The forgotten element - Interphones


One of the common complaints heard from radio amateurs over the years about WWII aircraft equipment is that they don't have enough audio gain. Given that they were never designed to drive a speaker, it's perhaps not surprising, but I have seen extraordinary hacks on some equipment like command receivers and the BC-348 in efforts to solve the problem. In my mind, the solution for anyone supporting the idea of minimal modification is to find an appropriate interphone amplifier of the period and use the radio to feed it. There is a wide variety of these amplifiers that usually go begging on the market, and using them permits the employment of their own fun accessories like jack boxes and the like. Beware, however, that they can begin to take on a life of their own and form a significant collection effort in their own right.

One thing not well understood about the interphone amps is that up until about 1944 they were not generally used to amplify the audio from receivers. The radios themselves were required to drive the multiple earphones of the crew directly, and the amplifier was only used to amplify the microphone inputs. In effect, the amp simply provided one of several audio sources for the various jack boxes to select. This deficiency in effectively using the interphone amp power led to reduced volume whenever several of the crew switched their earphones to any single radio position (to listen to Tokyo Rose on the Liaison receiver, for example.) Beginning with the AN/AIC-3, the number and variety of audio sources had increased to the point where it made sense to begin feeding them (on a switchable basis) to the interphone amplifier, along with the 'traditional' mic inputs. The subsequent WWII interphone systems (the AIC-4 generally used with the ARC-5 system, and the AIC-5 described below) followed that philosophy. As a result, to use the earlier Navy and USAAC/USAAF systems with your rig, some thoughtful if minor wiring changes from the original manual schematic may be necessary, along with the addition of some ~10,000 ohm balancing resistors to permit signal combining, similar to the approach shown here.

Some thought to switching may be necessary as well, if you want to use the amp for multiple sources. An example of a minor redesign is shown in RC-36 manual excerpts. This wiring change to a jack box allows the carbon mikes plugged into it to be amplified in the "command" switch position to adequately drive the command set modulator to 100% without the usual shouting required. The original microphone amplifiers in the three command set modulators don't normally have enough gain to accomplish this. With a level pot added to the circuit, it will usually permit a range of carbon microphone buttons, even those which are somewhat cemented by time and moisture, to be used effectively. (By the way, note that some are simply beyond amplification help.)

AAC/AAF amplifiers - just the basics

"Old Faithful", the BC-347 AAC/AAF standard in WWII. The PE-86 dynamotor to power it is below the amplifier.

Probably the most common of these interphone amplifiers is shown above, used in thousands of Army Air Corps/Force aircraft. It comes with the PE-86 dynamotor below it for those locations that do not have +250vdc available, but its simple, single tube design allows it to piggyback on the receiver power if necessary. It was included as a part of a whole series of RC-* (Radio Component) systems, depending on the jack boxes involved. By the end of the war, it was listed under Interphone installations RC-34, 35, 36, 51, and 175! As a side note, not all the RC-* systems were interphones, by any means. The designator was used as a catch-all for stray dogs and cats which didn't fit under a normal communications (SCR) nomenclature, like blind landing systems, IFF, certain special radars, and other oddball equipment. The arrival of the Joint A/N naming convention finally rationalized this confusing situation. A schematic and associated interphone wiring is shown in the RC-36 manual excerpts.

The primary jack box used with the BC-347 in the later aircraft was the BC-366, but later variations like the BC-1366, (with a VHF position included on the selector switch) are perhaps more common. I have seen a number of variations of the later box, with VHF or UHF engraved over the 'command' position.

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Quite a few older AAC aircraft were fitted with the BC-212 interphone amplifier below. Essentially a redesign of a 1930's amp with 1940 tubes, it was teamed with the 1930s jack boxes, like the BC-213. It was a central part of the RC-15, RC-27, and RC-45 Interphone Equipments, and shared some of the peripheral boxes for a time with the more modern BC-347 amplifier, like the BC-327, BC-334, and BC-345.
BC-212 amplifier


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The Navy Interphone amps - Experimenters Inc.

For some reason the Navy went bonkers with these systems, almost giving the impression that a new system was designed for every new aircraft. They assigned an RL-* desgnation to them, and only a couple of the more common ones are shown below.

Navy RL-24

The unit above should probably get the award for 'most needlessly complex' (at least for that point in time.) There was a separate part number and ID plate for almost every aircraft position jack box, like Pilot's, 2nd Pilot's (copilot), Navigator, Radio Operator, etc., with minor variations like a volume control or ICS/radio switch. (ICS = Inter-Communications System, a Navy-peculiar name for the interphone system.) It included two independent amplifiers, one for general crew application and one for the key crewmen who needed to hear the radio(s), like the pilot/copilot and radio operator. The Navy was fond of giving certain crewmen (like the radio operator) independent earphones, each side plugged into a different source, though it must have driven the wearer slightly nuts at times. However, it was prescient in providing the flexibility later needed for larger, more specialized flight crews. More on this is discussed in the Earphones and Headsets page.

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Navy RL-7 and one of its associated jack boxes

A more measured approach was the RL-5, 6, 7, and 9 series above, all minor variations on a capability that would later be reflected in the AAF contracted AN/AIC-2 below. These had rear mounted dynamotors rather than the separate mount dyno like the RL-24, so they were self contained. The large six pin plug in the center was to supply other equipment like an LM frequency meter if desired. Strangely enough, the station boxes didn't have a Navy nomenclature number - just a manufacturer's drawing number that varied according to the particular vendor involved.

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The later Interphone amps - the A/N systems


Beginning in 1943 a uniform nomenclature system was introduced to identify equipment, and the interphones were assigned an "AN/AIC-*" identification prefix. Shown here are three variations on that series.

AM-26/AIC-2 amplifier on its MT-28/ARN-5 shock mount and C-97/AIC-2 remote volume control

This amplifier was the late 1944 replacement for the old BC-347, and included enough power to drive interphone systems in aircraft with ten or more crew members, a task that in part was a failing of the BC-347. It was one of the few pieces of equipment that tried to lower costs by using another set's shock mount - in this case from the AN/ARN-5 ILS system. An integral rotary gain control switch, with the optional C-97/AIC-2 remote control head shown here, allowed the radio operator to increase the gain with increasing altitude to counter the loss of "loudness" in the thinner air. A schematic and associated interphone wiring is shown here.

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AM-40/AIC-4

The amplifier above was most often associated with the ARC-5 equipment in Navy aircraft (although the the RL-5 through -9 described above was still used to a wide degree.) It included an innovative feature in the form of a pressure switch that automatically increased gain with increasing altitude. The AIC-4 system used several station boxes, including those below:

C-173/AIC-4 "station" control unit. This one is missing the microphone umbilical.


C-174/AIC-4 "operator's" control unit. Notice the added number of channels on this one, including one for the sonobuoy channel when needed. A rotatable plate blocked it off when not in use.


A schematic and associated interphone wiring is shown here.

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AN/AIC-5 components: OA-53/AIC-5 amplifier on top, C-379/AIC-5 station box to the left, C-380/AIC-5 control box to the right.

The interphone system above was the last one 'type-issued' before the end of the war, and reflected the growing need for multiple channels, especially on patrol and countermeasures ferret aircraft, both to reduce the information load encountered using a common channel and to separate classified discussions from the flight crew segment. It also reflects the growing complexity demanded of the flight crew to get switches in the right position! It was powered by two separate command receiver type dynamotors on the rear deck, either of which could supply the entire set of five independent amplifier modules contained inside the set. Master volume for each of the five amplifiers is adjusted on the front of the set. Note the prewar five pin "auxiliary power" connector at lower left that was fairly ubiquitous on Navy sets of all types. It provided B+ from the dynos and 28 volts DC for peripheral devices such as frequency meters and the like.

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LS-184/AIC-10 aircraft loudspeaker

The operating spaces in WWII aircraft were so noisy that no loudspeaker was practical. It wasn't until several years after the war that an "official" aircraft loudspeaker was nomenclatured - a pair of speakers that look like they were drawn from Navy shipboard practice. The LS-131 driver, coupled with a straight trumpet; and the LS-132 folded horn, part of the AN/AIC-7. This system does not appear to have been widely installed, however. The first aircraft speaker designed and produced in any appreciable numbers was the LS-184/AIC-10. Incorporating a small "pencil" tube amplifier that needed 175v B+ from an external dynamotor set, it at least has the same black wrinkle finish as the WWII components, and sounds surprisingly good. This is one of the whimsical exceptions to the equally whimsical 1939-1945 acceptance window for the 'flight deck' in order to satisfy a special need, since my Wright 3350s are in the shop for repair anyway (earphones are okay, but not all the time....)

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