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I assume you're talking about regulating an AC signal. I'll also assume that you want to regulate the peak to peak voltage of the AC signal. The easiest way to do this would be to use two zener diodes tied together in series, with the connection also tied to the center tap of the transformer. This will prevent the voltages from swinging either too positive or negative. If the over voltage is due to a DC offset, you can either use a filter cap or a clamper diode. I hope that helps.
I think it's an SCR used in a crowbar circuit.
I think you need a 12V DC PSU with an over-voltage protection at the output state, so that in case of circuit fault, the output does not rise above 12v.
You can do this in a few ways:
1) If you want to use a crowbar, then, replace the thyristor with another of equivalent power.
2) Use an active crowbar: