BC-375E Technical Description

         
BC-375E

            The circuitry for the BC-375E and BC-191 is actually quite simple when compared to other transmitters of the time like the ART-13 transmitter. The block diagram of the BC-375E (and BC-191) is shown below. The tube line-up of BC-375E is definitely something from the 1920’s. The RF section is capable of putting out up to 75 watts into a 5 ohm load. The transmitter uses "real" AM modulation, that is to say the Class "C" PA is plate modulated by pair of  VT-4C in push-pull Class "B" operation. SCR-274N transmitters use a screen modulated AM scheme, which results in a weak and thin sounding audio. The ARC-5 and ART-13 transmitters use "real" plate modulated AM as well.

    The RF section consists of a VT-4C Master Oscillator (MO) whose frequency is set by variable capacitor "A". On tuning unit TU-26B, control  "A" is actually a variometer (variable inductor). On tuning units TU-5B, TU-6B and TU-26B an additional band switch "B" breaks the band in several sub-bands by switching in additional fixed capacitances into the MO and PA tank circuits.

    The MO feeds a second VT-4C tube used as a Power Amplifier (PA) stage.  Variable capacitor "C" is used to tune the stage to resonance. On tuning unit TU-26B, control  "C" is another variometer.  Output of the PA is selected from 6 output link taps by "D"  to obtain the best match for the internal antenna tuner unit.

    The tuning units contain all frequency determining components and the neutralizing capacitor. All tuning units were matched to the transmitter at the GE factory. If your transmitter has matched units, they would have the identical serial number as the transmitter.

    The internal antenna tuner has 4 controls: Variable capacitor "O" is used either to shorten the antenna electrically or serve a capacitive load, Roller inductor "M" is used to electrical lengthen the antenna in fine increments, Switchable Inductor "P" is used to electrical lengthen the antenna in course increments and antenna tuner mode switch "N"  set-ups the antenna tuner configurations (see the control section below for details of the four modes). This versatile antenna tuner allows the transmitter to load into almost antenna  (except a balanced antenna). On a typical aircraft  installation the BC-375E could be switched between a fixed antenna (spanning the aft section and rear stabilizer) or a variable length trailing antenna.

  In CW mode the telegraph would key the transmitter via a grid block scheme. Keep this mind because 1000 volts is always present on the transmitter tubes even when it is unkeyed.

    In Voice mode a carbon microphone (such as the T-17D) would drive the VT-25 speech amplifier which in turn would drive a pair of VT-4C used as a push-pull class "B" to modulate the PA.

    In MCW mode, the VT-25 would serve as a 1000 hertz tone oscillator which in turn would drive a pair of VT-4C used as a push-pull class "B" to modulate the PA.

     One very intelligent design aspect of this tube line-up is that you still can operate the transmitter (in CW mode only) with only 2  tubes. The two MOPA tube filaments are wired in one series string. The modulator tube filaments are wired in another series string.  And the speech amplifier has its own string with a dropping resistor. The MOPA tubes are lit up all the time. The modulator tubes are only lit up during MCW or Voice mode. The VT-25 is always lit up. Its function differs with the various modes: In CW mode, it is a sidetone oscillator, in Voice mode it is a speech amplifier and in MCW mode it is a tone oscillator for the modulator as well as a sidetone oscillator. I am sure many radiomen, with burnt out tubes at 25,000 feet, quickly swapped burnt a MOPA tube with one of the modulator tubes and continued operating in CW mode. You only need all 5 tubes if you operate this transmitter in Voice (plate modulated AM) or MCW (Modulated CW) mode.

 
    When the power switch is turned on (Red pilot lamp is ON),  26 volts is applied to the MOPA filaments, the VT-25 (via a dropping resistor) filament, the modulator filaments (only if Voice or MCW mode is selected) and the solenoid in the dynamotor is activated thereby placing 1000 volts on the tube plates. Keying is accomplished via a grid blocking scheme. The power supply is interrupted, if a tuning unit drawer is removed. This is done to remove the deadly 1000 volts from the exposed terminals on the transmitter, when the tuning unit is not present. A safety interlock switch opens the 26 volt line and kills power to the tubes and dynamotor. Although, after 60 years I would never place my faith 100% on the interlock switch.

TU-xx Tuning Units

    The BC-375E is designed to work with one of seven tuning units (drawers) for coverage of 200-500 Kc and 1.5-12.5 mhz .Both BC-375 and BC-191 can use TU-xxB series tuning units. Older tuning units with the “A” suffix (i.e. TU-xxA) are identical to the TU-xxB, except they have a pair of antenna terminals (connected to the tapped output of control “D”) on the upper right corner.

    I know of the existence of a third family of tuning units with the "N" suffix (e.g. TU-6N). The TU-6N, I have seen appears to be identical to a TU-6B unit with a minor difference, it has a plastic identification tag with a 1944 Philadephia contract number. I theorize the "N" might mean Navy as it does with the SCR-274N family.

          The strange naming convention leads one to believe TU-1B through TU-4B would are available as well. Looking at the documentation for the BC-191 (i.e. TM 11-800) reveals that TU-3A and TU-3B  for 400-800 Kc and TU-22A and TU-22B for 350-650 Kc were available as well.

         TM 11-800 makes also reference to the other missing pieces listed below, but notes that they are not standard equipment. Very few transmitters were issued these special units.

BC-306A Antenna Tuner

    The BC-306A external antenna tuner was used for frequencies below 800 Kc as is the case with the TU-26B tuning unit. It provides a variometer "F" and a  selectable tapped inductor  "E" to electrical lengthen the antenna even more for long waves (200 through 500 Kc). It is attached to terminal  Load "A" and  Load "B" on the antenna block of the BC-375E. When the BC-306A is not present, terminals Load "A" and Load "B" on the BC-375E were jumpered by a short piece of solid wire.

PE-73C, BD-77M and RA-34 Power Units 

    Depending on the transmitter (BC-375 or BC-191) as well as the application, three power units are available with following specifications. The RA-34 is a rectifier and the BD-77M and PE-73C are both dynamotor units.

BC-375 Controls

    The BC-375E has many controls on it. Below is the their individual function.

Control

A

B

C

D

E

F

M

N

O

P

Ammeter

Ammeter

Voltmeter

CW/Mod

Filament

Mode

ON/OFF

 BC-375E Equipment List

BC-375E Accessories

    BC-306A Antenna Tuner                                     PE-73C Dynamotor                                                        SA-13 Antenna Transfer Switch

                       T-17D Microphone                                                         J-47 and J-38 Telegraph Keys

                           TU-26B Tuning Unit with CS-48 Case                                                        PL-59, PL-61 and PL-64 Connectors

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

• BC-375 Introduction
• BC-375 Operational Considerations
• BC-375 & BC-191 Differences
• BC-375 Bibliography 
• Aircraft Radio Functions  
• Military Radio Links
• Swap Section 

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