Radio Boulevard
Western Historic Radio Museum

Successfully Operating the BC-375-E
on the Ham Bands Today

Brief History BC-375 & BC-191, Circuit Details, SCR-287 Station

Solutions for Specific Problems
Inadequate Power Supply Issues
FMing - How to do a Dynamic Neutralization
Non-symmetrical Modulation Problems

Other Suggestions for Operations
Carbon Microphone Options, Frequency Drift, CW, PL-64 Options
Details on PP-1104-C Power Supply, VT-4C Prices
630 Meter CW Operation with the BC-375

by: Henry Rogers - WA7YBS/WHRM

B&W Artwork from the original BC-375-E manual

Successfully Operating the BC-375-E on the Ham Bands Today

The BC-191 was primarily used as a field transmitter or for fixed radio set-ups in buildings. The very early versions don't have the antenna tuning section located on the right side of the transmitter but the basic concept of a simple MOPA transmitter using four large 211 tubes was present on those early versions. Sometimes the BC-191 was also used in vehicular applications. The BC-191 operates on +14vdc and can be powered by the typical battery-charger running the PE-53 dynamotor or it can also be operated using the AC powered RA-34 power supply. The BC-191 was used on Voice more often than the BC-375. There were other accessories for the BC-191 that allowed interfacing a telephone system into the transmitter. See the photo below-left showing the BC-191 setup with the RA-34 AC power supply, the BC342 receivers and the telephone interface with handset and T-32 carbon mike (also note the J-36 and J-38 keys.) The Army found the BC-191 to be a more versatile transmitter and many were still in use post-WWII.

The BC-375 rapidly became outdated for aircraft use, especially towards the end of WWII. Certainly the decision to build 100,000 transmitters was based on the planned requirements early in the war. Towards the middle of WWII, the radio engineers had come up with the ARC-5 series of transmitters-receivers and Collins Radio Company had introduced their famous and very modern ATC/ART-13. These transmitters produced more RF power in smaller packages and didn't require several tuning units for frequency mobility. Additionally, the ATC/ART-13 would autotune itself and had superb audio. These new transmitters were installed on the newest types of bombers while BC-375s were still being delivered. Near the end of WWII, crates of BC-375s were left unopened because the military viewed the transmitter as obsolete. However, that view was for bombers. The military still used the BC-375 in transports and in "search and rescue" planes well after WWII. They weren't the "latest and greatest" but they were reliable and worked well in those areas.  

Even though some BC-375s were installed on some types of aircraft during the Korean War, there were still thousands that remained in unopened crates in many locations. The BC-375 became available surplus immediately following the end of WWII. Though they cost the government $1800, the surplus market offered complete BC-375 transmitters with all accessories, all new in the crate, for around $40. All of the accessories were offered separately for only a few dollars each. These incredibly low prices attracted the attention of budget-minded hams and many bought the complete set-up as a "cheap way" to get "on the air." Probably some of the hams remembered that the BC-375 had been used successfully in B-17 bombers and since the superbly designed BC-348 receiver had also been in the B-17, well,...the BC-375 must be just as good. Since the BC-348 was relatively easy to convert to amateur use, it followed that the same must be true for the BC-375. Unfortunately, the hams were usually limited in their ability to power up the BC-375 with an adequate power source and rampant operational problems ensued. Within a few years, most dealers were aware that the hams were having trouble with the BC-375 and began advertising that they were great sources for parts. Prices dropped accordingly and more hams bought the BC-375 to try it out as a ham transmitter. In the post-WWII environment, television signals were on the increase and everyone received their TV signals via antenna. TVI became a common issue mainly due to the transmitter's pre-WWII design as an aircraft transmitter (or field transmitter for the BC-191) and the very specific voltage and current requirements for proper operation. The BC-375 wasn't a ham transmitter and never would be.

Eventually, the BC-375 garnered a reputation as an unusable relic that was only good for parts with CQ magazine's Surplus Conversion Handbook advocating that the BC-375 Tuning Units were particularly good for antenna tuner parts or other "home brew" amateur project parts. Most hams believed the Surplus Conversion Handbook's assessment of the BC-375 and many were "parted out" with only the "useful" parts saved. As a result, original and unmolested BC-375 had become a rare item by the 1990s. As more and more hams became interested WWII surplus radio equipment with the intention of actually operating that equipment as originally as possible, the BC-375 was given another look. Maybe it might be possible to operate this "beast" without enduring endless criticism of the signal.

The BC-375 uses a total of five vacuum tubes. Four tubes are VT-4-C triodes (type 211E) along with a single VT-25 (10Y) triode. The first tube (left to right in the photo right) is the VT-25/10Y which is the speech amplifier in VOICE mode, the 1000hz oscillator in TONE (MCW) mode and the sidetone oscillator in CW mode. The second tube is the VT-4 that is used as the Master Oscillator. The third tube is the VT-4 used as the PA. The remaining two VT-4s are the P-P Modulator tubes. Operating voltage is provided by the aircraft battery/charger system (+28vdc) that is used for the low voltage requirements in the transmitter and also used to power the PE-73 dynamotor that provides the high voltage (+1000vdc) to the transmitter. Lower B+ voltages are derived from resistive dividers operating from the +HV. The transmitter frequency range is determined by which of the seven Tuning Units is installed in the transmitter. The coils and condensers and connections for the MO and PA are contained in each TU. The range of frequencies is from 200kc to 500kc and from 1500kc up to 12500kc using the seven various TUs. The controls aligned vertically on the right side of the transmitter make up the wide range antenna coupler for the transmitter. The separate Long Wave tuner, the BC-306-A, is usually located to the right of the BC-375. The BC-306 allows a further extension of antenna matching ability below 800kc using the typical trailing wire antenna (200 ft. maximum length deployment.) Power output is between 50 watts and 90 watts. The BC-375 provided full break-in keying by allowing the elaborate internal antenna relay to control the receiver antenna and the receiver standby circuit. Additionally, external inputs via the PL-64 cable allowed remote receiver standby, side tone, remote microphone and key operation along with remote power control. The aircraft was usually set-up to allow the pilot to also have the ability to access the transmitter/receiver for various reasons.

