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Another use for electronic tubes (or valves as our British friends call them) is after the pre-amp stage of a guitar amplifier. Rectifiers are tubes (or solid state transistors) that change AC current to DC current that the power section needs to amplify the signal from the pre-amp. In the process of doing this, they add their own signature sound which is called rectifier “sag”. It’s commonly described as a slight (in the case of a solid state rectifier) to pronounced spongy feel with natural compression and sustain. This is a result of the rectifier experiencing a voltage drop as soon as the signal from the pre-amp hits it. After that, the tube recovers and gets up to full voltage. The dynamics of the player will also effect the rectifier’s response to the signal from the pre-amp. The harder you hit a chord or note, the more compressed it will be and as such have more harmonics.
Here are some sound samples
1) Strat DI’ed
2) Roland JC-120—widely regarded as the “cleanest” amp. It has a solid state rectifier
3) Fender Twin Reverb—The cleanest tube amp you’ll find. All the Fenders over 100 watts have solid state rectifiers. The Twin only put out 22 watts, thank goodness
4) Matchless—Hand-built boutique amp with tons of character. This is with the gain turned all the way down
5) Matchless Again– to demonstrate the dynamic response of the pre-amp and rectifier tubes, I played the first part with the volume on the guitar at about 6 or so and then turned it all the way up for the second part.
6) Marshall JMP—again to demonstrate the dynamic response first with the Strat’s volume at 6 and then at 10
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