Re: 1 watt or 1/2 watt Tube amps
Power scaling is a method of turning down the voltage, usually just the power amp stages (there are issues with doing the whole amp). Normally, the power stage is running full tilt, and volume is reduced by choking off the signal reaching the power stage (i.e. master volume). This certainly works for decreasing volume, but the signal entering the power stage is now too weak to cause it to overdrive.
Instead of choking off the signal, like other methods such as master volumes, power scaling instead makes the power stage act 'smaller' (for lack of a better term) by reducing the voltage to the power tubes.
When an amp is power scaled, one knob controls the volume (actually voltage to the power stage) and another controls the amount of signal reaching the power stage (drive compensation). Without the drive compensation knob, as volume is reduced (i.e. voltage is reduced to the power tubes), the power stage becomes overwhelmed by the incoming signal, and powerstage distortion increases. In effect, this knob (which electrically is identical to a master volume) controls the amount of signal slamming the power stage. So, the power scaling knob reduces the voltage and volume, and drive compensation tweaks the amount of power tube distortion.
For fixed-bias amps (i.e. the ones where *you* can adjust bias, such as Plexis, JC800s, etc.) things are a bit more complicated. As voltage is reduced, the bias also needs to be reduced. Power scaling kits from London Power (google it) have automatic bias tracking, so that bias is reduced appropriately as voltage is reduced. This is what I use on my Metropoulos Plexi.
For cathode-bias amps (e.g. 18 watt, a lot of Fenders, Ax84, AC30?), the design means that the tubes automatically adjust their bias with changes in voltage (i.e. the user never, ever, has to set the bias). In other words, you no longer need to worry about tracking voltage, and hence scaling is much simpler. The folks over at 18watt.com have come up with an open-source solution called VVR (Variable Voltage Reduction). It's a few caps, resistors, and a mosfet, and I easily adapted the design to my AX84 P1-Extreme. Also, several folks over there sell off-the-shelf boards/kits.
Either way, it requires modifying your amp. So, if you aren't comfortable doing it, you'll need to hire a tech to do it.
The 18watt Suhr Badger features power scaling:
http://www.youtube.com/watch?v=HBUxUB-6qjw
