[R-390] BFO transistor CLT

[David Wise]

Someone asked for a sketch, but the circuit is so simple that a
written description is enough.

Reprinted from IF_Deck section of Pearls Of Wisdom from www.r-390.net .

Date: Mon, 7 Nov 2016 18:47:47 +0000
From: David Wise <David_Wise at Phoenix.com> 
Subject: [R-390] Fw: Weak BFO Cured

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. :)

DC: Tube cathode current through 100 ohms sets base voltage.  Emitter voltage through 22 ohms
sets emitter current.  It varies some with temperature but it doesn't matter.
AC: The tube sees 100 ohms.  I don't know the coil impedance but the load is now 100 ohms lighter.
The coil sees about 20 ohms.  I don't know the 6BA6 cathode impedance but I'm pretty sure 20 ohms is lower.
So the tube is lightly loaded and puts out more voltage, and the coil is driven harder.

Regards,
Dave Wise