RadioBanter - View Single Post

April 30th 06, 01:44 AM posted to rec.radio.amateur.homebrew

wrote:
On Sat, 29 Apr 2006 18:01:58 -0400, ken scharf
wrote:

The regeneration control either varies the amount of feedback
(throttle cap, movable tickler coil, or pot across the tickler coil)
OR varies the gain of the valve (or solid state device) by varying the
plate voltage (pot or series resistor), screen voltage (consider the
screen of the tetrode or pentode as the plate of the triode and the
plate of the tetrode that of a cascaded second stage), or grid voltage

(yes you can also add negative bias to the grid to lower the gain,
rarely used in a regen using tubes but the ONLY way with FET's).

I found that was not the best with fets. However, fets work best when
run at best gain and the amount of feedback (regeneration) is
controlled via pot or throttle cap.

If the variable gain method is used, then the tickler coil must have
JUST the right number of turns so there is JUST the right amount of
feedback for regeneration to start when the valve (or other active
device) is operating at the optimal gain setting for detection. If
TOO much feedback is provided the gain of the detector must be set
so low that poor sensitivity results. Also adjustment of the regen
will be tricky. If too little feedback is provided regeneration may
not happen or the detector gain will have to be so high that the
detector will be unstable.

Mostly what I said.

For some reason most triodes seem to operate as detectors with
about 30-50 volts on the plate, and pentodes with 20-50 volts on
the screen. Using a regulated supply for the detector is a good
idea, especially if you want to try and receive SSB signals.

It does help. I have a 5 tube 80m RX I built that uses a regenative
detector. I didn't need the gain but for selectivity using very loose
couped IF. Stability was hard to maintain without a regulated
screen (6BA6 detector) source from interactions of agc and audio
amp pulling the supply. Once I nailed it down it performs extemely
well and is a bit narrow for AM phone with only 2 IF cans.

Also for radios using tubes with directly heated filliments dropping
the filiment voltage can tame the regneration a bit.

The fade out problem with your regen sounds familiar, I had this problem
on an old AA5 radio. It would stop playing after a while and came back
if I touched the signal grid lead (antenna tuning cap hot side) of the
12BE6 tube (I actually held one end of a .01 cap and touched the OTHER
side of the cap to the grid as it was a hot chassis set). Weird!

The problem was likely an open antenna or oscilator coil, I've seen
that before. The coil opens (broken connection) and the grid floats
till its self bias drives it nuts. Inject noise and you bleed off the
charge for a while.

I suspect the 6M grid resistor is too high for the tube used and it's
building space charge bias till it cuts off. I tried a regen using
some 1AD5 (submini 1.5V filliment tube) and it didn't like grid
resistors over about 4meg and it behaved the same way.
the solution was lower (2.2m) grid resistor and 100PF cap.

In general using a higher grid resister and smaller capacitor results
in increased selectivity and sensitivity with a loss in large signal
handling ability. Visa Versa for smaller grid resister and larger
coupling capacitor. Also the grid resistor can go across the capacitor
or from the grid to ground. If the grid coil's cold end is NOT grounded
the latter arrangement is required. In early tube manuals the
recommended grid leak resistance was usually 1-2 meg ohms, with a
capacitor value of 100-250pf. You have to watch the time constant
there or you might end up with 'super-regeneration' where the tube
breaks into a high frequency (ultra-sonic that is, around 10khz)
oscillation.