VOLUME 16 OCTOBER 1999 NUMBER 10
The Western Electric 7A Amplifier
WEB EDITIONWe are always glad to hear about an A.R.C.-inspired project. Sometimes an article does it; other times, as in Joseph Wood's case, an ad triggers a memory and away we go. No matter where the idea comes from, we're pleased to share it with our readers. (Editor)
BY JOSEPH WOOD
In my collection, I have a Western Electric Type 7A amplifier. I didn't even take time to study it until I saw an ad in Antique Radio Classified that read, "Wanted: a Western Electric amplifier."
I took my amplifier out to look it over and found that it is true to the Western Electric reputation for high quality work. For its time, it was one of the best amplifiers available.
I decided to examine the amplifier, shown in Figure 1, more closely. First, I removed the eight wood screws that held the panel to the sturdy, almost square birchwood case. The wooden case measures 12 1/2" x 10" at the base and 3 1/2" deep inside. I pulled the panel up and turned it over to stare at what is a real old-time collector's gem.The wire used in its assembly resembles the old-time No. 8 rigid house wire soldered with great care to each component.
The top panel is a 1/2" thick piece of black Bakelite insulating material and contains all of the components on its top and bottom sides. On the underside, we see a tap switch, an on/off switch, a C battery case, a small molded condenser, three well shielded Western Electric transformers, three tube sockets, four input/output terminals, three battery connect terminals and a ground terminal.
Inside the bottom of the wood cabinet is a schematic of the circuit. As I studied the circuit and its related parts, I found that three Type 216A tubes do the amplification. Two of the Type 216A tubes are used in a push-pull output stage for maximum amplification. Three iron-core Western Electric transformers are needed to match the tubes to the input and output of the amplifier. The 381A transformer is connected to the gain taps, and the small molded condenser has a number -- D44386.
Note that the Western Electric speech amplifier Type 216A tube is unusual in that it is spherical in shape and has a sturdy brass base. It is a 4-prong triode tube utilizing two prongs for the filament voltage, one for the grid and one for the plate.
This amplifier is battery-powered, and the tubes require 120 volts DC for the plate voltage, 6 volts DC for the filament voltage, and 7 1/2 volts for the grid bias voltage.The two C batteries are Eveready No. 751 9-volt batteries.
The top panel has a Western Electric tag that has a long list of patents dating from 1907 to 1921. The 5-contact tap switch gives away the item's vintage as belonging to the crystal set era.
(Joseph Wood, 6511 Chester, Hodgkins, IL 60525)
Joseph Wood, an Amateur Radio operator, has a degree in radio communications. He is a member of the Morse Telegraph Club and an avid collector of antique radios.
Figure 1. The Western Electric 7A amplifier.