutting edge tech they developed and sold an innovative, one-tube wireless remote, which they marketed as the "Mystery Controller."
The Philco Mystery Controller was the first mass-market remote
Then, came the war...
Following U.S. entry into World War Two in late 1941 the economy, still moribund due to the Great Depression, was jolted into a frenzy of war-related production. Philco was among the top 100 providers to the government for the next four years, as they pushed aside the design and manufacture of just about any product that was not war-related. In 1945 the war ended, and as restraints on commercial production were lifted manufacturers, Philco among them, looked for products they could quickly produce and sell to a country anxious to enjoy the benefits of peacetime. Both of the following radios are Philco model 46-250, meaning that they were produced that first full year after the end of hostilities. They look the same, don't they?
From 1946, two Philco model 46-250 radios (author's collection)
Both radios are housed in Bakelite cases with only two knobs; one was a combined On/Off Switch & volume control, the other for tuning up and down the AM radio band here in the U.S. Inside, both have an almost identical "All American Five" design found in most tube radios built from the 1930s onward. AA5 radios eliminated the big, bulky power transformers prevalent in the large consoles that were popular before the war, which greatly reduced the cost of manufacturing and owning a radio, something especially important during the Depression. The "trick" to an AA5 was, first, to reduce the number of tubes needed to the bare minimum. I'll cover this in a future blog, but for now it's important to know that in the 1920s and 1930s our improved knowledge of tubes (and what caused them to distort a signal) allowed designers to build tubes that combined two or more stages of the receiver into one tube, which helped to bring the tube count down to five. That typically included an RF converter, IF amplifier, audio detector/first amplifier, audio output, and rectifier which enable the most cost-efficient, best sounding radio.*
Was there a drawback to the AA5? Well, there was that nagging problem of electric shock. That's because one side of the power line was connected to the metal chassis, so you didn't want to touch it when the radio was plugged in and turned on. And who wouldn't want an appliance like that for the wife and kids? (Full disclosure: I received a couple of lessons on AA5 power supplies the hard way. Hurt like hell, too. You'd think I'd have learned after the first time. Or the second. Or the... never mind. That was one of the selling points of Bakelite, by the way - it's a great insulator.)
Okay, so now let's talk about these specific radios and how they play into the story of changing the "state of the art." As stated before both are the same model number (46-250) but, as we see from the stickers affixed to the base of each radio, there is a difference in the code numbers. These numbers are also called chassis numbers, which delineate versions of the basic circuit employed in each radio:
Product stickers for the two radios
The primary difference between the two chassis is that the 122 on the left uses a mix of "Octal" and "Loctal" tubes, while the 125 on the right uses Loctals and what is called a miniature tube. Octals, as you can see above, have a black base that was made with Bakelite. They had thick pins that fit snugly into metallic sleeves arranged circularly in a Bakelite socket that was mounted on the chassis. Octals were the workhorse of radios for decades. Loctals were a relatively new type of tube developed by Sylvania in the very late 1930s for use in automobiles. They had an aluminum alloy base that, as the name implies, locked into place in the socket. As detailed in a Wikipedia page on tube sockets, Loctals had the advantage of being "pin-for-pin" compatible with the older Octal tubes. Interesting to note that pin-for-pin compatibility is a selling point still used today in the field of semiconductor manufacturing.
Loctals are in both versions of the 46-250 we're talking about today, taking the roles of RF converter, IF amplifier, and audio detector/first amplifier stage. But chassis 122 on our left still has two Octal tubes: a 35Z5 for the rectifier and a 50L6 for the audio amp. This tells us that the 122 is an older model, as it is well-known that manufacturers such as Philco did not want to throw away their stock of older components. But, as the three loctal tubes indicate, Philco's stock of old Bakelite tubes for the first three stages must have reached a point where it was financially viable to use the newer loctal technology.
As it turned out, the move to Loctal tubes is a great example of incorporating "state of the art" that sometimes fails to live up to the promise. The tubes, according to Wikipedia, were "...prone to intermittent connections caused by the build-up of electrolytic corrosion." And if you tried to take advantage of the pin-for-pin compatibility, you found the smaller pins of the Loctal tubes in the bigger Octal sockets had a tendency to "wobble." So much for an upgrade. Those problems would not surface for a few years, and Philco would use loctals in many of the home tabletop radios they built after the war.
So loctals have kind of a sketchy history, and demonstrate one challenge of developing new technology that may, at first, be considered "state of the art" but later turns out to have many flaws that diminish its marketability. However, we will now see how the story of the audio output tube will, no pun intended, light the way forward not just for radios but all electronics. We turn again to Wikipedia, which explains that "in 1938 a technique was developed to use an all-glass construction with the pins fused in the glass base of the envelope. This was used in the design of a much smaller tube outline, known as the miniature tube..." The advantages of the mini tube are echoed today in the semiconductor industry where companies (such as Analog Devices) build and market products that use less power and therefore dissipate less heat (in the case of the mini tube, because the filaments were that much smaller.)
Despite the reduction in size the mini tube outperformed its predecessors and so for the next version of model 46-250, Philco designers tweaked the audio output section of the radio to accommodate a next-generation mini tube, the 50B5. Clearly the 50B5 presented enough of a cost benefit because, as you can see in this close-up of the chassis where the Octal 50L6 previously sat, Philco installed a mini tube socket in its place. One can see that an Octal-sized hole had already been punched into the 125 chassis, requiring the riveting of the smaller mini socket inside:
Mini Socket adapter the Audio Output tube
Lower power dissipation and smaller tube size meant that Philco, along every other radio manufacturer who wanted to stay competitive, would abandon Bakelite cases and start housing their radios in cheaper, lighter, and less expensive plastics developed during the war. They were not as resistant to heat, but didn't have to be. State of the art and commerce often walk hand-in-hand, which they did in the case of Bakelite vs. Plastic and octals vs. loctals vs mini tubes.
Me? I have a few colored plastic pieces in my collection, but I like the old school Bakelites with the bigger, hotter octal tubes. As one of my ADI colleagues is fond of saying, "real radios glow." You can hear the 46-250 125 chassis in action in this YouTube video.
* The AA5 design worked (shock potential aside) because the filaments of the radio's five tubes were connected in series and the voltage drops across the tubes came close to line voltage (a number that changed over the years - there's another blog for another day, for now let's just settle on 120V as the voltage drop goal). Since the leading number in a tube designation is the voltage drop across that tube, we can see the Philco 46-250 adds up to 106V. Philco added a 2 watt 80 ohm resistor to the series of tube filaments to drop the additional 9V. And yes, that meant that inside the cabinet not only did we have heat dissipating from the tubes, but also off that resistor. That's why radio backs had large openings, to provide air for cooling.
Close-up of the Philco 46-250 power supply showing the 80 ohm resistor