[AMRadio] D-104 element

[Donald Chester]

Let's try this again.

I think the best solution to the D 104 mic element problem if you regularly
attend hamfests  or flea markets, is to accumulate a few used D 104s
whenever you find one selling cheap. The normal selling price is $40-$80
each, but often you can find one, particularly an older one without the
crappy built-in pre-amp and all the exterior decoration they added to
attract the CB crowd, for $10-$20. Of course, some of those will have a bad
element just as some "good" hamfest tubes will be turn out worthless, but if
you collect a  few inexpensive used mics, you will probably find at least
one with a good element. Maybe you could then buy up some of those imitation
elements, refill the extras  and  re-sell them on e-pay. I have a couple of
good elements as spares, still mounted in the original mic head. I store
them in a cool, dry place, hoping they won't go bad with time, in case one
of my good ones fails. Actually hooking the mic up and using it shouldn't
have any effect on its life expectancy, as long as your operating position
is reasonably cool and dry. Excessive heat and humidity is what kills
Rochelle salt crystals.

I don't think those replacement elements have the same resonant peak in the
2000-3000~ range (aka "presence rise") that makes the D-104 response curve
unique and highly effective for AM audio. I run mine in balanced output
configuration which is possible because the stock D 104 bakelite case
element is non-polarised. All the replacements I have seen have a metal
case, with one terminal grounded, which doesn't lend itself very well to
balanced output.

You can get good low frequency response with a D 104 by running it into as
high resistance load as possible. I would recommend at least 5 megohms,
preferably 10. I run mine into a 20 megohm load by using a push-pull preamp,
each tube having a 10 meg grid resistor to ground. Many of the Hammy AM
transmitters allegedly designed for a xtal mic use input  resistances as
low as 250k, 100k or even 50k, which makes them sound like a tin-can
telephone.

A crystal microphone element is roughly equivalent to an ideal a.c.
generator with a 500 pf capacitor in series. The only way to get lower audio
frequencies to pass through that "capacitor" is to work it into a very high
impedance load.


Don k4kyv