[R-390] Fw: Weak BFO Cured

[David Wise]

?Some years ago my BFO gradually weakened and died.  Replacing the tube did not help.


The tube drives the coil as a cathode follower.  The cathode follower is low impedance but not zero.  I believe the design is marginal, with very low coil impedance at the driving point; a good 6BA6 can just manage it.  Q deteriorated, and eventually the tube couldn't maintain oscillation.


I improved the tube's driving ability by adding an emitter follower stage.  There's a sweet spot where the DC and AC requirements coincide, so you can do it with just two resistors plus power source.


Disconnect pin 7 from coil;

Pin 7 to 2N2222 Base and 100 ohms to coil;

Emitter to 22 ohms to coil;

Rectify and filter 6.3VAC to power the Collector.


BFO comes on strong and clean.  Good tube, weak tube, doesn't matter.  I call it the Bionic BFO.  :)


Dave Wise