Stupid question: "What is Gain?"

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bscepter, Aug 6, 2005.


  1. bscepter

    bscepter Member

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    Now, for years, I've used the word "gain" interchangeably with "volume," especially as it relates to the studio, e.g. "turn up the gain pot on the board."

    Now, as far as vacuum tubes are concerned, what exactly does "gain" mean? I keep hearing about "high-gain" 12AX7s, for instance. Does it mean they put out more power, making the amp louder? Or do they simply break up sooner, resulting in earlier overdrive?

    Help!
     
  2. drbob1

    drbob1 Silver Supporting Member

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    Ah, the fuzziness of the english language (especially when it's being practiced by a mixed bag of engineers from both sides of the atlantic)...

    Gain originally meant how much the input voltage to an amplifier or gain stage was increased in the output. So, for example, a 12AX7 has a gain factor of 100 times (although in a circuit it's not nearly that neat) so a 1 mV signal going in should come out as a 100 mV signal. That's the technical definitition. Amplifiers as a whole don't usually have a gain assigned to them, although you may find power amps labelled such that a 1 mV AC input will generate a 5v AC output (like Peavey CS800s-these numbers are BS examples BTW).

    In guitar amps, gain is often used to designate sustain or distortion-the assumption being that a stage with very high amplification or gain will overdrive the next stage into distortion. I'd say this is more of a marketing term than a strictly accurate engineering term, but it's been around so long no one argues about it. A high gain amp won't be louder than a low gain amp, but will have several stages of amplification overdriving the next stage to attain high levels of distortion and or sustain.
     
  3. bscepter

    bscepter Member

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    Thanks for the info.

    So... if I want the same volume out of my amp, but with less distortion, do I want a lower gain tube? For instance, do I want a 12AT7 instead of a 12AX7 in V1?
     
  4. TheAmpNerd

    TheAmpNerd Member

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    That was pretty good doc.

    bs, if you don't want so much "gain" you can try
    the following:

    amp
    tube - factor (approx)

    12AX7 - 100
    5751 - 70
    12AT7 - 50
    12AY7 - 40
    12AU7 - 20

    Some may sound better than others.


    Good luck and have fun.
     
  5. bscepter

    bscepter Member

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    Thanks, guys. I think I may try my Mullard CV4024 (12AT7) in the Top Boost position in my Vox.
     
  6. Blue Strat

    Blue Strat Member

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    The definition hasn't changed. However, guitar players now use "gain" to mean "overdrive/distortion", probably because "high gain" amps produce distortion for the reasons cited in drbob's original post.
     
  7. Old Fuzzface

    Old Fuzzface Member

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    As already mentioned, gain is the multiplier that an amp puts onto it's input signal to generate a larger output signal.

    This multiplier can be varied, and that's where you find a gain control. It should really be called a "variable gain" control. For a fixed input signal, the gain control will up the multiplier of the amp to give a larger magnitude of output signal.

    If the amp's gain is fixed at a constant value, you can still control the magnitude of the output by installing a volume control to decrease the size of the signal going into the amp. So the volume control should really be called "input attenuation".
     
  8. spentron

    spentron Member

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    Gain can control distortion or volume and usually ranges through both. But once you add another control that primarily only controls volume, this isolates the usefulness of a gain control towards distortion control.

    BUT, most gain or first volume controls also have a bright capacitor across them, so should actually be labelled Gain/Bright or actually "Gain OR Bright" since bright reduces with increasing gain. That's probably the biggest reason for swapping to lower gain tubes instead of simply turning down a knob, although different tubes -- and in different positions -- will of course also produce differences directly also.
     
  9. Wakarusa

    Wakarusa Member

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    A couple of posts refer to the amplification factor of several tubes and one uses the rating interchangeably with the term gain.

    That's not really an accurate way to represent voltage gain in the stage which, although influenced by the tube's amplification factor, is more a function of plate voltage, plate resistance, driven load, etc. Strictly speaking, gain in a stage is voltage out divided by voltage in (less any DC offsets).

    Your typical guitar amp 12AX7 stage with 100K plate load and 1.5K cathode resistor bypassed by a 20uF (or more) cap has an actual voltage gain significantly lower than 100.
     

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