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Question for Gurus - tube amp headroom

Discussion in 'Amp Input - Normal or Bright' started by bvdd, Dec 2, 2017.

  1. bvdd

    bvdd Strat-Talk Member

    54
    May 12, 2009
    New Hampshire USA
    I've seen this mentioned but not explained.

    15w 1x12 combo tube amp - 3 AX7's - 2 EL84's - 1 channel - Gain and Master controls.

    I'm running a clean boost pedal in the fx loop.

    Question - if the master volume is full up, does that automatically mean the
    power amp's/tube's headroom is maxed?

    Or is that not necessarily the case?
     

  2. stevierayfan91

    stevierayfan91 Dog Whisperer. Strat-Talk Supporter

    Feb 20, 2011
    Private.
    Hi :)

    generally to figure out at around which point the tubes will distort, the speaker efficiency rating is interactive with whether using lower or higher gain power tubes.
     
    Silvercrow likes this.

  3. fezz parka

    fezz parka The Wiggler of Sticks Strat-Talk Supporter

    Clipping can occur anywhere in the signal path: preamp, driver, power tubes, even smallish OT's can saturate. Then there's the speaker efficiency srvfan mentions above. Headroom (clean signal, no clipping) is a combination of a preamp that's clean, a driver that's clean, power section with enough watts to deliver the clean signal to an OT with big iron that won't saturate that drives a highly efficient speaker. In other words, a Twin Reverb.:D
     

  4. Tone Guru

    Tone Guru Senior Stratmaster

    Dec 13, 2011
    Music City TN
    Power stage headroom is affected by worn out tubes, a small output transformer, low or sagging B+ (rectifier), line voltage or load matching.
    A master volume reduces the headroom of the preceding stages.
    The phase inverter is usually after the master and considered part of the power section and therefore affects headroom.

    The big variable here is any active send or recovery stages and the device connected to the loop.
     

  5. Duotone

    Duotone Senior Stratmaster

    Feb 12, 2016
    Norway
    In most cases Master volume on full gives you the equalent of an amp without a Master dial.
    Mastervolume was put in to have a way to attenuate the preamp output before the power section.
     
    Last edited: Dec 3, 2017

  6. bvdd

    bvdd Strat-Talk Member

    54
    May 12, 2009
    New Hampshire USA
    Ok, so if the master volume is full up meaning the power amp/tubes is at
    maximum clean headroom, then turning up the boost pedal in the loop should only add more distortion, not clean volume - same as turning up
    the gain knob.

    Correct?
     

  7. johnnymg

    johnnymg Senior Stratmaster

    Sep 5, 2015
    Central Coast Ca
    Tube amps (generally) get most of their 'crunchiness' from the preamp section. So, your best clean headroom will generally be had when the master volume is maxed out and then adjust the 'gain' pot to taste/volume.
     

  8. bvdd

    bvdd Strat-Talk Member

    54
    May 12, 2009
    New Hampshire USA
    Got it. So here's a related preamp tube question:

    I'm running my master volume on full and the gain volume on 0-3.
    In this amp, there's a nice clean guitar sound when the gain knob is at zero.

    I replaced the 12AX7 (gain 100) in V1 with a 12AT7 (gain 60). The result is being able to turn the gain knob up higher before noticeable distortion.

    Question:
    With the gain knob so low, does that mean the 12AT7 is still sending distorted gain to the power tubes but it's just not noticeable yet, or is it sending 'CLEAN' gain until the knob reaches a certain level?
     

  9. candyapple1964

    candyapple1964 Senior Stratmaster

    Mar 9, 2014
    Australia
    You have it around the wrong way.

    Master on full. Set pre amp volume clean and loud but not 'maximum' volume level. It means there is clean headroom remaining. Turn on a boost pedal and you should get a boost in loudness and may a little bit of crunch.

    Set pre amp to full and adjust the master to the level you need. Typical modern amps will jave a compressed sustaining sounf and lots of distortion. Turn on the boost pedal and typically you get more sustain more distortion and not much loudness increase.


    Something that can complicate things is that some amps have a gain and a clean.channel. whe r e the gain channel has both a drive and a level control. Almost like a distortion or overdrive pedal built in.

    The basic rule of thumb stands though. The cleaner you run it the less compressed everything is and the more loudness boost you get from a clean boost pedal.
     

  10. candyapple1964

    candyapple1964 Senior Stratmaster

    Mar 9, 2014
    Australia
    The at7 is lower gain tube. End.
     
    Ebidis likes this.

  11. bvdd

    bvdd Strat-Talk Member

    54
    May 12, 2009
    New Hampshire USA
    Yes, I know - 60 vs. 100.
    I know that gain high and master low = more distortion/saturation and that
    gain low and master high = cleaner/less distortion.

    Not trying to be a PITA but I just want to be clear.

    What I'm asking is - when the 12AT7 gain is on say '1' is it STILL saturating the power tubes but I just don't hear it as soon as I do with the 12AX7?
     

  12. papergoblin

    papergoblin Strat-Talker

    Age:
    36
    323
    Sep 21, 2017
    Alabama

    Different tubes sound different and can give more or less headroom. You can have two of the same tubes, even same brand and each can have a different tone.
     

  13. bvdd

    bvdd Strat-Talk Member

    54
    May 12, 2009
    New Hampshire USA
    I appreciate your effort but I must not be asking my question clearly enough.

    Thanks anyway.
     

  14. Ebidis

    Ebidis Providing the world with flat bends since 1985 Strat-Talk Supporter

    Age:
    51
    Nov 14, 2013
    Alabama
    No it is not saturating the power tubes. If you hear clean tone, that means no saturation is happening yet. AS you turn it up, you should get to a point where you start to hear breakup. That is the end of your clean headroom. Anything beyond that threshold will be more break up until total saturation. The AT7 should give you more clean headroom than an AX7, but less max volume and gain.
     

  15. bvdd

    bvdd Strat-Talk Member

    54
    May 12, 2009
    New Hampshire USA
    Excellent! Thank you!
     
    Ebidis likes this.

  16. candyapple1964

    candyapple1964 Senior Stratmaster

    Mar 9, 2014
    Australia
    What gets power amp distortion is pure volume. Master full and drive it loud.

    Modern ampswitha compressing drive channel and a master volume we rarely if ever experience power amp distortion. So much so its a factor I dont think is worth considering.

    Using a lower gain at7 in preamp while continuing to control overall loudness with master - all you acheive is less gain on preamp and pretty much lower drive and distortion. The effect on power amp side is nothing.
     
    Ebidis likes this.