Saturday, February 11, 2023

to regen or not to regen?

 Mr Tom on TNRB posted an interesting question.

Why will this set not perform? If Mr Andersen says it is a good performer we will assume it is. Tom says it picks up local station weakly. What makes this circuit challenging is the ferrite core. L2 60 turns tapped at 10 from the bottom. With an air core coil the first thing would be "phase the reaction". If the two coils were wound separate we could just reverse phase by reversing the wires. With a center tapped coil it is a little more difficult. 

This was the fix. Observe the base bias flow out the base and up to the diode. The .01uf cap places the center tap at AC ground. If the signal flows up the lower coil and down through the .01uf cap the upper coil can conduct in either direction without conflict. 

If the base current flows into the center tap and out the bottom through the diode the center tap is negative. The upper coil will be in conflict with the lower and provide degeneration or in phase and will provide regeneration in the other phase.

The hard question with building the circuit is how are the coils phased. L1 is the transformer primary and the 2 secondaries will have a center tapped phase aiding or opposing.

One other thing with the ferrite core would be to make the coils separate and movable. Sliding the coil along the core will vary its inductance and allow some adjustment of the tuning and the reaction.

One other possible fix would be to reverse the diode and place it between the base and center tap. This would reverse the phase of the lower coil.

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