Armstrong Quad Regen Negative Quad Regen

Even though Q1 and Q2 are biased and connected in an unusual way they still operate within datasheet defined specification for most transistors.

They form Q multipliers (Armstrong/Negative Resistance) controlled by R2 acting on the LC tuned circuit. Just below the threshold of oscillation most of the losses in the tuned circuit are cancelled out - giving high Q, high gain and high selectivity.

The signal is detected by non-linear response of Q4 to a voltage source signal. Q3 forms a current mirror biasing circuit for Q3. Assuming Q3 and Q4 are approximately matched R4 should be half the resistance of R3.

You can replace R4 by a pair of headphones. If you need more volume you can try reducing R3. 

The 1 uF capacitor connected to R2 is optional and reduces noise but potentially can cause odd effects like super-regeneration.

The Negative Resistance Quad Regen has the disadvantage of Miller effect feedback for the collector of Q2 to its base. A solution is to add a third transistor to the differential pair (triplet) as per the Armstrong circuit.

Of course you must couple an antenna to the tuned circuit for example via a low value capacitor and for the Armstrong Quad Regen you must make sure the coil phasing is correct (or you will have to swap around the connections of one of the coils connected to Q1.)

The coupling coil ratios for the Armstrong circuit are similar to the Negative Resistance circuit at about 5:1 to 10:1.