|[AMRadio] Re: Mike|
ne1s at neandertech.com
Tue Mar 15 12:51:35 EST 2005
I'd put a 4.7Meg to 10Meg resistor from the 1st 6SJ7 grid to ground. A
crystal mike wants to work into a (high value) resistive load.
>> -----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 9:25 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 required.
>> 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 grid?
>> Your generous help is always appreciated.
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