> I know I could replace the element with one from Heil, but I would prefer > to > keep it as original as possible (it does have the preamp in the G stand > base).
Tom, Odds are the crystal element is still in good shape, provided it has not been subjected to a high humidity environment over an extended period of time. Also, anyone contemplating the use of the D-104 with the stock Astatic amp should consider an alternative buffer amp. The input Z to Astatic's 2-stage preamp calculates to 470K-ohm -- way too low for a crystal cartridge. The crystal cartridge should see a termination of no less than 5 megohm, and preferably greater than 10 megohm in order to preserve reasonable low-end response. The series capacitance from a typical crystal cartridge is anywhere from 800pF to 1600 pF. Calculate the - 3dB turnover point into various terminating resistive values and you'll see why it's important to completely unload the crystal cartridge. For simplicity and excellent performance, I favor a single transistor JFET, configured as a near unity-gain source-follower. The web is full of FET and op-amp designs for the D-104, but nearly all of them suffer from inadequate design, particularly with respect to crystal cartridge loading. Most designs I've seen attempt to bias the FET as the analog of a "grid leak" resistor used on a vacuum tube grid. As a source follower, the FET is completely self-biased when the correct source resistor value is used. The D-104 crystal cartridge can be attached right to the FET's gate with no other components necessary. For isolation, I add a 5-meg resistor in series with the gate, but this is not strictly necessary. If gate failure is a concern from static electricity, some ops have used a 10-megohm resistor from the gate to ground in an attempt to bleed static build-up.. I've never found this to be necessary. If all of the above sounds like too much work, simply insert a 10-megohm resistor between the D-104 cartridge and Astatic's 2-stage preamp. Signal to noise and distortion performance will suffer, but at least the cartridge will be unloaded. The Astatic preamp can more than compensate for the loss in level owing to the use of the series resistor. When the K3 is configured for ESSB transmit, the D-104 will sound nearly as good as an expensive studio condenser or dynamic mic. As a final footnote, I recommend ESSB only for use on uncrowded bands, and when signal strength is high. Otherwise, it's a waste of occupied bandwidth. Paul, W9AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