The BC-375 and BC-191 Today

By the 1990s, the vintage military radio collector groups were on the increase and the BC-375 was viewed with interest because of its importance in aircraft communications during the early parts of WWII. Several  ham-collectors worked on the problems of why the BC-375 seemed to be impossible to operate without "FMing" and poor quality audio. By careful neutralization of each Tuning Unit and use of robust battery/charger systems along with not trying to get maximum carrier power out of the transmitter, the resulting signals were pretty good. At least, good enough to encourage more military collectors to try the BC-375. Today, several BC-375 transmitters are being operated on the 75 meter ham band with notable success. Now,...the signals aren't Broadcast Quality Audio and the carrier power is relatively low but the signals are respectable and have what has become known as "the BC-375 sound." One word of caution though,...when operating a BC-375 or BC-191,...stay on vintage military radio nets or have QSOs with experienced vintage military radio operators. These hams know how the transmitter should sound when operating correctly and will not force you to endure endless audio critiques that are meaningless for the BC-375/BC191 transmitter's abilities.    

SCR-287 at the Western Historic Radio Museum (2009 to 2012) and in Dayton (2013)

In Virginia City (photo below) - The SCR-287 comprised a complete liaison radio station installed onboard various bombers and transports during WWII although it didn't look like the station shown in the photo below. I built the olive-drab desk set-up to better display the station for the Western Historic Radio Museum. Equipment is the BC-375, the BC-348-Q receiver (dynamotor operated,) the Lionel J-47 telegraph key and the Shure Bros. T-17 carbon microphone. The speaker is an LS-3, although these were never used in the SCR-287 or onboard the aircraft. Four BC-375 Tuning Units are mounted in their CS-48 containers on the wall. The home made console features a fold-down desk, a sound-proof (almost) compartment for the PE-73 dynamotor and a bottom shelf for the four storage batteries (four 12v batteries in series-parallel = 24vdc.) The panel to the left of the BC-348 has all of the remote connections for receiver audio output, receiver stand-by, xmtr CW sidetone select, xmtr microphone input and xmtr key input via the PL-64 connector. This station was on display in the museum until I closed in 2012. Although I tried to operate this set up many times, I was plagued with a multitude of problems that had their root cause in trying to run the BC-375/PE-73 on batteries.

In Dayton, Nevada (photo below) - Nowadays, this SCR-287 is set-up in our work shop in Dayton. I eliminated the batteries and went to a PP-1104-C high current power supply. I moved the PE-73 to the lowest shelf of the desk for better stability since the batteries were removed. I also installed casters on the desk so I could move it around the shop easily. I rebuilt the power cable (PE-73 to BC-375) into a fully shielded cable. The same shielding method was done to the BC-348 cable and to the auxiliary cable that is used for the remote standby and for the sidetone. With the new set-up, I still use the LS-3 but sometimes a 600 Z ohm headset is used which greatly reduces the noise from the PP-1104 and the PE-73 dynamotor (it's even noisier now since it's not in the sound proof box anymore.) I'm also using a Shure 102-C microphone now instead of the T-17. I use a J-38 key now. The PP-1104 provides a "solid" +28.5vdc with plenty of reserve current available allowing the BC-375 to produce a signal that is stable with no FMing and the carrier power can be raised up to 75 watts with no problems.

Successful Operation of the BC-375 Will Require These Steps

General Information and What's Important - The BC-375 and BC-191 transmitters are well-designed and well-built pieces of equipment. They performed well enough for their continued military use from before WWII up into the early 1950s. There is nothing "wrong" with the transmitter design if it is operated as it was originally intended, that is, operating on a robust charger-battery system with unlimited current available and with the operating voltage at the upper end of spec. Today, many hams usually can't operate the BC-375 or BC-191 in that manner and try using storage batteries. Although it's possible to get the BC-375 to operate with storage batteries as the power source, the setup will be a compromise at best and getting a decent will be a real challenge. Many of the surviving BC-375 and BC-191 transmitters seem to be in pretty good condition but they will still need a thorough check out and adjustment before actually putting one on the air. The antenna relay seems to always need a lot of attention and since it is in constant operation in CW and controls T/R functions in all modes, its proper cleaning and adjustment is critical. Be sure that the four VT-4/211 tubes are in good condition and that the VT-25/10Y is also good. All components need to be checked and the filament voltage adjustment links will probably need to be reset. It's a good idea to monitor the transmitter output with an oscilloscope at all times but especially during test and adjustment since the settings of the audio gain and the modulator bias have a significant effect on audio quality and power output on AM. Generally, operation on 160M and 80M can be trouble-free but operation on 40M pushes the BC-375 frequency high enough for instability and FM'ing to become nearly insolvable problems. Nowadays, if the BC-375 is operated "per the manual" with its proper dynamotor and an automobile battery set up you will certainly be the recipient of many "critical audio reports." The following is why,...

