A Simple Regenerative Shortwave Receiver

The following is a description of a regenerative, tube-based radio made out of spare parts.

I've listened to many shortwave services with this radio, including international shortwave

broadcasters, radio amateurs, and even a few pirate stations, and the tunable range of this

radio is easily changed by making plug-in coils with more or less turns.  The nicest thing about

this type of radio is the decent sensitivity without the hassle of the many tuned circuits found

in superhet receivers.  I am a relative beginner at reciever design, so if you have any suggestions

at all for improvement (or just want to point out a flaw), please send me an e-mail:

cwillis@guilford.edu.  Enjoy!

The plate voltage is conveniently developed from a diode and large filter capacitor, fed off the

hot 120 VAC cord.  Filament voltage is 6.3 V.  The preamplifier provides even more sensitivity.

A generic 2N2222 transistor, class-A amplifier works well, with a pot to control the amount of

gain.  I feed such a circuit from rectified and capacitively filtered filament voltage.  The first

6AU6A stage has positive feedback through the magnetic coupling of L1-L2, and frequency

selection determined principally by the grid LC circuit.  C1-C2 control regeneration;  One may

also vary the plate voltage at this stage.  C6-C7 can be increased for more bandwidth / lower

selectivity or decreased for less bandwidth but better selectivity.  The second tube stage is an AF

voltage amplifier, with the detected audio from the first stage coupled in via R3-R4-R5-C10.

R4 is effectively a combination AF input control and regen control, and R5 more of a volume

control.  The power AF stage develops slightly over a watt (according to tube specs).  This

PA is a very standard one and any pentode is likely to be easily adapted for use here.  By

far, the most trouble with this circuit is actually figuring out how to adjust the many controls

(which are not independent!) to tune in stations.  In general, you want to start with C2 at its lowest

value and R4 "somewhere in the middle."  Turn up C2 until radio noise is heard, then fiddle

with R4 for best sound.   To tune the radio, use one hand on C4-C5 and the other on C2, keeping

the rushing sound as you tune.  Not paying attention to C2 will either cause decreased sensitivity

or oscillation which makes a terribly aggravating screaming sound.  Good luck!

Figure 1   My radio, built in a junky-looking scrapped metal box.  The ~49 M band coil

is in place at left.  A 170-V power supply filter cap is on the right.  6AU6As (2) and

the 6AQ5A are also plugged into the box top.

Finally, a word of caution:  because they generate their own oscillations, regenerative receivers

should not be used for monitoring VHF aircraft bands or other services where any interference

is unwanted / dangerous.  It is possible to be shocked by the plate voltages in this circuit, including

very mild shocks from the poorly insulated tuning coil- so it's not recommended that very young

children use this radio.