[i]I've noticed that if I have to reach too far to change notes, I need to turn the pitch knob to the right... If I find myself overshooting notes, it's time to turn the pitch knob back to the left a bit...[/i]
Indeed, to turn the Epro's pitch knob to the left increases the note spacing while turning it to the right decreases the spacing. These adjustments are done to the LEFT of the zero beat knob position.
However... the Theremax is the opposite. The fine tuning adjustments are made to the RIGHT of the zero beat knob position and to move the knob to the left places the notes closer together and to the right makes them farther apart.
[i]...you really should let the theremin warm up for about 10 to 15 minutes before attempting to play it... This is because the magnetic fields around the antennae are constantly changing as they settle down and stabalize for about 10 minutes.[/i]
To elaborate, electronic component values change with temperature.
Before turning on a theremin, the internal components will be at the same temperature. When first turning on the Theremin, electrical current flows through the circuits and some heating is normal. As the parts heat up, their values (resistance, etc) changes. The size and materials in different components (resisters, coils, semi-conductor, etc) heat up at different rates. The instability is the result of uneven heating of the parts after power up. After the passage of time (say 10-15 minutes) the parts reach thier operating temperatures and all is stable again.
Another source of SERIOUS instability is excessive humidity coupled with rapid temperature change. (Photographers know about this, too.)
If your Theremin is in an air-conditioned environment and then you take it outdoors on a hot, humid day, moisture will condensate on/in the theremin. (I almost had to scrub an outdoor performance over this issue -- fortunately, I allowed enough setup time that things stabilized.) In the future, I will likely pack a compact mirror with the theremin (if the mirror collects moisture, then I'll simply wait till it clears up before power-up).
Oh... the result of the condensation? Total unplayability. The Theremin was playing rapid scales and notes on its own! For a few frantic minutes, my Theremin was jumping around from pitch to pitch -- like the sample/hold device on a synthesizer.