All military set-ups operated the BC-375 or BC-191 on the high side of the voltage range, that is, +28vdc for the BC-375 or +14vdc for the BC-191. This is really where the transmitters need to operate and automobile batteries without a charging set-up cannot provide enough voltage for stable operation. Suffice it to say that today the BC-375 and the BC-191 cannot be operated "per the manual" on the ham bands when operating on the low side of their voltage range. However, if you want to operate the transmitter as close as possible to original without enduring endless criticism of the signal and the lack of audio quality and car batteries are the only voltage source, you will have to reduce RF output power by at least 40% and you will have to recharge your batteries during "receive time." Extensive modifications to the original design concept of the transmitter (with the goal of achieving BC quality audio) seems to go against the very idea of operating vintage WWII military equipment in the first place. Modifications are not necessary for producing a decent military-sounding signal from either the BC-375 or BC-191. NOTE: Assume that when +28vdc is written, the voltage for the BC-191 would be +14vdc. When +24vdc is written for the BC-375 it would be +12vdc for the BC-191. This saves us from constantly including both voltages in the text. More details on the importance of the input voltage follows,...

Non-symmetrical Modulation - Though lack of serial number specific neutralization of the TUs is the most important step, another important problem is the inability of the transmitter to achieve symmetrical 100% modulation due to insufficient high voltage from the PE-73 because of its operation on +24vdc. In fact, due to the low +HV, 50% modulation is a common limit. To elaborate on the nonsymmetrical modulation, 100% positive modulation is next to impossible to achieve while 100% negative modulation seems rather easy. However, by reducing the loading to lower the output power, it is possible to improve the modulation symmetry. This reduced loading will also help the FMing problem. Usually a BC-375 can produce about 90 watts into a 50 ohm unbalanced load provided the tubes are fairly new and the transmitter is in good condition. At this level of power though, the load on the dynamotor is pretty high and since it is running on +24vdc, its RPM is too low for good stability. This results in severe FM'ing due to load fluctuation and the low +HV results in about 50% modulation. Most of your audio reports on VOICE will be "you sound terrible." A power output of about 40 to 50 watts will reduce the load on the dynamotor which reduces the FM'ing due to voltage fluctuations due to power load changes and this will improve the transmitter sound overall. Also, at the lower power level, the modulation level will be higher since the carrier envelope to be modulated is less. At 50 watts output power, 100% modulation is possible to achieve and with careful monitoring of the signal on an oscilloscope, negative modulation in excess of 100% can be avoided. Actually 100% negative is the limit, then you go into "cutoff" and that can be avoided with signal monitoring. So, when running on batteries only, the first thing to do is to reduce the output power to 50 watts. Remember, even though the carrier power is reduced your audio power is actually better (because of a higher modulation level) and overall your signal will be much more "powerful" sounding.

The Microphone Choice is Important - Options - Carbon microphones used with the BC-375/BC-191 are another source of audio problems and this problem applies for both +24vdc or +28vdc operation. The T-17 or T-32 carbon microphones were standard equipment. The T-17 was designed to provide intelligible communications from a very noisy environment, so almost all T-17 mikes will be noise-cancelling in design and therefore very limited in response.  A few very small openings in the mouth cup indicates a noise cancelling microphone. The T-32 looks like a "candlestick phone" and has a very large carbon button element. These mikes were commonly used with the BC-191. They can be a very good performing mike but many suffer from carbon packing, carbon fusing and other age-related and abuse-related problems. Typically, you'll find that almost all of the original carbon elements are practically non-functional nowadays, after all, they are pushing 70 years old! The usual procedure is to acquire as many T-17 mikes as you can and test them. After about a dozen or so, you'll find one that works well with lots of response. Be prepared to go through a lot of T-17s though. The small T-1 carbon replacement element can still be found in NOS packages and these usually will work fine too. Regular older telephone carbon mike elements work but will require testing to see if they will sound correct. Some do and some don't so testing is the only method to find out.

It's also possible to use more modern carbon microphones. These tend to have fewer problems with age and use related issues. I use a modified Shure 102-C carbon mike* with my BC-375 and the results are amazingly different than trying to use a T-17. Response is greatly improved and the audio gain can be reduced to about 6 or 7 and still achieve 100% modulation. The disadvantage with these more modern mikes is that they aren't noise-cancelling and the noise from the PE-73 in the background will be obvious. However, military radio enthusiasts enjoy hearing the dynamotor running in the background of your signal transmission, so it probably isn't that much of a problem.

There are also carbon mike substitutes, like electret mikes. These were commonly used as carbon mike replacements in some modern telephone equipment and are very common in microphone applications for computer use. Electret microphones consist of a condenser mike with a small FET amplifier for the output. They require a small voltage to power the built-in FET amplifier and that can come from the BC-375 carbon bias. They can be a little noisy. You will also have to add a transistor amplifier circuit in with the electret mike to isolate the carbon mike bias line from the FET amp and to provide a low Z output with the BC-375 input transformer acting as the collector load on the transistor. This will provide plenty of audio since it is audio driven low-Z and right in the audio line just like a carbon mike. There are several Electret circuits shown on the web.

