This dc-to-ac inverter is based on the popular 555. A 555 oscillator circuit drives a buffer amplifier consisting of Ql, Q2, and Q3. The circuit operates at 150 to 160 Hz. Tl can be a 6.3-V or 12.6-V filament transformer as applicable.
The frequency can be changed by changing the values of Rl and/or Cl.