[AMRadio] Re: Mike


ne1s ne1s at neandertech.com
Tue Mar 15 12:51:35 EST 2005


Steve, 

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. 

GL,
 -Larry/NE1S 

>> -----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. 
>>
>> Regards,
>> Steve
>> WA2TAK
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