Vox AC15 master volume question

Discussion in 'Glowing Bottle Tube Amp Forum' started by marcflores, Jul 8, 2019.

  1. marcflores

    marcflores Tele-Meister

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    Is the Vox AC15's master volume placed in the signal chain of the amp so that when it's completely cranked, it's almost like it's out of the chain entirely? I'm not sure if I'm asking that correctly. I've seen people say the same of the Blues Jr. So I'm wondering if it applies to most master volume amps (I've read it has to do where the MV is in relation to the phase inverters).

    Also, if cranking the MV to "get it out of the way" makes it more like a non-MV amp, when you turn up the volume knob to break up, is that break up more like the break up of a non-MV amp?

    Basically, when I turn my DR to about 5.5-6, it breaks up. Is the same effect happening when I turn the Vox MV all the way up then turn the volume up to when it starts to break?
     
  2. MuddyWolf

    MuddyWolf Tele-Meister

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    Ac15 mv is pre phase tube and mv is always in the circuit..
     
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  3. Jerry_Mountains

    Jerry_Mountains Tele-Holic

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    It's always in the circuit, I setup mine as you have said, cranking the MV and working with the chanel volume and it's the best tone that you could get from this amp, but it's very difficult to crank both I have achieved this only once in an outdoor venue and the tone was heavenly creamy but controllable with my guitar volume.

    But I suppose there's always a loss in the signal. I don't know if there is a mod or something to take out the master volume from the circuit, seems difficult.
     
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  4. Wayne Alexander

    Wayne Alexander Friend of Leo's Silver Supporter

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    To answer the OP's actual questions - yes, when you turn up the AC15 master volume all the way, it has very little effect on the signal. (Technically, it's a post phase inverter crossline master volume circuit, which uses phase cancellation between the two sides of the signal to reduce volume. When the pot is turned all the way up there is little to no phase cancellation thus very little volume reduction or tone/behavior change from a non-master volume amp).

    The behavior of the amp with the master volume all the way up is the same as the behavior of a non-master volume amp; the higher you turn up the volume control, the louder the sound and the dirtier the sound.
     
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  5. Snfoilhat

    Snfoilhat Tele-Afflicted

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  6. marcflores

    marcflores Tele-Meister

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    Thanks for responding. I thought that's what I read here where it says:

    "The Master Volume control was also located between the phase inverter tube and the output tubes."

    Source: http://www.voxshowroom.com/uk/amp/ac15c1_hood.html

    I assumed that meant it was post phase as you said since the output or power tubes seem to come immediately after. I just wasn't completely sure.
     
  7. MuddyWolf

    MuddyWolf Tele-Meister

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    My bad. It sounded like a preMV to me.
     
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  8. gusfinley

    gusfinley Tele-Afflicted

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    First of all, it matters which 'AC15' you are talking about.

    The 'AC15s' of the past 15 years or so have had master volumes placed just before the inputs to the power tubes. So, Yes, Post-phase-inverter-master volume.

    The Master Volume works much like the cut control, which mixes out-of-phase frequencies in order to 'cut' them from the signal. The Cut control changes the crossover frequency as you rotate it.

    The master volume doesn't filter certain frequencies and has the effect of cutting the signal level going to the power tubes. This is a cheap and simple way to add a master volume to an amp, but you phase inverter needs to be pretty balanced to do it.
     
  9. marcflores

    marcflores Tele-Meister

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    The newer AC15 models.
     
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