[Milsurplus] "Frnge Howl"

[John J Mccarty]

Hue and All;

Working from memory. Get a small, 6 oz tomato paste can. Use the normal can
opener to take off the top and bottom. Then cut it along its length. Find a
dowel rod or a broom stick smaller in diameter than the tube and squeeze the can
around it till it’s approximately the right size ( I can hear Mike H. in the
Garaj Mahal laughing at me right now ;-) ). Watch the sharp edge. Cut off the
extra and solder the seam. You will most likely have to cut a V notch in the top
for the grid wire top cap. Solder down the bent down sides of the V. Solder on a
tab at the bottom to attach the shield to a convenient screw. Don’t have to tell
you that a big soldering iron needed, like at least 100 watts with a good sized tip.

 I did a little reading and Hue, your right, from what I’ve seen the screaming
from the bc-474 isn’t fringe howl in the true sense of the term. Most of what I
read said that fringe howl is an Autodyne detector going into and out of
oscillation at an audio rate, making a loud racket since the gain of the
detector is at max. The 1D6 stage in the bc-474 is a detector with two stages of
audio amplification that was not supposed to go into oscillation. I don’t know
what kind of coupling causes the feedback but I suspect its audio feedback as
opposed to rf. Could the grid lead for the triode section catch enough signal by
running by the pentode section of the tube? Then sheilding the grid lead would
also be effective?

    Meanwhile by chance I’ve been tinkering with a copy of Frank Jones’ 19 tube
regen rx. Had what I thought was fringe howl when using the 40 meter coil. Tried
loading down the primary of the transformer without much improvement. I moved
the windings apart on the coil and it cooled things down considerably. Need to
rewind that coil.


73


John n9hrt


On 1/1/2011 10:48 PM, Hue Miller wrote:
> John, thanks for your input.
> So you cut the can down, horizontally, and then coiled it up more tightly, 
> or did it pretty closely
> fit without modification?
> Interesting - I don't even remember if mine had a shield on the 1D8 or not.
> So the mechanism seems to be capacitive coupling from transformer and output 
> wiring
> back to the grid, or magnetic coupling - less likely?? what do you think?
> -Hue 
> 
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