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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:  Enjoy!
C1____300 pF mica
C2____360 pF air variable
C3____82 pF mica
C4____150 pF air variable
C5____150 pF air variable
C6____22 pF mica
C7____75 pF air variable
C8____0.01 uF, 200 V ceramic
C9____100 pF, 200 V ceramic
C10___0.01 uF, 200 V ceramic
C11___0.01 uF, 200 V ceramic
C12___0.01 uF, 200 V ceramic
C13___0.33 uF, 16 V electrolytic
C14___0.01 uF, 200 V ceramic
R1____4.7 M
R2____100 K
R3____680 K
R4____2 M Pot
R5____2 M Pot
R6____100 K
R7____43 K, 1 Watt
R8____330, 1/2 Watt
R9____240 K
R10___470 K
L1____Tickler, X turns #24 closewound on bottom of 1 1/4" dia. form
L2____Main, ~1.5 X turns #24 above L1, spaced to 1" long
L3____Signal, ~3 turns hookup wire, loosely coupled, movable
L4 (inductor by C9)____A small pie-wound RF recieving choke
T1____Matches ~4500 ohm plate resistance to speaker, 1.2 W max
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.

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