Radio Boulevard
Western Historic Radio Museum

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

What's Important for Stable Operation
Non-symmetrical Modulation Problems
Neutralization Problems
How to do a Dynamic Neutralization
Microphone Options
CW Operation


by: Henry Rogers - WA7YBS/WHRM

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

photo above: SCR-287 on display at WHRM

SCR-287 - Photo above left

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. I built the olive-drab desk set-up to better display the station for the Western Historic Radio Museum. The transmitter used was the BC-375 along with the BC-348 as a receiver. The other components shown are the BC-348-Q receiver which does run on its original dynamotor from the battery supply, the Lionel J-47 telegraph key and the Shure Bros. carbon microphone, the T-17. 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. Again, the console is not a WWII vintage item - it's homebrew. It 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. This station was on display in the museum until I closed in 2012. Nowadays, this SCR-287 is in the process of being set-up to operate with the high-current adjustable power supply, the PP-1104. This allows getting rid of the batteries that had the station in a constant "discharge" state. The PP-1104 is now powering the station at +28vdc with plenty of reserve current available providing a signal that is stable with no FMing and 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 - Successful operation of the BC-375 family of transmitters on the ham bands is 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" and used with its proper dynamotor and an automobile battery set up you will certainly be the recipient of many "critical audio reports."

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, we do want to operate the transmitter as close as possible to original without enduring endless criticism of the signal and the audio quality and sometimes hams just can't provide anything other than car batteries for a voltage source. So, our endeavor will be to operate the BC-375/BC-191 with all original parts on series (or parallel for the BC-191) connected car batteries and achieve an AM signal that is intelligible and sounds more or less natural. 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 and +24vdc for the BC-375 would be +12vdc for the BC-191. This saves us from constantly including both voltages in the text.

Non-symmetrical Modulation - One of the main problems is the BC-375 does "FM" very easily when carelessly operated on +24vdc. This is one source of audio distortion in the VOICE mode. Another problem is the inability of the transmitter to achieve symmetrical 100% modulation (and this is mainly due to insufficient voltage from the PE-73 because of operating it on +24vdc.) In fact, 50% modulation is a common limit (when operating "per the manual," that is.)  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 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, severe FM'ing will occur very easily, 50% modulation will be about the limit and most of your audio reports on VOICE will be "you sound terrible." A power output of about 50 to 55 watts will reduce the load on the dynamotor (or power supply) 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 much easier 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, 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 and overall your signal will be much more "powerful" sounding.

Microphone 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 audio frequency bandwidth. One or two 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 and other age-related problems. Almost all of the original carbon elements are practically non-functional nowadays, after all, they are pushing 70 years old! There are 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 require a small voltage to power the built-in FET amplifier and that can come from the BC-375 carbon bias. It's a little noisy but hardly noticeable with all of the other BC-375 issues taken into account. 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.

Due to the biasing circuit in the BC-375's input, only carbon or electret mikes can be used unless the input is changed. But, you can use an inline microphone preamp to boost the audio level and also to interface a crystal or dynamic mike to the BC-375. You can couple with a capacitor to block the carbon mike bias. Usually the bass response will be lacking but it depends on your preamp design. Remember, the input Z for the carbon mike was pretty low, probably in the 50 ohm region. That's why higher Z outputs lose their bass response. Your pre-amp should have a very low output Z for best audio response. Though it goes against our philosophy of  running the equipment as original as possible, you might be considering a "slight" modification to the BC-375....

An Un-recommended Modification - By disconnecting the carbon mike bias line and running a shielded cable directly to the audio input transformer, now an amplified dynamic or crystal microphone can easily be used. The problem here is the very low input impedance of the audio input transformer primary will greatly reduce the bass response. However, the audio is very clean and highly intelligible. This easy mod can be done without removing any parts or doing any damage. It is just as easy to reverse the mod, if desired. Using an Astatic amplified TUG-8 mike base and a crystal mike head, the PTT can still be used and from the exterior no changes in operation or appearance can be noted. You do have to run the amplified base gain rather high - at about 80%.

