The D-104 or any other crystal mic needs to work into PLENTY of megohms of load resistance. Astatic recommends 5 megohms, but I run mine at 10 megs - probably about as high a grid leak resistance that you can use in a tube type preamp and maintain stable operation. I have plans in the works to replace the single conductor shielded mic cord with a two-conductor one, and connect the mic up in a balanced circuit for pushpull operation, and use a pair of high-mu triodes in a push-pull mic preamp. That way, each tube can have its own 10 meg grid leak to allow for a 20 megohm load on the xtal.
You can think of a crystal mic as an ideal a.c. generator with about a 500 pf capacitor wired in series. In order to get the best low-end response, the load resistance needs to be HIGH because of the high capacitive reactance in series. But there are inherent limitations to how high an input impedance you can achieve with a tube or FET input device. With the tube, if the grid leak resistance is too high, the grid will begin to hold a static charge that is not being drained off fast enough, and the bias voltage and plate current will drift around, resulting in distorted, flaky unstable output. 10 megs is about the limit. The instruction sheet that came with the earlier versions of the D-104 (the ones without the CB "power mic" feature) gave details on how to connect the mic up for a push-pull input stage. The stock xtal element is built as a balanced device. The unshielded bakelite case has two terminals, and either one can be used as "hot" or "ground" in the unbalanced configuration. For balanced output, connect the two wires from a two-conductor mic cord to the two terminals, and connect the shield to a ground point inside the mic head. Use a 3-connector mic plug, and at the mic preamp, let each conductor go to the grid (or gate) of one of the push-pull input devices (tube or FET), and ground the shield to chassis. The pushpull output is established by the two load resistors (grid leaks in the case of a tube pre-amp) of each amplifying device. They must be very close to equal in resistance. One end of each goes to ground. The two load resistors in series makes up the load resistor for the xtal element, and they act as a voltage divider to provide the balanced push-pull audio output for the amplifier. For a perfect match, I would recommend a pair of 10 megohm resistors, or two 5 megohm resistors in series for each, and using a DVM, match up the total resistances by adding smaller resistors series with the one with the lower resistance, until both give identical resistance readings. Besides improving the low frequency response of the mic, the balanced configuration makes it more immune to 60~ hum and rf pickup. Astatic recommends the balanced line for long mic cords more than a few feet in length. Don k4kyv _______________________________________________________________ This message was typed using the DVORAK keyboard layout. http://www.mwbrooks.com/dvorak/ http://gigliwood.com/abcd/ ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: AMRadio@mailman.qth.net To unsubscribe, send an email to amradio-requ...@mailman.qth.net with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
