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Preamp vs Phase Inverter vs Reverb vs Power vs Rectifier.

Many people get really, really confused when they try to understand anything about tubes. I spent a very long time myself trying to understand it enough to be able to perform basic maintenance and tweak the tone of my amplifier. So, the question for the community would be this:

What do the different tubes in an amplifier do? How does each tube affect the amplifiers tone?

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2 Answers 2

up vote 8 down vote accepted

The pre-amp valves do exactly that; they pre-amplify the signal from you guitar to a level high enough to be consumed by the power valves; tone adjustable's such as EQ, pre-amp gain and presence are all part of the pre-amp section, this is all done with analogue circuitry and the pre-amp valves amplify and apply gain to the result of you dialling your tone in.

The power amp valves then take this signal and 'really' amplify it; usually the more driven the amp (the louder you crank it), the more the power amp valves affect the tone, generally the more you crank the more break up/distortion/wideness of sound you get from the amp; the choice of power amp valve has a huge effect on this.

The rectifiers takes alternate current(AC) and convert it to the direct current(DC) required by the valves.

Check out this article The Secret life of valves

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And phase-inverters flip the signal 180 degrees, so when the signal is at its peak coming in, it's at its bottom exiting the circuit. This is useful for providing negative-feedback to the circuit, helping tighten the sound. –  Anonymous Jan 16 '11 at 21:54

DRL's answer is a good summary of pre-amp/power-amp/rectifier tubes. Here are some specific examples of tubes used and how they affect tone.

For the pre-amp, far and away the most popular tubes used are 12AX7's, although sometimes you see 12AT7's used to drive the reverb or effects loop (if the amp has these features).

For the power amp section, the tubes used fall into two camps:

  • 6V6 & 6L6: Fender is known for using 6V6's in their smaller amps and 6L6's in their larger amps. Either way, these tubes have a lot of clean headroom, so you get more volume from the amp before they start to distort. When they do break up, however, the sound can sometimes be a bit harsh.
  • EL34 & EL84: Marshall is known for using EL34's and EL84's in their heads. These have less clean headroom, so they'll distort at lower volumes, but the distortion is usually warmer and smoother.

This explains why, historically speaking, Fender amps were/are popular with country/western guitarists who wanted a cleaner tone, while Marshall heads were/are often used by rock musicians wanting a great distorted tone (as with all generalizations, of course, there are exceptions).

Another factor involving tubes and tone include whether the power amp uses a Class A or Class A/B circuit design. A Class A design is typically less efficient and thus produces less volume, but with (subjectively, of course) better tone. Vox amps and smaller Fender tube amps often are Class A. Class A/B amps get more volume due to greater efficiency, but their sound isn't quite as sweet. Larger Fender amps and Marshall amps are usually Class A/B.

Finally, a hybrid amp will typically use tubes for the pre-amp section, but a solid-state power amp. The idea here is to use tubes for the portion of the amp that affects the tone the most, and then save money and weight in the power amp section, which is designed to be as clean as possible. Of course, these amps can't provide the really wonderful power-amp distortion of a great all-tube head set to 11, but that's the trade-off.

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Just reading this makes me want to go and turn my (1970 Marshall) right up and hear all the stuff you've mentioned dripping out of the speaker. Great info ! –  user2808054 Jun 4 at 11:55

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