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The earliest guitar amps used a vacuum tube diode (a “valve rectifier”). From the late ‘50s
onwards, amp designers began using more efficient, more stable silicon diodes instead (a
“solid-state rectifier”). While the type of rectifier used has no direct effect on the
fundamental clean tone of the amp, it does impact on dynamics and playing feel as soon as
things are cranked.
Specifically, valve rectifiers can struggle to keep up with the demand for power created by
sudden, loud notes. Referred to as rectifier “sag”, this phenomenon has the effect of
slightly compressing the signal, smoothing out the initial note attack often in a pleasing
way. Though silicon rectifier equipped amps “sag” too, the effect is much less pronounced,
leading to less compression and a firmer, less spongey feel that some players prefer.
The RevivalDRIVE makes both of these flavours available in a very subtle and realistic
way. The VALVE RECTIFIER channel offers the characteristic valve sag effect. The initial
attack of the note is slightly compressed and sustained notes appear to “bloom” as the
rectifier recovers. In contrast, the SILICON RECTIFIER channel delivers more
aggression, less sag and a tighter, more precise and responsive feel overall.
For extra flexibility, you can switch the VALVE RECTIFIER channel to behave like a
second SILICON RECTIFIER channel using the MODE DIP switch on the rear of the
pedal.
RECTIFIERS, SAG AND GHOST TONES
Because both sag and ghosting are generated in the power stage of the amplifier, there is a
degree of overlap and interaction between these two effects. As you would expect, this is
fully reflected in the RevivalDRIVE.
Firstly, the choice of rectifier channel influences the sound of the ghost tones produced.
This is because the waveshape of the rectified power signal is slightly different for each of
them – something close to a smoothed triangle wave in the case of a valve rectifier, and a
sawtooth wave in the case of a silicon diode. You can see these wave shapes represented on
either side of the channel names printed on the pedal’s control panel. Though the effect is
subtle, you should experience harder-sounding ghosting characteristics when you switch to
the SILICON RECTIFIER channel.
Secondly, adjusting the GHOST control will also change the timing of the sag effect.
Turned fully clockwise, sag is immediate. Like a compressor set with a very fast attack
time, the reduction in level happens so quickly that you may not hear the change occur. By
contrast, with the GHOST control turned fully counter-clockwise, the onset of sag is very
slow. A medium to medium-fast setting offers a realistic real-world sag characteristic.
To understand what is happening here, we need to remember that the GHOST control
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