As of 2021, there is a "designed for the T-17" electret mike module that is for sale on eBay for about $30. These modules are easy to install into the T-17 with only two wires needed for the connection. The results are quite good with lots of response and very good audio. I believe the design was developed to work best with the "noise cancelling" type of T-17, that is the one with the three tiny holes in the cup. If the electret module is used with a large opening then the mike has to be held about a foot away and the breath noise is very apparent. Also a spit cover seems to adversely affect the audio quality. Use these electret modules with the common "noise cancelling" T-17 for best results. NOTE: The manufacturer of these electret modules has indicated that he will no longer be supplying the modules by any venue. So, no longer available as of June 2021.

If you are contemplating bypassing the bias voltage source and running an amplified mike directly to the audio input transformer - don't. The primary of the transformer is very low impedance, around 50 Z ohms or so. It is so low that it will limit any bass response in the mike and unless the mike is amplified you'll never have enough output for full modulation. Stay with the carbon mikes and try to find one with good response. The BC-375 was designed for a carbon mike and performs best using a carbon mike.

Other Information That Might be Useful

Today, hams usually want to use Voice mode and make transmissions that last several minutes. On 160M and on 75M this isn't a problem. Drift above 4.0mc will become a problem because of the length of time the transmitter is on. If you are contemplating 40M operation, you will need to make very short transmissions - 30 seconds would be about the maximum length before frequency drift becomes quite noticeable. Some users find that removing the tube cover plate will help with the drift since it keeps the temperature of the tubes cooler. Generally, drift problems are negligible on 160M and 80M but can make operation on 40M impossible.

CW Operation - Using a BC-375 on CW will generally garner several comments relating to "chirp" or "blooping" of the CW note. Like drift, on 160M and 80M, the CW note is pretty good although there will be some noticeable blooping if the transmitter is operated using a battery-only set up. On 40M, the CW note is pretty bad with severe blooping accompanied by rapid frequency drift. The better your power source is the less likely that blooping will be noticed on 160M or 80M. A high current power source like the PP-1104 will eliminate most CW issues on 160M or 80M. On 40M, it doesn't seem to matter very much. Using a BC-191 and RA-34 combination will produce some ripple to the CW note due to the "split" 12 volt supplies in the RA-34 that power the BC-191 tube filaments with 12vac. It's not bad but it is noticeable.

CW operation is accomplished by keying the PTT line operating the sending relay within the transmitter. This is rather noisy and unless a headset is used for reception (and sidetone) the racket can become irritating after awhile. Also, the sending relay limits the CW speed to 15 WPM or less. Again, during WWII and onboard the airplane the transmitter noise was masked by all of the other noises going on during flight. Also, short, one word CW transmissions didn't allow the radioman (who always used a headset) to notice the sending relay noise. In actual use nowadays in a typical ham shack, with the PE-73 running and a PP-1104 fan spinning, the sending relay noise is not all that noticeable.

The sequence of keying operation is very important for minimal "blooping" of the CW note. There was an accessory piece of equipment that may have only been available to depot repair technicians. It was connected to the BC-375 or BC-191 and monitored the keying sequence. Hand operation was necessary to very slowly rotate the sending relay arm and this was accomplished with a couple of auxiliary tools (a clamp and an adjusting tool.) The "relay sequence tester" had a series of jeweled lamps that had to illuminate in the correct order for the sequencing to be correct. There are adjustments on the relay that position the contacts to allow proper sequencing. When the keying is sequenced correctly and the transmitter operated on its dynamotor running off of a PP-1104C power supply, the CW note will be solid with no blooping. This piece of test equipment is seldom seen because apparently it was built by the depot technicians per instructions that are in the manual. There's no Signal Corps identification other than "BC-191 Relay Sequence Tester." For more details with photos on this equipment go to AAFradio.org - click on "TOOL" in the "B-17 Corner" section of the website.

1. Remote Standby - access pins 21 and 22, provides NC during receive and NO during transmit from BC-375 sending relay (component #1189)

2. Side Tone Output - access pins 33 and 32 (Gnd), provides 1000 cycle note with key down when in CW mode (Hi Z output)

3. Remote Microphone Input with PTT - use the following pins: Ring (Mike Input) is pin 23, Tip (PTT) is pin 34 and Shell (Gnd) is pin 24 - references a PL-68 phone plug

4. Remote CW Key Input - access pin 34 (PTT line) worked against pin 32 (Gnd)

5. Remote Power ON/OFF - access pin 20 worked against pin 24 (Gnd) - remote switch closure will turn on transmitter-dynamotor

Remote Standby and Sidetone interfaced with the BC-348 receiver, muting the receiver during transmit and providing the radioman a side tone for monitoring his CW sending (which was heard through his headset.) Remote mike and key along with remote power ON were to allow the airplane's pilot to access and transmit on the BC-375 from the pilot's seat - something that might be necessary if the radioman was unable to operate the transmitter. The pilot's position had a Remote Junction Box to provide the mike and key input along with the power ON control.

