I have two questions.
1) Will I kill this this little pre amp with a 15v AC supply when it calls for 12v AC? (no +/- range indicated).
2) Could/should I use a resistor to get down to 12V?
Hi valley001
1) Most probably be OK, but not worth the risk.
2) Yes, but it would require a bit of messing around and the transformer effective secondary output impedance would be a touch on the high side.
But I have a cunning plan, not elegant though: connect a string of eight 1N400x diodes (you must use 1N400x diodes) in series, with all diodes facing the same way. Then make an identical string of eight 1N400x diodes in series. Connect the two strings of diodes in parallel, except with the diodes in the strings facing in opposite directions. Call this circuit a voltage dropping diode assembly (VDDA).
Connect the VDDA in series with the secondary winding of the transformer.
As I say, not elegant, but effective, easy to construct, and cheap: the 1N400x diodes should cost under £2.0US. You may be wondering why not use two back-to-back 7V5 Zener diodes. The reason is the peak current demand of the valve amp is not defined: the input could be a diode capacitor rectifier, which would take huge gulps of current and would destroy a Zener diode (rectifier diodes are designed to handle huge peak currents).
The 1N400x diodes may get hot, so mount them in free air for cooling.