The circuit is basically a multivibrator constructed around a double triode ( I used ECC83 ), and oscillates around 1KHz.

At the output I’ve connected a 2000 Ohm telephone loudspeaker, but with a transformer, a 4 Ohm speaker could be used as well.

The frequency can be adjusted from the 100K variable resistor.

The ECC83 works well even at low voltage ( ~20V ).