There are distinct advantages to wiring a crystal microphone for balanced output; this is easy with the classic D-104 with the stock crystal or ceramic element, which is non-polarised. The case is bakelite with two terminals, and no polarity is assigned to the two terminals. This ambiguous polarity allows the mic element to be wired to provide balanced output to feed a push-pull preamplifier. Balanced output with some other types of crystal microphones may not be practicable, since the case of the element is frequently made of metal with one terminal grounded directly to the case, and sometimes the shell of the microphone itself is made of plastic and therefore provides no shielding.
Balanced output is achieved by using a two-conductor shielded mic cord. Each of the two conductors is wired to one of the terminals on the crystal element, and the shield is grounded to the interior of the shell of the microphone head. Each tube in the first stage of the push-pull preamp has a grid resistor to ground, therefore the two grid resistors are effectively in series, with the common point of connection grounded, to act as a voltage divider to provide the 180° out-of-phase signal required for the grids in the push-pull circuit.
The push-pull pre-amp has two advantages over the more conventional single ended circuit: (1) There is substantially less rf and 60~ a.c. pick-up, and (2) the two grid resistors in series allow the crystal to be loaded down with twice the maximum resistance as with the single ended circuit. There is a limit to how much grid resistance can be used in a tube type amplifier stage before the bias voltage and plate current become unstable. Astatic recommends using 5 megohms with the D-104. In the RCA Receiving Tube Manual, the recommended maximum for most tubes is only 0.5 megohms! Up to 10 megohms is usable, provided that the tubes are carefully selected for stability. With the push-pull circuit, the total load resistance can easily be set to 10 megohms (5 megohms per tube), and by carefully selecting and matching tubes in the first stage, up to 20 megohms load (10 megohms per tube) can be used.
Using the highest practicable load resistance allows the maximum low frequency response of the microphone. A crystal microphone element can be thought of as an ideal a.c. generator with approximately 500 pf of capacitance wired in series. In order to pass lower audio frequencies through 500 pf, an extremely high load resistance is needed. In some "vintage" commercially built amateur AM transmitters, the grid resistor is way
too low. I understand that in the Heathkit Apache it is only 50K. No wonder that transmitter has a reputation for being "scratchy".
The push-pull pre-amp is not my creation. The original data sheet with the D-104, dating back to the early 30s, describes this circuit and recommends it for runs of mic cord over a few feet, to reduce hum pickup. As expected, I observed that it also reduces or eliminates stray rf pickup from the transmitter.