Tube Amp Question

Discussion in 'Guitar Gear Talk Forum' started by bcrich, May 5, 2011.

  1. bcrich

    bcrich New Member

    I couldn't find a definite answer on any website about this but, can anyone tell me:

    How does the number of vacuum tubes affect the tone/output/gain? Like 2xECC83,
    it means two ECC83 tubes, but what is the purpose of increasing/decreasing the number of tubes in an amp?
     
  2. rickkkyrich

    rickkkyrich Guest

    I think it will effect the output. More number of tubes may lead to high gain output..
    i'm just guessing... let flood come in for the exact answer..
     
  3. flood

    flood New Member

    ok, short question with a very long answer, which i hate. gonna try and avoid that. here goes:

    to think of a vacuum tube purely as an amplifier is wrong - it is essentially a functional component.

    the 12AX7/ECC83 is the most ubiquitous tube in the guitar amplification world. it is called a dual triode, because it encloses two triodes in one glass envelope. for information on triodes and how tubes work... look up wikipedia and NEETS module 6.

    a 12AX7 can be used in a number of ways in a guitar amp:

    1. gain stages - as an voltage amplifier, basically, whose gain and frequency characteristics can be controlled by the component values around it. small voltage in, larger voltage out.
    2. cathode follower - impedance conversion to drive a tone stack - the typical marshall/fender/vox passive EQ section, which is used in practically every amp out there with a few exceptions, introduces a loss of about 10dB in the signal path. this loss is greater if the source is high impedance. very often (but not always), this EQ is fed from a cathode follower section, usually using up an entire 12AX7 tube, with one half as a gain stage and the other as the "real" cathode follower.
    3. as a phase inverter to drive the power tubes in a push-pull amp- one half of a 12AX7 amplifies the incoming signal, the other half amplifies the INVERTED signal to drive the power tubes.

    there are other uses and topologies - the cascode/totem pole amp, cathodyne PIs etc. but these are fairly rare nowadays so i won't get into that.

    a more common use is using an entire 12AX7 for a tube-buffered effects loop.

    now this is where things get trickier to explain...

    let's say the amp has 5 12AX7 and 2 EL84 tubes, boasts 18W and a tube buffered effects loop.

    working backwards:
    1 12AX7 is the phase inverter
    1 12AX7 is the effects loop, cathode follower + makeup gain stage
    1 12AX7 is the clean channel
    2 12AX7s for the dirty channel (3 gain stages + cathode follower)

    that gives us 5 preamp tubes.

    this is an oversimplification for sake of example though... it gets messier:

    usually a high gain channel of an amp, e.g. the bogner ecstasy uses about 5 triodes or 2.5 12AX7 sections. sometimes, multichannel amps have a dedicated mixer stage based around a 12AX7.

    the trainwreck express uses 3 gain stages but gets shitloads of gain on full bore.

    one misconception i will clear is that slamming one tube after another will not give you a good high gain sound - high-gain is tricky. you need to design one stage after the other, not just paying attention to the amplification but, surprise surprise, the attenuation. that's right - after amplifying the signal you need to CUT it for those kvlt chugga-chugga-chugga sounds. and that's not all. your choice of capacitors goes a long way, because you need to tailor the frequencies too. so you're shaping the tone that way.

    even in a fairly simple amp with one or two gain stages with a lot of gain (the marshall 1974x comes to mind) the choice of coupling capacitors goes a long way. if you allow too much bass to go through, you will have farty overdrive.

    the underlying principle is that you can't have infinite gain in practice. so more tubes doesn't necessarily mean more power. the maximum voltage on a 12AX7 is, in practice, about 250-300V in the later stages of an amp.

    gain is multiplicative - so with a 100mV signal, you would theoretically see an output voltage of about 5-10V peak-to-peak after the first stage, 400-800V after the second stage, 3200V + after the third.... but in practice, this is not happening cause the second stage has a maximum voltage of around 150V! this is where the tube gets saturated and clipping starts to occur. that's all there is to distortion. then there's frequency-dependent distortion etc. etc. which we won't get around to discussing....

    for the actual power of the amp, the most critical factors i would name would be:
    1. the design of the phase inverter
    2. the biasing of the power tubes
    3. the plate and screen voltage seen by the power tubes.
    4. the efficiency of the output transformer and speaker.

    that's as far as i'll go today. happy to have confused you further :)
     
  4. bcrich

    bcrich New Member

    Lol!

    A simpler explanation required please..
     
  5. flood

    flood New Member

    hahaha. that's as simple as i can make it! the short answer to your question would be "yes but no" which in itself is great phail.
     
  6. thehundredthone

    thehundredthone New Member

    What's so hard to understand? Fine, here's a simpler but slightly flawed explanation.

    For more output (wattage), you need more tubes. But more tubes don't necessarily mean more output, because they are used for a variety of purposes.
     
  7. flood

    flood New Member

    no.

    you do NOT need more 12AX7 tubes for more output. you can drive a power stage to full output (at lower THD) with one or two 12AX7s. hi-fi amps don't have a lot of tubes.

    the word "gain" itself is a bit misleading too.

    what you need multiple preamp tubes for is for multiple stages and to "sculpt" your distortion, drive EQ sections, as mixers, etc.

    you need more POWER tubes for more power, obviously. EL84, EL34, 6L6, 6V6, KT88, KT66, whatever.
     
  8. flood

    flood New Member

    no.

    you do NOT need more 12AX7 tubes for more output. you can drive a power stage to full output (at lower THD) with one or two 12AX7s. hi-fi amps don't have a lot of tubes.

    the word "gain" itself is a bit misleading too.

    what you need multiple preamp tubes for is for multiple stages and to "sculpt" your distortion, drive EQ sections, as mixers, etc.

    you need more POWER tubes for more power, obviously. EL84, EL34, 6L6, 6V6, KT88, KT66, whatever.
     
  9. thehundredthone

    thehundredthone New Member

    I didn't say more preamp tubes = more power. Perhaps I should've said more power tubes, but I thought that was implied.
     
  10. bcrich

    bcrich New Member

    That's what I was asking about.. Thanks flood and thehundredthone!

    I'll save the first post of flood on this thread for future reference.. thanks!
     

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