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Friday, December 14, 2012

The role of rectifiers

The job of a rectifier is to convert alternating current (AC) to pulsating direct current (DC). The filter capacitors that follow it in the circuit remove the pulsations, and provide a smooth DC power source. It seems pretty basic, yet how this is done has an effect on tone.

There are two types of rectifiers: tubes and solid state diodes. In the early vacuum tube days, solid state diodes weren't powerful enough or readily available, so rectifier tubes were used. In the 1960's, solid state technology had advanced enough where silicon diodes could be used. Sometimes several would be strung together to get the voltage rating needed, but now that's no longer the case.

One difference between the two devices is that a vacuum tube rectifier has an internal resistance (which is different depending on the type of rectifier tube used). Another is that tubes are sensitive to the amount of capacitance that is used to filter the pulsations out of the DC, and have a limit to the amount that can be used.

The way these two factors affect tone shows up as:
  • sag or compression in volume when playing loud, and
  • the amount of bass the amp can reproduce.

Both loud volume and/or bass frequencies pull more current through the power supply. Current through the resistance in a rectifier tube produces a voltage drop at the tube (Ohm's law), so the power supply voltage sags below what it is when the amp is idling. If the capacitors following the rectifier tube can't make this voltage up, the B+ voltage dips and the volume goes down slightly. When you're playing loud, or hit a loud note, this sounds like a little bit of compression. Or if there's a lot of bass, the tone changes because the bass is amplified less than the other frequencies.

Diodes don't do this. The voltage drop across a diode is constant and very small in comparison to a tube. They also don't care how much capacitance is at their output, so larger capacitors are often used. This makes an amp play "tighter." Metal players usually prefer diode rectification for its tightness, while many blues players like the looser tube rectification because the compression makes the amp sound like it is singing.

Diodes are cheaper and will last longer then rectifier tubes, but amps still use rectifier tubes for tonal purposes. Most of the time the rectifier type is built into the amp design, but some amps (Mesa) allow switching between tube and solid state rectification. There's also a mod that uses a diode to isolate the preamp and power amp capacitors. This allows the preamp to stay tight, since it stops the power amp's filter capacitors from drawing on the stored voltage in the preamp capacitors when the amp is working hard. It's also possible to build  or buy a rectifier tube socket with solid state diodes wired into it to change the rectification method. (You need to be careful here as this also raises the B+ voltage slightly and may stress aging filter capacitors.) And there are even some solid state rectifiers that have a resistance built into them so you get solid state longevity with tube-like tone.

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