A few years ago I had a notion to figure out why most kit regenerative
receivers just didn't work, despite their reputation for great sensitivity,
bla bla bla.

I looked at a few schematics and a few points really jumped out at me.
Basically, most standard Heathkit/KnightKit circuits are just Hartley or
Armstrong oscillators with variable plate or screen voltage, provided by a
potentiometer.. At first this sounds reasonable, but the goal of a
regenerative receiver is really that it NOT oscillate.

Most of the circuits have 2 fatal flaws, both of which lead to the same
result. The 2 flaws are grid-leak bias and resistance coupling to the
following audio stage. The problem is that these techniques cause a shift in
tube operating points as the signal strength changes, making it "snap" into
oscillation. This occurs because any signal, or impulse, or anything else
that drives the grid instantaneously positive causes grid current to flow
and charges the grid capacitor, biasing the grid negative and reducing the
plate current. This wouldn't be so bad except that the reduction in plate
current causes the plate voltage to rise, due to the coupling resistor in
the plate circuit. As the plate voltage increases the gain increases and the
thing snaps into oscillation, ready or not.

I put together a test unit using the Armstrong (tickler) circuit, but I used
NO grid condenser or resistor. Instead, I returned the grid circuit to a
0-20 volt regulated power supply (properly bypassed for RF, of course)so I
could control the grid bias manually. Also used choke coupling, actually the
primary of an output transformer in the plate circuit. Used a 6T4, and the
circuit would still work even with plate voltages below 30 volts.

I didn't fool around with it too much after I proved my point, but the thing
really did work pretty well, far better than any "kit" regenerative I have
ever seen. With regulated power supplies I could get some real sensitivity
and a smooth roll into regeneration. I was impressed. Never did make a real
project of it, but it was a good experiment.

If you go back and look at some of the pre-historic regenerative circuits
you will notice the same concepts at work. Most were choke coupled, and many
used variometers or "throttle condensers" to control regeneration. Again,
these techniques do not allow the DC operating point of the tube to change
with variations in signal strength.

Just food for thought.






"Dale Parfitt" wrote in message
news:Bdeyf.1445$he.1382@trnddc03...
Hi Group, I am entertaining building anohter regen. My first attempt,
although pretty, was a mediocre performer- using a 3 section compactron
tube.
http://parelectronics.com/pics/regen2.jpg
Design goals a
1. plug in coils for band changing MW-20 MHz or so.
2. speaker level audio
3. smooth regen
4. prefer miniature tubes to octal.

Anyone care to post their favorite schematic/ links/ thoughts?

Tnx,

Dale W4OP