Western Electric 91E - Notes from the Manufacturer; Patented Steered Current Source Technology; Type 91 Amplifier Family History

TYPE 91E
INTEGRATED AMPLIFIER
2
NOTES FROM THE
MANUFACTURER
PATENTED STEERED CURRENT SOURCE TECHNOLOGY
e 91E is the result of years of research and eort by Western Electric engineers to provide
listeners with the absolute nest in SE tube sound. is achievement is made possible by a patented
(U.S. Patent Office 10256776) steered current source (SCS) circuit, explained below.
e circuitry embodies an enhanced parallel feed design, wherein the load (primary winding of the
output transformer with the secondary winding connected to the loudspeaker) is AC coupled to
the plate of the 300B tube. DC bias is removed from the transformer and allows for smaller, simpler
transformer designs with no need for a gap in the core. e welcome side eect is the absence of
unsymmetrical magnetic distortions caused by DC current through the transformer. e circuit
topology also strengthens the coupling between tube and load.
Unlike traditional parallel feed topologies, the tube and output transformer are supplied current
by the SCS. Working in Class A mode, and with the transformer AC-coupled, the SCS provides a
constant idle current for the tube’s plate, ensuring a very stable operating point for the tube.
With the reduction of tube current comes decreased AC power dissipation, providing more power
to the load without overstressing the tube. Additionally, the SCS design can achieve an output
power of more than double the conventional 8-10 watts.
TYPE 91 - FAMILY HISTORY
In 1936, under its lm sound subsidiary Electrical
Research Products Incorporated (ERPI), Western
Electric commissioned the 91A amplier for use in
the wall-mounted Type 500A amplier set (cabinet
pictured right). Amplier sets like the 500A combined
with projectors, loudspeakers, and other tube
ampliers to make up “Wide Range” and subsequent
“Mirrophonic” sound systems in the 1930s. Western
Electric manufactured, installed, and maintained these
early sound systems, thus introducing a burgeoning
Hollywood movie industry and moviegoers worldwide
to the magical, yet naturalistic wonder of high-
delity sound reproduction. Even small-to-midsized
hometown theaters, for which the 91A was designed,
could be equipped with lifelike, synchronized sound
for motion pictures.
e 91E continues the legacy of Western Electric
sound. Its proud place in the Type 91 lineage is rooted
in an idea—What is the best possible 300B amplier?