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November 1949 Radio-Electronics
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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Those of us old enough
to remember the classic Simpson volt-ohmmeter (VOM) from the 1970s will look at
this 1949 model appearing in Radio-Electronics magazine and probably not
notice much if any difference. The basic case design is similar and it appears to
be about the same physical size. The selector switches and potentiometer knobs look
familiar as well. The primary difference is what is inside - a vacuum tube rather
than a field-effect transistor (FET). The Simpson Model 303 is a vacuum tube voltmeter
(VTVM - actually a VTVOH). Prior to the availability of FETs with their very high
input impedance characteristic (10 MΩ or greater), a vacuum tube input
stage was needed to isolate the device (or circuit) under test (DUT) from the relatively
low impedance of the resistor-based volt-ohm meter (VOM - as with the Simpson Model
260) meter circuitry. The problem is that a low VOM impedance relative to the DUT
results in erroneously low meter movement displays due to the voltage divider effect.
For instance if a DUT has an output potential of 1 V and an output impedance
of 10 kΩ, and the internal resistance of the meter on the selected voltage
scale is also 10 kΩ, then what would otherwise be shown on the VOM as 1 V
would appear as 0.5 V instead. If you read some of the vintage magazine articles
posted on RF Cafe, the authors admonish against discounting test equipment internal
resistance lest measurements be skewed and the wrong result is assumed.
Simpson Electric Company Vacuum Tube Volt-Ohmmeter Ad
 Simpson
Instruments That Stay Accurate
Presents the New! Model 303
Vacuum Tube Volt-Ohmmeter
... A Worthy Companion of the 260
Specifications
Smaller and Handier for Greater Portability
A worthy companion of the world-famous Model 260 is this brand new addition to
the Simpson line - the Model 303!
Skilled Simpson engineers spent months of painstaking research in the laboratory
to produce the Model 303, which is one of the most versatile instruments ever made
for TV servicing. This ruggedly constructed instrument offers the maximum in portability
because it is approximately 60% smaller than other vacuum tube volt-ohmmeters. However,
no sacrifice has been made in read-ability. The 303 has a large 4 1/2" meter, despite
its handy compactness.
One of the many features of the 303 is its low current consumption. The AC voltage
range is wider than on any other similar instrument - from 1.2 volts minimum to
1,200 maximum. Like all other instruments bearing the Simpson name, the Model 303
is an instrument of highest quality at an amazingly low price.
Simpson Electric Company
5200-5218 West Kinzie Street; Chicago 44, Illinois
In Canada: Bach-Simpson, Ltd., London, Ontario
Posted December 1, 2021
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