Nowadays just about the only thing that would be used would be the Remote Standby for the BC-348 receiver. Use pins 21 and 22 on the PL-64 and pins 2 and 6 on the PL-103 for the BC-348 to allow the BC-375 to provide remote standby. If you want to have the Side Tone, you can connect PL-64 pins 33 and 32 to BC-348 pins 1 and 5. This connects the Side Tone to the audio output of the BC-348. When the BC-375 is transmitting the BC-348 is muted and the Side Tone operates the 'phones (or loudspeaker.) When the BC-375 isn't transmitting the Side Tone is off and the BC-348 operates the 'phones (or loudspeaker.)

Reference the BC-375-E military manual (Technical Order 08-10-139 is the manual number - original manuals were available from Fair Radio Sales) and the schematic for complete diagrams of the PL-64 connector in the transmitter and the PL-64 plug and cable assembly.

You can make your PL-64 cable using multi-conductor shielded cable. Neither Remote Standby or Side Tone requires much current so fairly small conductors are okay. 22 gauge would be about the smallest that should be used. If you use a carbon mike for a remote mike, you don't have to shield it anymore than the cable shield. The carbon mike is very low impedance and fairly immune to RFI.

The BC-375 Experiment Using PP-1104-C Power Supply

Much of the BC-375's "bad reputation" was impart due to CQ magazine's "Surplus Conversion Handbook" assessment that transmitter was obsolete and only good for parts. Certainly the 1950s was a time when the bargain prices and unlimited availability of WWII surplus gear made it seem that "hacking up" an old relic didn't amount to much historical damage. After all, there were thousands available and they were cheap. CQ magazine was the main advocate for "modification mania" and convinced droves of hams that they could "out design, out engineer" and "knew more about radio" than the teams of professional radio engineers that were employed by some of the largest radio manufacturers in the world. This belief was further popularized by magazine articles featuring the latest "hack job" and with glowing reports of how great everything worked after a ham had performed his modifications. However, what had worked fairly well for the BC-348 and a few other military designs wasn't going to work for the BC-375. All attempts to modify the BC-375 into a usable ham transmitter failed - mainly because the transmitter was designed for a specific purpose in military aircraft or vehicular applications. No modifications seemed to work and, since it was a simple MOPA military transmitter, in CQ's opinion, it was worthless. Remember though, CQ was advocating that the BC-375 should perform like a ham transmitter and not that it didn't perform like a military transmitter. Since it couldn't be modified to function as a ham transmitter it must only be good for parts.

If one believes that the WWII radio engineering teams "knew what they were doing" and designed the BC-375 to function properly without distortion or instability. If one knows that the transmitter was successfully used by the military for two decades and that must account for performance that was acceptable. Then the BC-375 performance problems must be rooted with how the 1950s hams were trying to operate the transmitter. I decided to try and operate the BC-375 as closely as possible to specifications and at the proper input voltage and current levels and see what kind of performance would result. As a power source I chose the PP-1104-C high current adjustable power supply. The PP-1104 will supply well over 50 amps at +28vdc - nearly double the current requirement for the BC-375 operated by the PE-73 dynamotor. I also chose to run the voltage slightly higher (at +28.5vdc) as it seems likely that the battery-charger buss on the airplane would be on "the high side" with the airplane aloft and running "at speed." Next, I had to neutralize the Tuning Unit because it was not a matching serial number TU but was an "orphan" TU that was obtained separate from the transmitter. Since this TU had been neutralized to a different BC-375, it would have to be neutralized again to my BC-375. I performed a dynamic neutralization on this TU. All power cables to both the transmitter and the receiver (BC-348-Q) were shielded. I believe the original cables were shielded but this is not specifically shown on the schematics. Most transmitters are found without cables today. The shielding prevents stray pick-up and stray radiation. A good earth ground helps too, although while aloft a good ground is hard to obtain! The airplane frame acted as a return ground for all of the equipment and as a counterpoise to the trailing wire antenna, basically an unbalanced end fed wire. However, I was going to be using a tuned dipole presenting a 50 Z ohm unbalanced load with very little reactance (mostly resistance.) A digital frequency counter was used to monitor frequency and an oscilloscope was used to monitor modulation. So that was the set-up.

Testing with local monitoring showed that 100% modulation was easy to obtain with good condition microphones. Typical was about 90% modulation using a T-17 microphone with the audio gain set to 8. A more modern Shure 102-C carbon mike achieved 100% negative modulation with the audio gain set to just 6.5. The wave envelope pattern viewed on the oscilloscope was symmetrical and 100% positive modulation was also apparent. No FMing was detectable unless the local receiver BFO was turned on and then only a very slight frequency excursion was noticed on some voice peaks. Without the BFO on, no FMing was detectable. Power output was running at 75 watts carrier power output. Audio quality monitored locally using an SP-600 receiver with a headset sounded very good with all of the characteristics expected from a carbon mike. I set up an impromptu "on the air" test after one of the Sunday morning Vintage Military Radio Nets on 75 meters (3974 kc.) This allowed me to have about six or seven ham military radio enthusiasts critically listen to my BC-375's signal. My test transmissions were copied by all of the participants most of whom were around 150 to 200 miles away. All comments were very positive with "I would have never guessed that was a BC-375" being the typical reply. Additionally, none of the stations detected any FMing unless they switched on their receiver's BFO and then the FMing was only slight and only on some voice peaks.

