homebrew side-tone oscillator

I like very low- and independent CW side-tone from my transceiver. When I wear my headphone it is difficult to hear a side-tone from high frequency piezo-buzzer buit in my old hb Z-80 message-keyer. This is a sound of homebrew simple side-tone oscillator with a frequency ranging 200- to 1000 Hz. Keying of the tone is achieved by a mute transistor and the key-click is much reduced with a CR filter on its base circuit. The accompanying high tone is a side-tone from a piezo buzzer of my elec-key for the reference. The photo shows a oscillator board with a dual 50k-ohm VR from ALPS for freq adj, twin-T osc and keying circuit.