Posted By: Robert Nickels (W9RAN) Posted: 09/254/2022 Heat up the iron 0 Comments 09/254/2022 |
The National MB-40-SL tuning assembly |
It's probably safe to say that no company produced more components for radio builders than the National Radio Company of Malden Mass., many of which were designed by radio engineer extrordinaire James Millen W1HRX. Having acquired a degree in mechanical engineering at Stevens Institute, Millen always stressed solid mechanical design principles in National's equipment and components. National started out making electrical components and then using them in it's own receivers and other equipment gave the company an edge in knowing which features and functions were needed. The MB-40-SL is the "small" version of the MB-150 tank circuit, rated for 40 watts of RF power and aimed at a variety of applications, from interstage coupling in a higher powered trasnsmitter to the output matching network in smaller rigs. It's most notable feature was the ability to cover all amateur bands from 80 through 10 meters with a single tuning control and no bandswitch (with a small gap in between 40 and 20 meters). Copies of the Application Notes and a copy of the patent can be downloaded from the "Attachments" section below to better understand how it works but the MB-40-SL seems to be a clear example of the Millen-National RF ingenuity at work. But a bit of research suggests that this very National-like product orginated not with Jim Millen or even it's own engineers, but an outside inventor named Thomas Murray Ferrell of East Hempstead, LI, New York, who originally applied for a patent on his "Electric System" in 1940 and was issued patent # 2,370,724 on March 1, 1945. The patent was assigned to National Company Inc. but the nature of Mr. Ferrell's relationship with the company is unknown. It would be interesting to know if Mr. Ferrell assigned any other patents to radio manufacturers but the USPTO database does not allow searching by name prior to 1976. An early SSB transmitter-receiver design came to the attention of Hal Goodman, who was President of Cosmos Industries in Long Island City, NY, a small electronics manufacturing company making among other things, PTOs for the Collins-designed R-390 series receivers. Goodman was showns a one-off handbuilt "bilateral" transceiver that was designed by Butch Mason W6KAG and liked it enough acquire the manufacturing rights. The prototype was called the "Portsider" and was aimed at mobile operation, but Goodman used it as the basis for an "all in one box" single sideband home station transceiver that was released in 1958 as the Cosmophone 35. (Another suitor for Mason's design was Faust Gonsett, who ended up starting SBE to build his own bilateral transceiver a few years later, the SB-33). Not much is known about the Portsider, including whether or not it used the MB-40-SL, but as the design of the Cosmophone evolved, it was chosen as the output matching network. According to "Mr. Cosmophone" WA5UEK, Hal Goodman got a "good deal" on a bunch of them, with the intention of using two 6146 in parallel to achieve 100 watts output. Unfortunately, National wasn't kidding when they rated the MB-40, as it would arc over when the power exceeded 75 watts, thus the "Cosmophone 35" resulted. Had this choice been made differently the Cosmophone would have been a much more viable contender for barefoot operation. Ironically, the MB-40-SL was invented on Long Island and in the end, the only know commercial user of it was also located there. As the ap note explains, the tuning sequence as the knob is rotated does not gro from low to high, but rather tunes in the sequence of 80-20-15-40-11-10 meters, making the PA tuning very unique. Find a low power homebrew transmitter that tunes in that sequence and you can be sure there's an MB-40-SL behing the panel! Unfortunately not many (if any) commercial transmitter manufacturers used the MB-40, probably because it was cheaper to put in a chunk of Air-Dux and a couple of cheap (at the time) air variables, and add a deck to the bandswitch they probably needed for the exciter stages. But it remains an interesting piece of technology and if you find one on a hamfest table (like I did) you'll know what it is!
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