Power converter project

I'm taking a lab class where we have to build a Power Converter using 2 MJE15032 power transistors to convert a 12 DC signal to AC which runs through a hand wound transformer with 2 sets of primary windings with 20 turns, 2 sets of feedback windings with 6 turns, and 1 secondary winding with 210 turns. The transformer works and the circuit oscillates, but it's oscillating at a speed of about 700KHz when we need a frequency of 30KHz. I'm thinking that decreasing the number of feedback windings would decrease the oscillating frequency, but I'm not sure. Maybe I should increase the voltage divider so that less direct voltage is running into the transistor bases. I'm not really sure, but if someone could point me in the right direction of how to decrease the oscillating frequency here, I'd really appreciate it. Thanks.

PS: here's a copy of the circuit schematic.
And we currently do not have the bridge rectifier hooked up. We are testing the circuit with a 100K resister on the output.

 

Just looking at the schematic it appears that you have a 'blocking oscillator'. Usually the construction of the transformer itself is what determines the resonant frequency. If it is a lower resonant frequency you are aiming for, I would suggest increasing the number of turns of the secondary output winding.
Another possibility (although not certain), is to measure the inductance of the secondary output winding, then calculate the necessary capacitance to create a 'series resonant circuit' at the desired frequency, then place that capacitance in parallel with the secondary output winding.

Anyways, good luck, be safe, and have fun!

 

What are you using for a transformer core? Air? You'll reach saturation very quickly that way.

If you wound it on a ferrite toroid or E-core that was insulated with tape (to prevent early saturation), your resonant frequency would dramatically decrease.

 

I believe we have an E-core pot. It's got two halves with a plastic ring that we wrapped the wires around before inserting into the two halves. The only problem with increasing the number of secondary turns is that we wound those first. So, that would mean rewinding the whole transformer. I was really hoping we could change the number of feedback windings or the voltage divider since those would be a lot easier than rewinding the entire transformer. Thanks for the response guys.