The very first PA systems used a straight tapered funnel arrangement, referred to as a bull horn, megaphone or loudhailer, without any form of electronic amplification. They certainly “reinforced” the user’s voice to some extent but they were quite directional.
When valves later became readily available, PA amplifiers capable of a few watts were quickly developed. Those early units were used with carbon microphones, while the speakers were larger, more powerful versions of the horn speakers used with early receivers. Horn speakers were very efficient compared to modern loudspeakers but their frequency response was quite restricted and the distortion from these units was relatively high.
An old exponential horn speaker as used in many PA systems during the 1960s. Similar speakers are still used in modern PA systems.
As a result, early PA systems were only suitable for voice communication.
PA amplifiers of various output powers were later used during World War II, some so big that they used
radio transmitter valves in their output stages to feed the loudspeakers. By contrast, the commercial PA amplifiers used for the general public following the war were quite modest, with output powers ranging from around 5W up to about 25W.
As an aside, when I first worked in the radio service industry in the late 1950s, my employers hired out a PA system. This used a 25W amplifier (similar to the unit described here) and was teamed with a dynamic microphone, a record-playing turntable and up to four reflex-horn speakers. The speakers could be located some distance from the amplifier, as the output was fed to a 100V (medium impedance) speaker line.
The connecting cable consisted of a length of twin-lighting flex which had low losses at the speaker impedances used. Standing under the speakers when the unit was operating at full power was a deafening experience and it could be heard up to 3km away (depending on wind direction).
AWA PA1002 PA amplifier
The patch terminals on the rear panel allow the amplifier’s output to be matched to many different impedances for both 100V and 75V lines and are connected according to the table. In addition, there are two line output terminals plus antenna and earth terminals for the optional tuner.
My vintage radio collection includes several valve PA amplifiers, the largest of which is the AWA PA1002, a 50W unit from the mid-1960s. PA amplifiers improved considerably after World War 2 and top-end 50-100W valve PA amplifiers quickly reached the pinnacle of their development.
Unfortunately, I had no luck in obtaining a circuit diagram for the PA1002 although I do have circuits of other AWA PA amplifiers of the era. AWA obviously experimented with many different circuit designs because none of the circuits I have are anything like the PA1002. As I result, I eventually traced out the output and driver circuits of the amplifier and this revealed a push-pull output stage that’s quite different to what’s normally expected. In addition, the chassis has holes punched for two additional output valves, so that a 100W version could be manufactured using as much common circuitry as practical.
The amplifier itself weighs a hefty 14kg without any accessories and it measures 430 x 230 x 230mm (W x D x H), not allowing for the knobs and mounting feet. A feature of the unit is that there is plenty of room on top of the chassis for accessories, such as a monitor speaker and a radio tuner. These accessories are not fitted as standard but could be easily added as their connection sockets are already wired.
As can be seen from the photos, access to the parts for service (or restoration) is relatively easy. To gain access to the valves, it is only necessary to remove four screws from the ends of the chassis that hold the U-shaped perforated steel cover in place. It is then quite practical to carry the amplifier by one or both of the metal rods located at the top of the cabinet that attach the front and back panel to the chassis.
The circuitry under the chassis is accessed by turning the unit upside down and then removing the four self-tapping screws holding the rubber buffers and the bottom shield in place, followed by the baseplate itself. It’s then quite easy to access most of the parts although some parts are tucked in under a ledge at the back.
The bottom of the cabinet is fitted with rubber anti-scuff buffers and there are also ventilation holes in the bottom cover sheet.