Mod details: First, remove the back of the transmitter. You don't have to take the top off to remove the back. Next, locate the audio input transformer. You'll note that the four terminals are marked with numerals 1 through 4. Unsolder the wires going to terminals 1 and 2. Tape these lead ends with friction tape to insulate. Unsolder just one end of the 200 ohm resistor across terminals 1 and 2 and tape the loose end. There is also a TC lead from the transformer going to the gain pot that needs to be unsoldered at terminal 1 and taped. Now, all wires and components going to the audio input transformer are disconnected and their ends taped for insulating purposes. Now solder a shielded cable (RG-58U is okay) with the center conductor going to terminal 1 and the shield going to terminal 2. Route the cable down the right side of the transmitter panel as viewed from the rear. Turn the transmitter upside down and now you can easily see the mike input jack. Unsolder the two brown wires that connect to the "ring" terminal on the jack and tape their ends. Solder the shielded cable center conductor to the "ring" terminal and solder the cable shield to the "shell" terminal which has a TC lead to chassis. While the transmitter is upside down, install two 20uf to 50uf electrolytic capacitors across the two 1uf tub capacitors that are the grid bias filters for the 10Y and the 211 modulators. Note that the positive lead on these capacitors has to be connected to chassis. Turn the transmitter rightside up and re-install the back cover. This completes the "slight" mod. No components were removed and the transmitter PTT still operates as normal. The only change is now you can't use a carbon mike. You'll have to use a crystal or dynamic mike with a preamplifier. The easiest to use is the Astatic TUG-8 base with a crystal mike head. You'll have to turn the gain on the stand base up about 80% for enough output to drive the very low input impedance of the BC-375's audio input transformer. It's also possible to use a separate mike preamp that has some compensation in the circuit and has a transformer output to match the low Z audio input transformer and achieve better bass response. With just the TUG-8 base amplifier there will be very little bass response and the audio with be more or less "communications grade" audio. Reversal of the "slight" mod is very easy since no original parts were removed and no holes drilled.

I don't recommend this mod since we are trying to run the BC-375 as original as we can. It's presented here because I have tried it and it is a quick way to achieve decent audio. I have to also say that after I tried this modification, I quickly returned the transmitter to stock. This decision was quickly reached after listening to the unnatural sounding audio in the monitoring receiver. Extremely devoid of any bass response but very crisp and intelligible - not like a BC-375 at all.  I recommend searching for a high quality carbon mike or modifying a mike body to use an electret mike as a substitute for the carbon element. This way the BC-375 will sound pretty original and give the listener an idea of what military radio ops had to listen to.

To sum it up, the most important things to do to your BC-375/BC-191 transmitter are:

1. Reduce power output to around 50 watts, 60 watts maximum, if operating on +24vdc battery set-up. 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, RF power can be increased to 75 watts.

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.

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. Make sure your Battery-Dynamotor set-up is operating correctly and that the batteries are at full charge. Batteries are really not a very good source of consistent high current +28vdc power. Without the charger system (that was always running when the airplane was aloft - or the motor running in the case of the BC-191) the batteries by themselves will very rapidly discharge. Additionally, at full charge the batteries 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. 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 20 amps.

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 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 follow the same suggestions as operating the BC-375 on "pre-flight" +24vdc. 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. Remember, the "BC-375 technology" was archaic when the transmitter was new, so today it really is "Ancient Modulation" but when set up properly it can certainly provide listeners with the authentic WWII sound of the BC-375. CW is possible but it's very noisy due to the sending relay (but if you're running a PP-1104 and a dynamotor you'll hardly notice the increased din.) The sending relay also limits CW speed to about 15WPM. All of your BC-375/BC-191 operations should be on specific Vintage Military Radio nets where the operators are familiar with the military sound of carbon mikes and have even heard quite a few BC-375 and BC-191 transmitters. With operation in these kinds of nets, you'll usually get good advice on your signal quality that isn't based on the desire for Broadcast quality audio.