I couldn't adjust the POTs to get an output voltage beyond 11-12 volt here

[Kyyo]

I am new to electronics and have been exploring easy to make DIY things for a while now. I am using the following circuit to make a voltage regulated high current DC power supply. The problem is, I couldn't adjust the POTs to get an output voltage beyond 11-12 volt. The DC output is always between 11 - 12 volts, neither less, nor more. By the way, the trafo is 12-0-12 5 Amp, and I am measuring no-load voltage with a digital voltmeter.

My questions are:

1. Is the circuit correct, regulated voltage at the base of 2N3055 will regulate the voltage at its emitter - is this principle true?
2. Is keeping one of the terminals of the POTs open (Not Connected) a good idea? Should they service the purpose in this circuit?
3. If I want to troubleshoot, what are the test points and what are the expected voltage or other attributes I should look for.

Thank you a lot for your help, in advance.