When I was growing up in the 1950s, my dad abbled in radio/TV repair. His shop was trewn with all kinds of electronic parts. Add
to this a small book of radio projects compiled from
1940’s issues of Popular Science Magazine and I was
provided with many hours of experimentation and fun. I
particularly remember a shortwave design with the
intriguing title “Europe on One Tube.”
I am embarrassed to admit how many hours I spent
trying to build radios based on these wonderful articles.
Most of the designs used a regenerative circuit invented
by Edwin Armstrong in 1912. A few years ago, I stumbled
on an unusual regenerative design that operated on 12
volts — not the 100 or more of conventional tube designs.
The radio I built turned out to be one of the best ever.
In this article, I will describe how to build and operate
the broadcast band version. Should you decide to tackle
it, I can promise you many hours of fun both in the
building and in the listening for distant radio stations.
Vacuum tube technology dates back to the time of
Thomas Edison and the light bulb. In 1883, Edison noted
that he could get electrons to flow between the hot
filament of an experimental light bulb and a positively
charged metal plate. The so-called Edison Effect only
occurred in the near vacuum of the light bulb.
In 1904, the British scientist, John A. Fleming used the
Edison Effect to produce the first practical tube, or
“thermionic valve.” Fleming’s diode valve passed electrical
current in only one direction, making it useful as a radio
frequency detector and a rectifier for converting
alternating current to direct current.
American inventor, Lee de Forest added a third
element to the vacuum tube design and produced the
triode, or “Audion” as he named it. He interposed a grid
of wire mesh between the filament and metal plate that
provided a way to control the flow of electrons.
The significant feature of his invention was that small
changes of voltage on the grid would produce much
larger changes of voltage on the plate, resulting in voltage
amplification. Thus, a weak audio or radio signal could be
amplified, which had many practical applications in
telephone and radio communication.
As time went on, other advances were made in tube
technology, including the addition of an indirectly heated
cathode and other grids. For our purposes, the triode
vacuum tube will serve as the heart of the regenerative
Radio detector circuits take a variety of forms. The
simplest is the diode detector mentioned earlier in relation
to Fleming. When the triode came along, other detectors
were invented including a design called a plate detector.
When a radio signal was applied to the control grid of a
triode, detected audio could be taken from the plate
circuit. The regenerative receiver takes the plate detector
one step further and adds a small amount of positive
feedback, resulting in “regeneration” that substantially
increases circuit gain and selectivity (ability to separate
nearby radio stations).
The result is a very simple circuit consisting of only
one tube and a handful of components that produce
amazing results. Add a couple of stages of audio
amplification and you have a radio design that provides
hours of fun and listening pleasure!
The basic circuit consists of a dual triode 12AU7.
While this and other similar tubes are meant to operate at
plate voltages of 90 volts or more, the 12AU7 performs
amazingly well in the current application at only 12 volts.
Dangerous voltages normally associated with tube
projects are eliminated.
One disadvantage of low plate voltage operation is
that it is not possible to develop sufficient audio power to
drive a speaker or dynamic earphones. An LM386 IC
power amplifier serves this purpose, making the overall
design a hybrid mix of vacuum tube and semiconductor
technology. The tube circuit consists of two sections: the
regenerative detector and a low level audio amplifier.
Refer to the schematic in Figure 1.
The radio frequency (RF) signal from the antenna
(binding post J4) is applied to winding L1 of the spider
web-wound coil. Winding L1 inductively couples the RF
signal to a second winding L2 that — along with variable
capacitor C1 — forms a resonant circuit covering the AM
broadcast band (550 to 1600 kHz).
Capacitor C2 couples the tuned RF signal to the
control grid of the triode V1-A. Resistor R1 provides a DC
path to ground and “leaks” electron charge that would
otherwise build up on the control grid and prevent the
tube from working. A tap on winding L2 provides a small
May 2015 27
Antique Electronic Supply - www.tubesandmore.com
Digikey - www.digikey.com
Home Depot - www.homedepot.com
Jameco Electronics - www.jameco.com
New Sensor - www.newsensor.com
RadioShack - http://radioshack.com
Video introductions to soldering: