CF mic based around Sony C37A

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Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
Hi guys

I came across the Sony C37A schematic and all the little threads all around about this microphone. I know there is nothing spectacular in this schematic but since I'm in a Learning stage about tube circuit design, I decide to starting all the design from the beginning with a new tube, new resistors values, new capsules, etc. Since it does not look too complicated, it should be fun and really instructive.

So, I want to build the microphone around a 12ay7 tube (good choice??), M7 or K47 type capsule. The polarisation voltage as to be around 60V, isn't it? So I start with this in mind and manage to work around the other parameters. I bias at around 1ma, and it gave me -1.0v. My Vpeak is about 35V (is it ok for a microphone???). If, for exemple, my HT is 120V, I trace my load line and get around 2.0ma and a Rk of 60K. Then, I found my Rb from the bias I choose: 1v/1ma = 1K. Not that bad I think.

If my grid voltage is 60v + -1V = 59V, then HT-Va will give me about  61v. Am I right?

From now, is it a good way to design this circuit? Is it better to bias half way the HT, or I should, for exemple, put a 180v ht and manage to bias to get my 60v polarisation and a higher Vout (around 120v)??

Now I get back to my design. Vdrop across Rb is 1ma x 1k = 1v. So, my Vgk is 1v. Then, I have 60v to drop across the 60k at 1ma. at the jonction of Rb and Rk, I have the grid leak resistor (100M for the sony C37A)

This is where I bug. How can I manage to find this value. I'm not shure about it's fonction, too. Maybe I'm wrong somewhere in my maths, thought.

Thansk for your time and your help. :)



 

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