The point of this experiment was the demonstrate that there is nothing wrong with the BC-375 design. If the transmitter is powered up with the same voltage and current capabilities that were found on the airplane and if it is operated in the manner that it was intended, it performs admirably. Now it certainly won't pass the "Broadcast Audio" test because it isn't that kind of transmitter. A properly powered BC-375 will produce "military communications grade" audio with no distortion and, of course, it will have the typical "carbon mike" sound. That's normal and what should be expected from the BC-375.
 

UPDATE - October 12, 2013 - To prove to myself that the results of this test were not specific to my BC-375 transmitter, I performed another test using KØDWC's BC-375 transmitter. The KØDWC BC-375 needed to have its TU-6 Tuning Unit dynamically neutralized and the microphone gain needed adjustment but everything else was fine with this transmitter. The transmitter was at my QTH in Dayton, Nevada, so I was using my PP-1104 for the power source, my shielded cables, my Shure 102-C microphone and my antenna-ground system. I was operating the KØDWC BC-375 at 75 watts of carrier power output. This time, rather than testing after the Net, I just checked in using the KØDWC BC-375. The reports back from the other Net check-ins were the same as the previous test with my BC-375. Typical comments being "I'd have never guessed that was a BC-375. It sounds great!" and "Only a very slight frequency change and only on some voice peaks."

I think this is pretty convincing evidence that, when operated at +28.5vdc with 50+ amps current availability along with dynamic neutralization of the TU, shielded power cables and a good condition carbon mike, the BC-375 is capable of good quality military communications audio at a respectable power level.

Using the PP-1104-C to Power the BC-375   -   PP-1104 Basics

Curing RFI in the PP-1104 - Some PP-1104-C versions are rather noisy in the RF spectrum, mainly due to the magnetic amplifier that will produce some RFI. The PP-1104-C can be set-up with shielded AC lines coming in and shielded cables for the output. Be sure that the case of the power supply is also connected directly to the station ground system with a 10 ga. ground wire. Additionally, it's a good idea to bypass the AC line in with .01uf capacitors to chassis and to also bypass both the positive and negative output terminals with a .22uf tubular and a .01uf ceramic disk capacitor connected in parallel to chassis. Be sure to check the 10-32 screws that mount the top cover, the bottom and the back cover to see if good grounding contact is being achieved. Usually the paint is very heavily applied and quite hard and this sometimes insulates the covers from making good contact to the main frame. It's usually necessary to use a small sanding disk to remove the paint where each screw head makes contact and remount the screws using toothed lock washers. This will provide an "RF tight" enclosure which helps considerably in reducing RF noise. Thanks to Jerry Fuller W6JRY for the PP-1104-C RFI suggestions.

photo right:  The PP-1104-C. This one was built by Gladding Keystone Corp. in 1967

Another PP-1104-C Suggestion - In looking at the photo of my PP-1104-C above you will probably see that I have it setting on a small furniture dolly. I bought the small dolly at Harbor Freight Tools but I think they are available at almost any large hardware or tool store. The small dolly is a perfect fit for the PP-1104 and allows it to be moved around the shop with ease.

And Another Suggestion - The PP-1104-C operates much more efficiently when powered by a 230-240vac line voltage. These power supplies were originally delivered set-up for 230vac operation. The cover on the right side of the cabinet can be taken off to reveal TB1 terminal strip and if terminals 2 and 3 are jumped, the input voltage should be 230vac. If terminals 1 and 2 are jumped and terminals 3 and 4 are jumped, the input voltage should be 115vac. AC line input is on terminals 1 and 4 (and 4 is jumped to 5 at all times.) The momentary output voltage drop when a large load is switched on is greatly reduced when the PP-1104-C is operated on 230vac. However, the PP-1104-C does operate fine in practically all cases on 115vac. I've been running the Gladding-Keystone on 120vac for over ten years and only in one case did it have a problem running a DY-12 dynamotor. Although that dynamotor ran fine once at speed, on many occasions the PP-1104C output voltage dropped so low with the dynamotor's instantaneous starting current that the dynamotor's start-relay would "chatter" a few times before the motor started to move and the current demand dropped. In all other operations, the PP-1104C operated on 120vac with no problems. A test of the "problem" DY-12 operating on a PP-1104C running on 240vac didn't produce any "chattering" when powering up an ART-13 transmitter.

Operating on 630 Meters CW with the BC-375

The TU-26 should have its calibration chart on the front panel that shows the proper MO Tuning setting for the desired operating frequency. It's a good idea to check the neutralization even though CW will be the intended mode of operation. Use the dynamic method as this is the most accurate and easiest to accomplish.

When set-up for operation below 500kc, working in conjunction with the BC-306 and the TU-26, the BC-375's internal antenna tuner is bypassed. Controls O, P and M have no effect on transmitter operation.

I used one half of my "2 half-waves in phase" antenna. That resulted in 77 feet of one side of the 450 ohm feed line and 135 feet of antenna (one leg of the 270 feet length,) resulting in a total of 212 feet of antenna. This is connected to the Antenna post on the side of the BC-375. Ground should be connected to the proper post or to the BC-375 chassis or cabinet. This antenna is actually slightly longer than the trailing wire maximum of 200 feet but I'm able to easily match its load to the BC-375 on 473kc.

