Pentadyne Regen Receiver

Q1 and Q2 form a differential pair that supplies negative resistance that is coupled to the tuning tank circuit C3 and T1 secondary. You can couple an antenna to the tank magnetically with a couple of turns of wire or with a light capacitor link.  

Adjusting R2 controls the amount of negative resistance (the amount the current at the collector of Q1 drops with an increase of voltage there.)

At just below the point oscillation is induced, the negative resistance cancels out most of the losses in the LC (really LCR) circuit resulting in very high LC Q providing gain and high selectivity.

Q3 buffers Q1 and Q2 and leads into a current mirror AM detector comprising of Q4 and Q5.

If Q4 and Q5 were exactly matched the combined DC currents flowing through Q1, Q2 and Q3 would be exactly mirrored at the collector of Q5.  

The RF signal at the collector of Q3 is directed by the RFC L1 into the base of Q5 where it is detected by the non-linear behavior intrinsic to transistors.  C2 smooths out the resulting detected envelope.

If Q4 and Q5 are matched L2 can be replaced by a resistor, half the value of R1 would be fine.

Alternatively you can measure the maximum current at the collector of Q5 and choose a load resistor based on that.

You may need additional RF filtering depending on the audio amplifier you connect the circuit to.

C6 is optional and could possibly cause super-regeneration but if the circuit will tolerate it is helpful.