Using the BC-375-E Today

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 will be trouble-free but operation on 40M pushes the BC-375 frequency high enough for instability and FM'ing to become an insolvable problem. Nowadays, if the BC-375 is operated "per the manual" and used with its proper dynamotor and a good battery set up for the 24vdc you will certainly be the recipient of many "critical audio reports." When the BC-375 was designed for the military from the older BC-191 (and its even earlier kin) the same concept of robust construction and limited audio capability was used. The idea that the transmitter needed to be instantly operable with ease and to be "indestructible" due to mis-use or mis-adjustment meant that over modulation must be avoided since this could rapidly destroy the modulation transformer. Another rumor was that the military didn't want the transmissions to propagate too far, so the modulation was kept deliberately low. Hmmm, maybe. Suffice it to say that today the BC-375 and the BC-191 cannot be operated "per the manual" on the ham bands. 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. 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. So, our endeavor will be to operate the BC-375/BC-191 with all original parts and achieve an AM signal that is intelligible and sounds more or less natural.

The main problem is the BC-375 does FM very easily when carelessly operated. This is one source of audio distortion in the VOICE mode. Another problem is the inability of the transmitter to achieve symmetrical 100% modulation. 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.

Next, the tuning units that are used absolutely need to have their neutralization adjusted for minimum FMing. If the transmitter is not neutralized, then some minor parasitic feedback in the master oscillator will cause frequency instability, or FMing, causing more audio distortion. The easiest method requires a receiver capable of FM detection that will tune to the BC-375 transmitter frequency. These types of receivers are always limited to more modern types of communications and SWL receivers. I borrow a Yaesu FRG-8800 from a neighboring ham which does have the capability of receiving FM on all frequencies. Tune in the transmitter's signal on the FM receiver and talk into the microphone. If the BC-375 is FM'ing, you will hear yourself quite easily. It's best to use earphones for monitoring. Adjust the TU neutralizing control until you no longer hear your voice modulation and the FM'ing and audio distortion should be at a minimum for that tuning unit at that frequency. You might have to adjust for the minimum audio response as voice peaks always seem to cause a little bit of FMing. The difference can be checked by switching the receiver over to AM detection and listening to yourself transmit. All of the "fuzzy" sounds should be gone and the AM signal should sound clear and intelligible. If you have the proper kind of FM detector-receiver, it's easy to check the neutralizing often and the results are simply amazing. There is no other easy way to set the neutralizing on the tuning units.  NOTE: I have tried other methods to set the neutralization for minimum FMing, like using a standard communications receiver with the BFO on and listen for deviations in the carrier frequency but this doesn't work at all. About the closest to FM monitoring is to use earphones to monitor your BC-375 AM signal on a standard communications receiver and adjust the neutralization for the best sounding audio. This will get you kind of close but not as close as the FM receiver does. Thanks to Tom Horsfall, WA6OPE for the neutralizing information using an HF FM receiver.

Carbon microphones used with the BC-375/BC-191 are another source of audio problems. 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....

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%.

Slight 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 the slight 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.  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 transmitter are

1. Reduce power output to 50 watts. This will improve modulation symmetry.

2. Neutralize the TU at the frequency you intend to operate the transmitter. This is the most important step for good audio.

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.

4. Make sure your Battery-Dynamotor set-up is operating correctly and that the batteries are at full charge.

With these steps, your BC-375 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.

Henry Rogers WA7YBS - October 2009, October 2010, December 2010