For a starting point, use the settings in the manual for 500kc and this will allow you to at least "dip" the plate current. From there, various settings can be changed to see what allows matching the load to the transmitter. Use the calibration chart to set the MO frequency, I used 473kc (475kc is shown on the chart.) The object is to get the PA current to about 180mA to 200mA at resonance. If the PA current drops down below 100mA at the "dip" then more antenna loading is needed. It took about 10 minutes of trying different combinations to find settings that worked for my set-up. This was mainly because the manual suggestions are for an antenna that is shorter than 200 feet since it recommends setting N to 3. Since my antenna was 212 feet long, setting N to 4 (longer antenna) worked great where N to 3 didn't work at all. Once I switched over to N to 4, proper loading was accomplished in less than a minute.

The CW note on 473kc is very different from the note on HF operation. No blooping or other instabilities. Just a pure "DC note." No noticeable drift either. I'm not loading the transmitter to its full rated PA current of 220mA as this really "pushes" the loading on the PE-73. Adequate output power is achieved at about 180mA and the loading on the dynamotor is reduced which results in good stability.

BC-375, TU-26 and BC-306    
Settings for 473kc,... 212 ft end-fed wire
 

  A = 3      -      Band Change on TU-26

  B = 20.067    -   MO Tuning on TU-26

  C = 71.5        -    PA Tuning on TU-26

  D = 2         -    Ant Coup SW on TU-26

  N = 4         -    Ant Circuit SW on BC-375

  E = 2        -     Ant Vario SW on BC-306

  F = 8.5        -      Ant Vario  on BC-306
 

          Total PA I = 180mA
 

  O = Not Used   -   on BC-375

  P = Not Used    -   on BC-375

  M = Not Used    -   on BC-375

The test showed that the BC-375 can be used on 630 meters successfully. It has a very good, steady, pure DC note to its CW using the TU-26 tuning unit. During the problematic conditions of midsummer and early evening, the BC-375 was still able to be copied at a distance of over 250 miles. The other thing this test showed was how important the receiving location is. Tom is plagued with local neighborhood RFI on MW. Even a shielded loop didn't help. However, going to the Internet SDR, located away from RFI, he could hear my signal.  >>>

If you're interested in using vintage military gear on 630 meters, go to the web-article "Vintage LW Receivers - Part 4" and go to the section on "Operating Vintage LW Gear on 630 meters." I have a lot of information there on getting thru the process of dealing with the UTC to find out if your planned 630 meter operations will be compatible with your local utilities. Also, information on using the ART-13/CU-32 combo on 630 meters. Use the Home/Index for navigation.

To Sum it Up

 The most important things to do to your BC-375/BC-191 transmitter are:

1. If you're going to operate using a +24vdc battery setup then reduce power output to around 40 to 50 watts. This will improve modulation symmetry. You can increase the power to 65 watts for operation of the BC-191 on the RA-34 power supply. If running a BC-375 on a high-current +28vdc power source, like the PP-1104, RF power can be increased to 75 watts on voice.

2. Dynamically Neutralize the TU at the frequency you intend to operate the transmitter. This is the most important step for "clean sounding" audio without the typical BC-375 "fuzzy" sound. This step is necessary no matter how you are powering the transmitter. If you have "matching" TUs for the transmitter, that is, the serial numbers all match indicating that the TU to be used is the original one for the transmitter, you might find that the neutralization is very close already. However, it's very strongly recommended that the neutralization be set by the "dynamic" method, even on matching TUs, to assure the absolute best sounding and cleanest audio from your BC-375.

3. Find a high quality Carbon Microphone or use an Electret substitute. Quality sound will usually require a new Carbon element in order to have the response necessary for good audio modulation. Remember, we are trying to achieve "clean" audio with the "military" sound - not Broadcast Quality audio. If you're looking for Broadcast audio, you'll never find it in a BC-375 or BC-191.

4. If you're using batteries as the power source, make sure your Battery-Dynamotor set-up is operating correctly and that the batteries are at full charge. Be aware that batteries alone will NOT supply +28vdc power. Without the charger system (that was always running when the airplane was aloft - or the vehicle motor running in the case of the BC-191) the batteries by themselves will very rapidly discharge. Additionally, at full charge the batteries alone will only supply about +26vdc maximum and that begins to decrease immediately with the transmitter in operation. Typical batteries will provide +24vdc for the majority of your operation. Turn off the BC-375 between transmissions to conserve the battery's charge as much as possible. This would be for net operation only and probably not practical for a "one-on-one" QSO. You can also parallel a DC power supply that can somewhat act like a generator, providing charging while operating the transmitter. The power supply should be capable of at least +30vdc and have current capability in excess of 40 amps but even if you can't supply that much current, 10 amps or so will help keep your batteries charged while you're in the receive mode.

5. The BC-375 runs much better at +28vdc. Use of a high current power supply such as the PP-1104 will allow the BC-375 to operate at +28vdc with ample current reserve. It's been my experience that when the BC-375 is run at a solid +28vdc you can increase power output to 75 watts without non-symmetrical modulation appearing and your signal won't FM - at least on 160 meters or 75 meters, it won't. Since the dynamotor is operating to spec, the output voltage is in excess of +1000vdc and because the dynamotor is running faster with plenty of reserve current, that +1000vdc is pretty stable to load variations, thus the lack of FMing and better all around performance. Dynamic neutralization is still necessary, though.

