BC-375E Aircraft Liason Transmitter

 

    Welcome to my BC-375E web site ! I was always fascinated by USAAF aircraft radio equipment from the World War II era such as the ART-13 and the SCR-274N / ARC-5 series command sets. Collecting WW2 aircraft radio equipment combines three of my interests (aviation, military history and vintage electronics) into one. The BC-375E has a special charm for me. The BC-375E is the epitome of a real transmitter; 4 x big tubes, real AM modulation, a 1920 radio design, lots of knobs, switches and meters.


Figure 1.
  BC-375E, serial #74000. This particular unit is undergoing restoration with parts from around the globe.   It originally came from Manitoba with many bits missing.  The tube cover comes from Australia, meters from California and Oregon, tuning unit from Texas and tubes from Ontario and Quebec.

 

    The BC-375E is a liason transmitter that was primarily used on medium and large aircraft (i.e. bombers) during World War 2 by the United States, Canada and Great Britain. The BC-375E's ground based cousin, the BC-191F, is almost identical to the BC-375E. It has a different operating voltage as well as other minor differences. The BC-375E is wired for 24/28 DC volts aircraft power systems and the BC-191F is wired for 12/14 DC volts systems found in vehicular installations. Below is a list of BC-191F and BC-375E radio installations with the application, matching receiver and useful range (in miles) for various modes of operation.

Application

Installation

Transmitter

Receiver

CW

MCW

Voice

Ground

SCR-177-B

BC-191

 

BC-312 or BC-314

100

70

30

Ground

SCR-188-A

BC-191

BC-342

100

70

30

Vehicular

SCR-193

BC-191

BC-312

60

40

20

Aircraft

SCR-187-A

BC-191

BC-224

750

500

250

Aircraft

SCR-287-A

BC-375

BC-348

750

500

250

 

    The BC-375E was commonly found on aircraft such as the B17, B24, B25, B26 and early B29's. The BC-375E makes a cameo appearance several times in the movie about the most famous B17, “The Memphis Belle”. Later B29 aircraft used the ART-13 transmitter paired off with the BC-348 and this installation was known as the AN/ARC-8.  I believe the most famous B29's  the "Enola Gay" and "Bock's Car" had AN/ARC-8 and not SCR-287 installations.


    Although, the BC-375E was used extensively during most of World War 2, even in early B29 installations, it would be eventually replaced by the more “modern” and easier to use, ART-13 made by Collins Radio. The BC-375E transmitter requires the manipulation of up ten different controls (“A” through “D” on the individual tuning units, “M” through “P” on the internal transmitter antenna tuner and “E” through “F” on the external BC-306A antenna tuning unit, when used on frequencies below 800 Kc) to set it to a specific frequency. On an ART-13, once pre-programmed, the radio operator could select one of 10 pre-set frequencies with the flick of a switch. Then an elaborate motorized tuning system would move all the frequency determining components (variable capacitors and variable inductors) into their preset positions inside the transmitter within 30 seconds. One might ask "Why wasn't the BC-375E obsoleted earlier ?" I would suspect the same argument used for mass producing B17 and B24 bombers for most of World War 2, in lieu of the modern B29, applies to keeping BC-375E as the main liason transmitter as opposed to the ART-13. Simply put, why risk a new unproved technology, when the old stuff is well characterized, working well and easy to manufacture.

    The genealogy of BC-375E dates back to the 1920’s. Anyone familiar with World War 2 radio equipment would tell you the BC-375E seems out of place when compared to more “modern” equipment such as the SCR-274N / ARC-5 transmitter with a direct frequency dial VFO or ART-13 with autotune functionality. The BC-375E required a separate drawer (one of seven) for a specific frequency range and then the operator had to set several knobs to select a specific frequency from the calibration tag . The MOPA (Master Oscillator Power Amplifier) design for the RF section looks like something out of a 1928 ARRL handbook. The BC-375E was preceded by the BC-375D (pre-Dec 7, 1941). From photographs I have seen, the BC-375D appears to be identical to the BC-375E. The only thing that is different is the contract date. BC-375D’s have pre-Pearl Harbor dates like June 30, 1941 and the majority of BC-375E’s have 1942 contract dates. See tags below for details. I would assume BC-375A, BC-375B and BC-375C also existed at one time, although I cannot find any information about them at this time. The BC-191 appears to have preceded the BC-375 by a couple of years.

 
  

Figure 2.  BC-375D (pre-Pearl Harbor) and BC-375E (post-Pearl Harbor).  Black tag is found on the front of the transmitter on the center right . White tag is found inside behind the VT-4C tubes and is seen when the tube cover is removed.

    The predecessor of the BC-191 is the BC-AA-191 (very similar to the BC-191, except the entire antenna tuning unit on the right column is missing; i.e. the Antenna Current meter, Antenna Inductance Tuning “M”, Antenna Circuit switch “N”, Antenna Capacitance Tuning “O” and Antenna Inductance Switch “P”). The BC-AA-191 dates back to 1935. It used an external antenna tuning unit known as the BC-AA-194. The drawers on the BC-AA-191 are quite similar to the BC-191, except the transmitter antenna terminals are on the upper right of the actual drawers as opposed to the upper left side of the transmitter (as is the case with the BC-191 and BC-375).


    The predecessors of the BC-AA-191 appear to be the BC-110, BC-114 and BC-127 dating back to circa 1925. The BC-127 covered 150-350 Kc and contains four VT-4 tubes. The BC-114 covered 400-850 Kc and contains one VT-2 and three VT-4 tubes. The BC-129 covered 850-1500 Kc and contains one VT-2 and three VT-4 tubes. These are fixed frequency band units and do not have plug-in drawers like their successors, the BC-AA-191, BC-191 and BC-375.

   
    The BC-375E and most accessories were manufactured by General Electric of Schenectady, New York. GE, like Western Electric and Stromberg Carlson, was a prolific manufacturer during World War 2. GE is an interesting company that today has many diverse business operations like jet engines, medical equipment, capital leasing, appliances, etc. It was formed in 1892 out of the merger of several of Thomas Edison’s companies. It is the only company today that makes up part of the Dow Jones Index, that also was part of the Dow Jones index in the 1890’s.


    Unlike many war surplus item available from 1946 to 1976 , the BC-375E was unfortunately rarely used in its original form. Many amateur radio magazines referred to it as “a wonderful source of parts". Amateurs fall into two categories: 1) The prolific recycler: "I can really use the parts on this thing for such and such a project" or 2) The pack rat: "I better save this thing, it might come in useful one day". Fortunately, for the category 2) hams, many BC-375E have survived the last 60 years. With the exception of few articles, most published articles describe how to recycle the  parts of the BC-375E. See my reference section for a comprehensive list. 


    What is a BC-375E worth ? Based on current Ebay pricing anywhere from $150 to $500 US. The BC-375E could be had in 1946 for $50.00 and in 1948 for $12.50 (see the ad below) on the surplus market. Typically, the tuning units go for about $25 US each currently. Unfortunately, the BC-375E contains power triode tubes highly sought after by audiophiles: 4 x VT-4C (aka  211A) and 1 x VT-25 (aka 10). I suspect this is driving prices up considerably. These tubes were once consider useless beside the "modern" Eimac-Varian tubes and were available at one time for under a dollar.

 

Figure 3.  September 1948 CQ advertisement. Those are not typos, but are the actual prices. Picture courtesy of Mike Kelly, VK3CZ. They errornously say the BC-375E can be converted to 10 meter operation with slight modification. This is false, VT-4C (or 211) tubes have a maxmium useable frequency of 15 mhz. This conversion might be possible, if the MOPA tubes (VT-4Cs) are replaced with 838's which are identical to 211's, but have a maximium useable frequency of 30 mhz.

    I was very fortunate to get my first BC-375E (complete with tubes, three PL-xx connectors and a TU-9B) about 10 years ago at an amateur radio auction for $30.00 Cdn ($20.00 US).  Usually, I am a victim of listening to other people’s  “deal of a lifetime” stories. I am glad, I at least have one of those stories for myself. I had always wanted a BC-375E for quite sometime, since I first saw one previously in ad in an old 1950’s QST.  One day, after 15 years of searching and quite unexpectedly, I was at an auction and the contents of a silent key's barn attic, were cleaned out after 30 years and were up for auction. The first item was a liason receiver (R1082) from a British Lancaster bomber. Everyone was “ooh”ing and “aah”ing at it and it fortunately distracted everyone from the next item in the lot. The R1082 went for about $150.00 Cdn and everyone was still looking at it after the auctioneer’s gavel fell. The next item, a gorgeous BC-375E, started at $5.00.  Myself and this other character were the only two people bidding on it. I brought the price up to $30.00 and waited in agony as the auctioneer slowly said “Going once …….”, “Going twice  ………” and finally after what seemed like an eternity he said “Sold”. Only then did the crowd, distracted by the Lancaster receiver, start to notice the BC-375E as I was hurrying to my car trunk with it.

 

    How many BC-375E are left ? My educated guess would be 10,000 to 20,000. My guess, based on the quantity of other equipment (i.e. SCR-274N) manufactured to be used in the same installations as the BC-375E, is that anywhere from 100,000 to 150,000 BC-375E's were manufactured by General Electric. The lowest and highest BC-375E serial numbers I have seen are #7739 and #74000. The lowest and highest tuning unit serial numbers I have seen are #769 and #94747. Knowing that tuning unit serial numbers were matched to transmitters, this would confirm at least 94,000 BC-375E's and 658,000 tuning units were made.

 

    Another interesting way to calculate the total of BC-375E is by total US bomber production during World War 2.  My assumption is most bombers (including patrol planes and patrol bombers) would have had BC-375E transmitters (except most B29s).  Between July 1, 1940 and August 31, 1945  the US military accepted 97,810 bombers into service: 12,726 B17 Flying Fortresses, 19,203 B24 Liberators, 9816 B25 Mitchells, 5157 B26 Marauders and 3970 B29 Super Fortresses. From this information, I would estimate 90,000 to 95,000 BC-375Es manufactured would probably be in the ball park, assuming most medium and large planes used BC-375E’s.

 

    The US total numbers, B24 and B25 numbers were taken from “A Pictorial History of Military Aircraft” edited by John Pimlott. The B17 and B29 numbers come from the Boeing  B17 Website  and Boeing B29 Website, respectively.  The B26 numbers come from Warbird Alley Website 

 

    How many BC-375E survived World War II ? Maybe 30,000 to 45,000. Then how many survived soldering iron wielding hams ? Maybe, 10,000 to 20,000. Many more BC-375E were taken apart because of the wealth of RF parts, then say their cousins, the SCR-274N / ARC-5 command transmitters or ART-13 transmitter which could be converted for amateur radio very easily.

 


 

For more BC-375 information, click on the following links.

 

Suggestions or comments:  [email protected]

 

 

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