vacuum tube

A vacuum tube is a robust electron device. It may stand reasonable signal overloading
and some abuse also. Solid-state devices are delicate and need protection circuitry. The
J310 JFET of Fig is not an exception. We need to include a source resistor for
protection and biasing reasons. Without this resistor the quiescent current would be too
large, some 30mA according to the manufacturer. Really too big for our purposes. The
gate-source junction would be also vulnerable to very strong signals that eventually
could reach our receiver.

In the figure above, the 18-kohm source resistor regulates the drain current ID to
0.157mA when the power supply is 4.5 Volts DC. So we get 2.83 Volts for the voltage
drop across the resistor. The gate-source bias-voltage is then -2.83 Volts. For a 9-Volt
supply, the JFET’s drain current increases only to 0.16mA. We get a very steady
operating-point current with respect to power supply variations. The resistor needs RF
decoupling and that is the reason for the 1nF capacitor. It is recommended a Mylar type.
Greater capacitance values are not recommended, as they can make the circuit operate
as a relaxation oscillator for high regeneration levels. This is an unwanted feature.

This said, “grid-leak”- type of detection is virtually cancelled. A VERY strong signal
would be needed to overcome the gate-source bias and make the corresponding junction
to conduct. Detection now is of the square-law type. The non-linear characteristics of
the JFET make this possible. The “grid” R-C components have no effect on the
detection action and have been maintained there for nostalgic reasons only. The receiver
seems to be a bit more selective if these components are left in the circuit, although no
explanation has been found for this.

The modulation is extracted from the junction of the RF choke and the primary winding
of the Mouser TM003 audio transformer acting as an audio choke. The 3.3-nF
capacitor, also a Mylar type, filters any residual RF component leaking through the 3.3-
mH RF choke and that may be entering into the AF amplifier.

The two-transistor amplifier has a voltage gain of approximately 1000 and has an input
resistance of 6k ohms at 1kHz. A piezoelectric ceramic / crystal earphone gives
comfortable listening.

One final comment is that a ground plane underneath the circuit’s layout is needed for
stable and hand capacitance-free operation. Connect the circuit’s common ground to this
plane.