tube V1, turn the radio on with the volume control. Rotate
the gain control fully clockwise. Use an ohmmeter to
measure resistance between pins 1 and 2 of J1. After a
few seconds, it should read 1,000 ohms or higher. A low
reading (less than 100 ohms) suggests incorrect wiring and
should be investigated.
Next, insert V1 into its socket and repeat the
measurement. Now, expect a low resistance of 27-30
ohms. When satisfied with these checks, you are ready for
the “smoke test.” Turn the radio on and set the volume
control, gain, and tuning dials to mid-position. Apply either
a 12 volt battery or AC-to-DC source, and note whether
the heaters in V1 glow a dull red.
After warm-up (about 30 seconds or so), you should
hear static in the speaker. Rotate the gain clockwise until
you hear a squealing sound, indicating that the
regenerative detector has passed into full oscillation.
Normally, you will operate with the gain set below this
point. On especially weak signals or when full selectivity is
needed, set the gain just below the point of oscillation.
In urban areas with strong AM stations, the spiderweb
loop will be all the antenna needed. Best reception of
distant stations will be at night using an outside antenna
25’ to 50’ in length in conjunction with an earth ground.
Here again, the Internet will provide lots of advice on
installing long wire antennas and earth grounds. Here’s a
tip when finding really weak stations.
First, use earphones rather than the speaker to
eliminate distracting sounds around you. Next, rotate the
gain control until the radio just breaks into oscillation. As
you rotate the tuning knob, you will hear whistles that are
heterodynes or beat frequencies of the radio’s oscillation
and radio stations. Rotate the tuning knob very slowly, and
note that the beat frequency starts at a high pitch and
decreases as you rotate the tuning knob. When the pitch
is very low or disappears completely, you are tuned
directly on the station’s frequency. If you tune too far, the
pitch will begin rising.
When “on frequency,” reduce the gain until the
detector just falls out of oscillation, and you should hear
the station. It will likely be weak and fade in and out, so
you will have to listen carefully to hear the station
identification and get the call sign and location.
My initial choice of frequency coverage was the AM
broadcast band, but the radio can tune the shortwave
bands by simply changing the spiderweb coil design.
Figures 10A and 10B show a coil designed to cover the 4-
14 MHz shortwave bands, including international
broadcasts and amateur radio. It requires heavier gauge
wire (#16) and larger slits in the spiderweb coil. Because
of the very wide tuning range, you may have difficulty
tuning signals precisely.
Shortwave receivers often have a second tuning
capacitor of a much smaller value in parallel with the
existing one. This “band spread” capacitor provides easier
and more precise tuning once the general frequency is
tuned with the main tuning capacitor.
Rather than adding another variable capacitor, a
varactor diode could be used with the band spread tuning
accomplished by a potentiometer controlling the reverse
voltage of the diode. I have not tried this yet, but see no
reason why it wouldn’t work.
I hope you will enjoy building and playing the retro
regen radio as much as I have. Though I have not
accomplished it yet myself, maybe we will succeed finally
in accomplishing the goal of “Europe on one tube!” NV
34 May 2015
■ FIGURE 10. Template of spiderweb shortwave coil.