6. The BC-191 can be operated with its specific military AC power supply, the RA-34, which eliminates the need for batteries and the dynamotor. The RA-34 will not operate a BC-375 however due to its +12 volt LV design and the different wiring of the low voltage section of the transmitter. The RA-34 provides dual 12 volt supplies (both AC and DC) that power different sections of the BC-191 transmitter (AC to the tube filaments, DC for the other LV requirements.) The BC-375 design ties these two low voltage sections together in parallel and runs them on +28vdc with dropping resistors as needed. The RA-34 will provide stable, quite operation of the BC-191 but the RA-34 does have a quirk or two. If you use a BC-191 and RA-34 combo on CW you'll have quite a bit of ripple on the CW note since parts of the transmitter are being operated with 12VAC. It's likely that the majority of military operations using the BC-191 were on VOICE, so the minor ripple on a rarely used CW mode didn't bother the Army. It was still readable, but be prepared for critiques when operating CW with a BC-191 and RA-34 combo. More than likely, you're going to have to be careful on VOICE and don't try to load the transmitter to maximum output. You usually can run the BC-191/RA-34 combo at about 65 watts output without any problems.  

With these steps, your BC-375/BC-191 will sound about as good as it can. 

I'm sure that today the majority of VT-4/211E tubes are NOT being used. The current audiophile/vacuum tube investor purchasers have no intention of ever powering up a VT-4. They won't be installing them into any type of audio amplifier. The only intention is to hoard them for future sale - an investment. And, because of this hoarding/investment and its effect on VT-4 prices, today, (2021) it's rare to hear a BC-375 or BC-191 on the air. I don't know of any that are undergoing restoration. Most military radio enthusiasts figure "why try to operate a transmitter that is so expensive to restore and so difficult to get to sound good when, for a fraction of the cost, an ART-13 can be purchased and it runs twice the power output - and sounds really good doing it - and the tubes for it are cheap and of no interest to audiophiles or investors."

If you're patient, with a little luck in checking swap meets and ham radio listings, it might be possible to find VT-4 211E tubes "one-at-a-time" for a more reasonable price. Among most military enthusiasts the VT-4/211E does sell for much less than eBay prices but it's still an expensive tube regardless of the source (typical "mil-collector" price usually runs from $75 to $125 for a known-good tube but this typical sale would usually be "in person" and definitely not on the Internet.) The key is patience. It might take quite a while to find a complete set of tubes at a reasonable price but it is still possible.

UPDATE: March 8, 2021 - I may have to "take back" what I thought about the value of a "tube-less" BC-375-E. One just sold on eBay for $405.00.

More information, including a possible source of cheap VT-4/211E tubes next.

The End of Operational BC-375 Transmitters?

Shown to the right is the interior of the BC-375 showing the four VT-4/211E tubes along with the 10Y/VT-25 tube to the far left. The demand by audiophiles (actually by "vacuum tube investors") for the 211E has had a catastrophic effect on many future restorations or rebuild attempts of the BC-375 transmitters. It's not at all uncommon for a single 211E in good usable condition to sell for $250 (that's Fair Radio's 2021 price) and, if the tube is NOS in the original box, it would sell for double that on eBay. Power triodes that interest tube investors always sell for more in a set, so a good condition "quad" of 211E (necessary for a "tubeless" BC-375) could easily sell for $1200 to $2000. Luckily, these insane prices are only found on eBay and only when dealing with tube investors from Asia (or USA sellers that ONLY want to sell to tube dealer/investors in Asia.) Between military radio enthusiasts a more common price is about $125 a piece for a good usable 211E but that's still $500 for a quad if you have a "tubeless" BC-375 (check Aug 2021 update below.) The 10Y is still not expensive (except the Western Electric VT-25 version.)

There is a fairly common VT-4 substitute tube, the 805. However, although the 805 is an identical tube to the 211E, the external structure is different in that the 805 employs a plate cap where the 211E uses a base pin connection. There's ample room for plate leads without drilling holes so the incorporation of 805s into a BC-375 can be accomplished fairly easily. The only problem is that the tube dealers have now discovered the 805 also and the price of that tube has started to climb. 805 tubes are still cheaper than the 211E,...but for how long?

The high prices of power triodes like the 211E or the 805 have certainly halted or, at least, slowed down most BC-375 rebuilds. Today, the "standard condition" is to find most BC-375 are for sale "without tubes." Unfortunately, the high cost to "retube" a BC-375 has relegated "tubeless" transmitters to a "parts set" status. And, if a complete "tubed" BC-375 is for sale, the "tube investor" price will certainly be factored in.

The military radio enthusiast should take a look at the easily available ART-13 transmitter. These transmitters use tubes that are of no interest to vacuum tube investors. ART-13s are easy to rebuild, easy to get going and they produce a 100 watt carrier, 100% modulated with excellent audio.

1. BC-375-E Manual - Technical Order No. 08-10-139 - January 5, 1943

2. Thanks to Tom Horsfall WA6OPE, Jerry Fuller W6JRY, Dennis Duval W7QHO, Jack Antonio WA7DIA, Chuck Cusick KØDWC and many other WWII military radio enthusiasts for their contributions and suggestions that have ultimately resulted in the BC-375 and BC-191 transmitters becoming good-sounding, usable military transmitting equipment.