November 1946 Radio News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Selenium rectifiers were the first widely used replacements
for vacuum tubes in commercial electronic equipment. Since amplification
was not possible - that came in late 1948 with the invention
transistor, compliments of Shockley, Brattain and Bardeen
- diode action in AC-DC power supplies was its primary application.
Typical reverse breakdown voltage is in the neighborhood of
20 volts and current handling capability depends on the interface
surface area. Cost kept the selenium rectifiers from being widely
adapted early in their history (1933),
but by 1946 when this article was published it had dropped enough
to make their use economical. Eliminating one or two vacuum
tubes in a radio or television power supply made the sets more
efficient and increased reliability since the selenium rectifiers
rarely suffered failures if designed into the circuit correctly.
magazines of the era published many articles about selenium rectifiers, including
After Class: Working with Selenium Rectifiers,
Selenium Rectifiers for Home Receivers,
Applications of Small High-Voltage Selenium Rectifiers, and Using Selenium Rectifiers.
New Selenium Rectifiers for Home Receivers
By George Eannarino
Field Eng., Federal Telephone & Radio Corporation.
Selenium rectifiers have been designed to replace all conventional
types of rectifier tubes used in home receivers.
Fig. 1 - Typical portable receiver used
to illustrate step-by-step installation of selenium rectifier.
Markedly improving the performance of home receivers and
completely trouble-free, the new miniature selenium rectifier
stack, developed by Federal Telephone and Radio Corporation,
Newark, N. J. means more "customer satisfaction" plus increased
profits to the up-to-the-minute serviceman. One of the first
advancements made in home radios since the end of the .war,
this stack replaces all conventional rectifier tubes, is simple
to install, and is guaranteed to outlive the receiver, thereby
ensuring a minimum of power supply failures.
With competition in the service field mounting, this latest
development should provide a "shot in the arm" for many alert
radio repairmen for it has such excellent selling points, such
as instantaneous starting for both a.c. and d.c., less heat,
longer battery life (in portables), improved tonal quality,
trouble-free power supply operation, pilot light failures reduced
to a minimum, and more audio output (in 35Z5 circuits). In every
case this new type of rectifier means customer satisfaction.
Fig. 2 - Chassis is withdrawn from cabinet
and rectifier tube removed. Only four tools are needed for complete
Selenium rectifiers have been used in electronic circuits
for many years but due to their relatively high cost and large
space factor they were not used in home receiver rectifier circuits.
However, by virtue of a new process developed by Federal, this
cost and space factor has been reduced to the point where use
of the miniature selenium rectifier is entirely practical.
The rectifier stack has two distinct poles, positive and
negative, corresponding to the plate and cathode of the vacuum
tube and can be inserted into the circuit as such. The positive
side, denoted by a "+" sign, is equivalent to the cathode while
the negative side functions as the plate. Soldering the stack
into the set in this manner constitutes the entire replacement
operation unless the filament of the tube was linked to other
parts of the circuit, in which case a resistor is used to replace
the rectifier tube filament.
Fig. 3 - Two extension leads are soldered
on the selenium rectifier lugs. Positive side is shown by red
wire, negative by yellow or black.
Installation of the miniature #403D2625 selenium rectifier
can be made right in the customer's home as indicated in Figs.
1-6. The set shown is a typical three powered portable using
a 117Z6 rectifier. As is evident from the schematic of the power
supply (Fig. 7) the filament of this tube is not interlocked
with any other component in the set. Therefore it is only necessary
to insert the stack into the circuit along the lines outlined
in the previous paragraph. Solder the positive side to the cathode
terminal on the tube socket, the negative side to the plate
terminal, and the installation is over.
Only four tools are required to perform the entire 7 minute
operation, a soldering iron, screw driver, socket wrench, and
a pair of long nose pliers. First the chassis is withdrawn from
the cabinet and the tube is removed from the socket (Fig. 2).
Extension leads are then soldered on the rectifier stack. It
is recommended that the positive lead be covered with red wire
so as to distinguish it from the negative lead, which is usually
made yellow or black (Fig. 3).
Fig. 4 - Here the leads are soldered
on to the appropriate pins. The red wire goes to pin 4 while
yellow or black wire. whichever is used, is soldered to pin
At this point it should be noted that, whenever possible, the
stack should be installed underneath the chassis. However, in
this case, as is the case with many portables, though the stack
is only 1 1/4" x 1 1/4" x 11/16", it did not fit underneath
the chassis and was inserted from above in the space that was
formerly occupied by the tube. In this latter case some type
of protective covering must be provided.
Fig. 4 shows the leads which were drawn through center of
the tube socket and soldered to the appropriate pins, the red
lead to the cathode terminal and the yellow one to the plate
terminal. The set is then turned on and if the rectifier has
been installed correctly, it should start operating immediately,
(Fig. 5). Finally a protective covering, which is supplied with
the rectifier, is placed around the stack and tightened to the
chassis via a screw and nut (Fig. 6), The chassis is then put
back in cabinet and the job is done.
Fig. 5 - Rectifier operation should
now be checked by turning the receiver on. The set should operate
immediately without the customary warm-up period.
After insulation of the rectifier, check filament voltage.
If this is too high for normal tube operation, insert a 27 ohm
resistor in the line just before the rectifier to bring the
operating voltage of the filament back to normal.
Three operational improvements have been achieved with the
installation of this rectifier. In the first place, since rectification
is now immediate, the set operates as soon as it is turned on,
in contrast to the filament warm-up period previously required.
Secondly, in view of the low internal impedance and high efficiency
of the stack, the ambient temperature of the set is reduced
by approximately 35° F. which results in increased battery
life. Finally, the long life of the selenium rectifier means
that power supply troubles are reduced to a minimum.
Federal has developed replacement sheets covering every possible
power supply used in home receivers, which give specific, easy-to-follow
instructions, on what steps are necessary to install this rectifier.
These sheets are available to all servicemen and can be obtained
by writing to Federal Telephone and Radio Corporation, 200 Mt.
Pleasant Ave., Newark, N. J.
Fig. 6 - Final operation consists of
tightening rectifier to the chassis and placing shield over
it. Chassis is then returned to the cabinet with conversion
Fig. 7. Schematic diagram shows method of
substituting a selenium rectifier for a conventional 117Z6 tube.
Posted October 30, 2014