Guitar amp tube definitions, functions...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tedjac, Jan 16, 2006.

  1. tedjac

    tedjac Guest

    Hi all,

    I'm new to the tech side of amps and I'm hoping that you can educate me or tell where to turn to get that education. I know that there are preamp tubes, and power tubes, etc... but what I don't know is how they all work together and which tube does what with what effect on the amp sound (tone, amplitude, etc.).

    For example, there are four 12ax7 type tubes in my future amp (Fuchs ODS 30 SLX). What does each one do? There are four 6V6 power tubes... how do they work together and effect each other and the preamp? I'm not really interested in the actual deep electronics theory, but more interested in a practical understanding of what makes a guitar amp work and how the components come together to do what they do. What tube characteristics would be best in which tube socket to get a particular end result?

    Do these questions make any sense?

    Thanks for any and all help with this...

  2. Chiba

    Chiba Gold Supporting Member

    Feb 18, 2002
    State of Denial
    Go - right now - and get Dave Hunter's new book, The Guitar Amp Handbook. Even a sketchy first reading will answer some of your above questions. It hasn't been out long, and people may suggest a myriad of other books, but if you want a basic, easy to follow understanding of how the different parts of a tube amp interact, this is your book.

    To partially answer your question ... your preamp tubes are used to provide gain; they make the tiny little signal coming from your guitar louder. They also do 'things' to it, like drive a tremolo/vibrato or reverb circuit, not to mention adding that thick, rich distortion tone we're always praising tube amps for.

    Usually the last 12An7 tube before the power tubes is the phase inverter, which kind of drives the power section.

    Of course, power tubes take the preamp's signal and make it even louder, occasionally (and sometimes even purposely) adding even more distortion to the sound.

    The cool thing about the twin triodes (12An7 types) is that since they're basically two tubes in 1 housing, you can do all sorts of neat things with them.

    That type of tube is *generally* interchangeable, so you can also fool around with how much gain your amp creates and where it happens by subbing in anything for the 12AX7. Try some AT7, AU7, and AY7 tubes in there :)

  3. tedjac

    tedjac Guest

    Thanks Chiba... I ordered the book today...

    Anyone else want to share some hard earned wisdom on this subject??
  4. drbob1

    drbob1 Silver Supporting Member

    Aug 9, 2004
    Guitar signal-->first gain stage (this is the first part of the first preamp tube and gives much of the overall tone and gain of the amp-put your best tube in here)-->more gain stages-->EQ-->gain recovery for the EQ (loses volume in the EQ stage)-->more gain?-->phase inverter (this tube splits the sine wave of the signal into + and - halves and sends each 1/2 to the right or left side of the "push-pull" output tube pairs)-->6V6 power tubes (these amplify the signal much more and send it on to the output transformer)-->OT (which reasembles the sine waves and sends them to the speaker).

    Tubes are one way devices, they can't amplify a -voltage without some fancy footwork (like setting them up so that they have a steady output that the sine wave drives more or less negative), so you can get more power with a pair (or two pair) of tubes, each driving part of the sine wave thru the OT to the speakers...
  5. fabiomayo

    fabiomayo Member

    Jun 14, 2005
    Niteroi - RJ, Brazil
    I'm yet to order some books on the subject. Meanwhile this helped a lot understanding the basics of how an amp works...

    Some here may disagree with Randall Smith's politics and concepts but this text is a pleasure to read and very informative for the dummies like me....

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