WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I may be wrong, but 12V seems just a little low for correct operation. What is this circuit to be used for?
edit: Are you using this for a distortion pedal? If so, it should work fine. Just don't use electrolytics for coupling.