First of all there should never be an element of any tube that is left open as far as DC is concerned, and typically, crystal microphones have no measurable DC leakage so it is an open circuit to DC. If a Crystal microphone is to be used on this circuit the tube would soon go to cut off or thermal grid emission run away, depending on the tube condition. If the tube has no cathode resistor for bias then I would recommend a few mega ohms of grid leak resistor for contact bias and a coupling capacitor in case a microphone such as a dynamic or some type of microphone that has a DC path to ground is used. If the tube has a cathode resistor, then a lower value of grid leak could be used as a load for the microphone. A 5 Meg potentiometer is useful here with the wiper to the grid like a rheostat to ground. Some microphones do better with a lower load resistor for damping but crystal microphones will lose all low frequency response if loaded too heavy with out equalization.
Matching impedances is confusing. If you match the source impedance with the load impedance, maximum power will be transferred from source to load. With microphones, we are not concerned in transferring power. It is the response fidelity and ringing factors that we are concerned with. There is a best load that is needed but it is hardly ever the impedance of the source. See http://www.qsl.net/wa5bxo/power.html allow a lot of time for the picture to finish loading. Good Luck John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Tuesday, March 15, 2005 8:25 AM To: amradio@mailman.qth.net Subject: [AMRadio] Mike Thanks Guys: 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 guess the first question is: what impedance number is required for a 6SJ7 grid? Regards, Steve WA2TAK