Accessed times since July 14, 1998.

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Photo by David Hershberger, All Rights Reserved
Thanks also to Daniel L. Dickey of
Continental Electronics
for permission to publish the information and photo on this page.

The photo and information herein may not be republished without
permission from David Hershberger


Doherty tube that is a museum piece in the "high power broadcasting" department at Continental Electronics in Dallas. This tube was used in a Mexican "border blaster" transmitter built by a predecessor of Continental in the late 1930s. There were 8 of these tubes used - 4 "carrier" tubes and 4 "peak" tubes. The total power output was 500 kilowatts, so the station had the same power as WLW.

The tube is a water cooled triode, Western Electric, type 320A. There were only 9 of them built - 8 were used in the "border blaster" and one was a spare.

This tube used a "bright tungsten" filament, meaning there was no thorium present to increase electron emission. It took a lot more filament power, but on the other hand, tubes with "bright tungsten" filaments could last almost forever.

In the picture, you can see two plaques, one on either side of the tube. One plaque tells the story of the station, and the other plaque is a set of specifications on the tube.

Dave Hershberger
Principal Engineer of
Continental Electronics, Inc.
Nevada City, California



Eight of these tubes were put into service in 1938 in the 500 kilowatt
transmitter, XERA, at Ciudad Acuna, Mexico, designed by Jim Weldon.

The tube was developed by Bell Laboratories in 1937. The design engineer
was Doctor H. Eugene ("Dick") Mendenhall, who was present at the site
during initial tests.

The high efficiency "Doherty" linear power amplifier was used in each of
the two 250 kilowatt amplifiers which were parallel combined.

Mr. William H. Doherty inspected the installation about a week or so
before preliminary testing started.

Only nine of these tubes were ever built and no tubes ever failed in
service nor during the four years operation which followed before the
station was closed down. It has a "bright tungsten" filament (no

A crack in the lower glass was found to have occurred in one tube
during the day's idle period and none of the Mexican janitors could
remember hitting it with a broom handle.

Tube Data Sheet

320A Vacuum Tube

The 320A Vacuum tube is a 3 element water cooled tube designed for use
as an oscillator, modulator, or amplifier at the higher power levels
and high frequencies.

Filament ratings: 35 volts at 435 amperes

Average thermionic emission: 90 amperes

Characteristics at 18 kV plate voltage and 8 amperes plate current:

Amplification factor: 30
Plate resistance: 965 ohms
Transconductance: 31,100 micromhos

Maximum plate ratings:

maximum plate voltages: 12.5 kV modulated
18 kV non modulated
20 kV rms AC
current: 15 amps DC
dissipation: 150 kilowatts

Maximum grid dissipation: 2 kilowatts

The 320A vacuum tube must be mounted vertically, with the filament
terminal end down. A plumb bob should be used for vertical alignment.
When a standard socket or mounting is not provided the following
mounting suggestions should be adhered to. The tube should be supported
from the under side of the upper bulged section of the water jacket.
Care must be taken not to deform the water jacket by clamping.

WE-320A Under test a Western Electric Tube in New York 1940
From: cover of "electronics" magazine, July 1940 McGraw Hill