[AMRadio] Mike

Jim candela jcandela at prodigy.net
Tue Mar 15 10:32:33 EST 2005


     I just was reading about this in my "Radiotron Designers Handbook"
where a simple preamp using a 6SJ7 was used with a crystal microphone. In
short you can use a higher grid resistance to change the frequency response
(more lows), but the text describes the limitations involving reverse grid
current, dc operating point shift, signal to noise ratio, dynamic range, and
so on. There also is some alternative circuitry shown to give you some ideas
if you want to do some extensive modifications. I find it interesting that
they show the 6SJ7 in triode mode using two different techniques.

    I scanned in several pages, and have posted them on the net at the
following link. The text is sideways, and you can turn image 90 degrees CCW
with your Acrobat reader. Hope this helps.



-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of
StephenTetorka at cs.com
Sent: Tuesday, March 15, 2005 8:25 AM
To: amradio at mailman.qth.net
Subject: [AMRadio] Mike

Thanks Guys:

This mike is for a vintage 1948 AM rack rig homebrew by W2PLY who moved into
Senior Residence.
The speech amplifier/modulator schematic is right from August 1947 QST.

I got the rack and the 5 sections directly from the flat bed trailer at the
estate sale...and now just about finished doing what very little fixin

The article cites a crystal mike; it feeds directly into the grid of a 6SJ7
with no grid leak resistor.

As I understand these things, the mike - what type of cartridge - needs to
put out 'so many' microvolts necessary for the tube into which it feeds.

And the impedance needs to 'match'.

I understand there plenty of gain after the 6SJ7...and one of the 6SN7's has
a "tone control" potentiometer in the grid.

Personally, my voice is a little higher from center in pitch..and I'd prefer
more lower than higher freq. response...which is generally a problem due to
low freq. rolloff in the mike curves...that's why I'm eager to try this
'tone control'.

I guess the first question is: what impedance number is required for a 6SJ7

Your generous help is always appreciated.

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