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Tube amps: Help to understand Master/Non-master and grit/dirt/distortion?

Discussion in 'Amps and Cabs [BG]' started by StuartV, Jan 27, 2012.

  1. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    I've read a lot of posts about, just as an example, old Ampegs (SVT or V-4) with no Master volume versus newer ones that do have a Master Volume.

    And I've read a lot of posts about "tube breakup" and "grit" and "dirt" and "tubeyness" and "clean".

    I THINK I understand what all this means and then every now and then I'll read a post that doesn't make sense to me and calls everything I thought into question.

    So, here's how I think all this stuff works. Can somebody please tell me what parts are wrong?

    Some amps have an input gain adjustment and a master volume (like a new SVT CL) and some only have a channel volume (like an original SVT). On the ones with only a channel volume, that volume is actually adjusting the input gain and the "master volume" is built into the amp internals and runs wide open.

    When people talk about "dirt" or "grit", technically they are really talking about distortion. And in an all-tube amp (or any amp, really), distortion can come from 2 places - the preamp and the power amp.

    So, on a non-Master Volume amp, you have no control over how much distortion you get. When you set your volume level (i.e. the amount of input gain), you will get however much distortion you get from the preamp and the poweramp. If you want more or less, your only options are to use a hotter or less-hot bass or different cabs that have a different amount of sensitivity.

    In contrast, a Gain/Master amp will give you some control over the amount of distortion at a given volume. If you want more distortion, you turn up the Gain and turn down the Master. But, this only gives you distortion that comes from the preamp. Distortion from the power amp is still only going to be increased by turning up the actual volume (not just turning up the volume knob). If you turn up the volume knob, but turn down the gain knob to maintain a certain volume, you will decrease the preamp distortion but you will not increase the power amp distortion because the output power is directly related to the actual output volume, so if you keep the volume the same the output power is the same, so the power amp distortion is the same.

    If an amp has different input sensitivities, then you can use those to increase or decrease distortion, to a degree. The Ampeg V-4 has a 3-way switch for Input Sensitivity that is 0dB, -6dB, or -9dB. If you have it set for -9dB, and you have the volume maxed, and you want more distortion, you could get it by changing the input sensitivity to -6 or 0. But, the utility there is limited because, if you want a lower volume level, increasing the input sensitivity will increase the volume, so you'll have to turn the volume knob down some and end up with the same distortion as before.

    Does all that make sense? If so, here are some questions that I hope somebody can enlighten me on:

    Preamp distortion generally will sound a bit different than power amp distortion, right?

    And when people are talking about messing around their settings to get more or less distortion they're generally just getting more or less preamp distortion, right? Because the power amp distortion is going to only come from running the amp wide open or nearly so. Anything less and the power amp will be running clean with no clipping (i.e. distortion), right?

    When people talk about tube "breakup", that generally is referring to power amp distortion, right? Or is that a general term that would also apply to preamp distortion?

    Can you characterize the difference in the sounds of preamp distortion versus power amp distortion?

    If power amp distortion is particularly desirable, why don't (more) tube amps come with a switch that lets you control how much power they're putting out? Either by controlling how many power tubes are in use or, like the old MusicMan HDs did it (I think) by reducing the plate voltage to the power amp tubes? That way, if you want lower volume, but also power amp distortion, you could switch it down to a lower power setting and get what you want.

    And if you read all that.. thank you! Double thanks if you can shed some light on the subject for me.
  2. RickenBoogie


    Jul 22, 2007
    Dallas, TX
    You are certainly on the right track with your assumptions. Tube breakup can be from either/or, and full on grind is usually a wide open power section. Preamp o/d is more "wooly" and less compressed than power amp o/d, generally, but different amps behave differently, (no surprise), so overall generalizations may not apply to every tube amp circuit.
  3. From my understanding, which is limited, if you keep the master on full and use your gain to adjust the overall output you essentially turn a master volume amp into a non-master amp. Personally I don't like preamp distortion and prefer amps that can reach power distortion with minimal preamp distortion.
  4. mofeezy


    Mar 30, 2011
  5. Your other option on a non-master amp is to turn your instrument volume down (If you don't want to switch basses). I do this occasionally for recording purposes with my SVT if it gets too furry sounding for the song.
  6. oinkbanana


    Oct 20, 2011
    logically this seems true
    but in practice it isn't.

    i know an SVT-CL with the master gain maxed still doesn't sound like an SVT-VR.

    i wonder if an amp with a master volume bypassed, sounds better then just having it dimed. any amp techs wanna chime in?

    edit to correct my topo.
  7. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    That would depend on what the Master knob is actually doing in the circuit. If it's just a passive attenuator between the preamp and power amp stages, then there should be no noticeable difference between having the pot "wide open" vs bypassing the pot. If the Master is controlling the actual gain stages, or something similar, then the difference is not so clear-cut and not likely to be the same between different amp models.
  8. I'd say you are hearing things, as the CL and AV are the same amp with different cosmetics.
  9. I'm guessing he meant VR not AV.
  10. oinkbanana


    Oct 20, 2011
    bassboysam is correct.
    av and CL are the same amps.
  11. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I don't know that I buy that 100%, but in all honesty, I've yet to try it. That sounds to me like one of those things that should work out on paper but something along the way changes.
  12. Then there's the Mesa D-180 master, which works as a current controller on the loudness while keeping all the gain. I'm no amp tech but it sounds like MusicMan might have inspired that system.
  13. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    I assume you meant a CL and a VR don't sound the same with the CL Master wide open.

    But, that may be because I think I've read that the CL is not really the same amp circuit, etc, at just a VR with Master Volume added.
  14. Korladis

    Korladis Banned Supporting Member

    Yeah, it's definitely an interesting configuration. Not one I've really seen on anything else.
  15. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    in every amp that i've worked on, diming the master volume control sounded very close, but didn't sound exactly the same as if i remove it out of the circuit completely.

    as far as the CL/AV verses a VR, IMO, they're different animals, mostly in the front end (i.e. preamp).

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