Power Selection Switch


Does anyone know how some companies accomplish a power selection switch? I'm referring to ones found on amps like the PRS MT15 and the Peavey 6505 mini head. The Peavey even has 3 power settings. I did some web searches but couldn't find much. Not sure what is typically done for this.

Things I thought of is maybe a pentode/triode mode switch or integrating an attenuator of some kind.

There are several ways how amps achieve these things. Roughly most of them are divided to 1) adjusting headrooms via supply voltage, 2) adjusting headrooms via biasing or loading, 3) pentode triode switching or filament voltage reduction 4) attenuating the output signal.

Headroom adjustments via voltage usually alter either plate supply voltage of the power tubes, screen supply voltage of the power tubes, plate supply voltage of the phase inverter stage, or simply just all of the above and even supply voltage of the preamp.

Decrease the voltage and the headroom is lowered because the amp starts to clip earlier, and rated output power is therefore reduced. Depending on where this "starving" occurs the resulting clipping stage (that limits the output power) may be a power tube, the phase inverter or even just one of the preamp stages.

Bias modulation is usually directed to either power tubes or the PI, phase inverter being probably more common. The stagr bias is adjusted so that headroom of the stage decreases and clipping begins earlier.
"Loading" is less frequently employed practice but in practice it's based on the idea that headroom of a stage decreases with increased loading; e.g. headroom of a 12AX7 is less with 10k plate load than with 100k because the tube can not supply the increased current draw. A common method is to "load" the PI stage excessively therefore causing reduction of its headroom.

Converting a pentode to operate in triode mode, or overall just controlling the (reduction of) feedback via screen, can be used to decrease headroom of the device.

Filament voltage reduction is getting to be a rather forgotten method but it can effectively decrease headroom of a tube.

Final, and most simple, method is just attenuating the signal that drives the loudspeaker.

All of these can be made to have continuous or stepped control, or even some interacting dynamics with, say, envelope of the audio signal, though latter practice is most commonly exploited in schemes that modulate either plate supply or screen voltages.

I'm not familiar with 6505 Mini or MT15 but I doubt they invented anything original.
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