Educate me please.... MV vs. attenuation

Discussion in 'Amps and Cabs' started by chrisjw5, Jan 30, 2006.

  1. chrisjw5

    chrisjw5 Supporting Member

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    I posted a request for help in finding something that would get "the sound" I hear in my head at less than "honey call the police" volumes. I thought I needed a Master Volume amp.

    I got some great responses here and at the BaM board, but some people telling me that a MV isn't the way to go. I guess I just don't understand the concept the way I do. I thought a MV would let me push the power tubes harder at a lower speaker volume. And I thought an attenuator did the same thing to a non-MV amp.

    I know you'll never get the same sound at 105 db that you will at more neighbor-friendly levels, but I'm guessing I'm missing something. I thought MV and attenuation were close to the same thing, and were a "90% solution" to getting that saturated power tube tone.

    Can someone explain to a non-physics major? Many thanks in advance. This is my first time really getting into researching amps, and I'm liking it, just a bit confused.
     
  2. trisonic

    trisonic Member

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    Christopher,

    MV's usually act on the output of the preamp section before the signal hits the power tubes. On the other hand attenuaters affect the whole amp (incl. the power tubes) by limiting the signal to the speakers.

    That's the simplest way I know of putting it.

    Best, Pete.
     
  3. electronpirate

    electronpirate Member

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    trisonic is right.

    Some amp makers do better than others with this. A good master volume *can* be better than an attenuator. I use both, since with the MV up, I get a good mix of the preamp and power tube distortions. Thus I don't have to add as much 12ax7 preamp gain to the tone. I have one of the best MV's out there, but still I get a better mix when I have it up, and attenuated.

    This is all VERY dependent upon the amp, and the attenuator.
     
  4. rockstah

    rockstah Member

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    come over to www.metroamp.com forums - lots of food for thought there.
    Mark
     
  5. Softpaw

    Softpaw Member

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    A master volume let's you hit the *preamp tubes* harder, and NOT the power tubes. An attenuator let's you hit the *power tubes* harder and NOT the speakers.

    Guitar signal --->> preamp tubes (-->MV lowers signal to)--->>power tubes--->>output transformer(-->power attenuator lowers current to)-->>speakers

    That's a simplified version of things, of course.

    One key thing to realize is that the preamp section increases the voltage of the signal (and shapes it etc), The power tube/transformer section takes the weak signal from the preamp and creates enough current to drive the speakers speakers. That's why it only takes a relatively small resistance to make a level control for the output of the preamp, while it takes a BIG unit of some sort to cut down the *current* coming out of the transformer heading to the speakers. The current creates a lot of heat in an power attenuator, hence the name "hotplate" for example.

    Master Volume Control between Preamp and Power Section


    Now if you put a volume control after the preamp, you can drive the preamp tubes hard, get distortion there, then cut down that signal as it leaves the preamp so it does NOT hit the power tubes too hard and therefore create too much volume.

    signal--->preamp tubes distortion/compression--> volume control-->power tubes.

    The preamp tube distortion can come from 1 to 4 or more "stages". The more stages, generally the more compression and smoothness of the distortion. High gain amps have more preamp tube stages ("gain stages") than low or medium gain amps. Each stage adds a bit more distortion and no one tube is driven to extreme nastiness.

    You can also use a "clean boost" or overdrive pedal to hit the first preamp tube harder to get more distortion from that first preamp stage.

    signal--> increased by OD->> preamp tube hit harder/distorts more-- etc.

    compare

    signal---stage 1 preamp tube --> stage 2 (add a bit distortion)-- stage 3 (add more distortion) etc.


    You can also think of a distortion pedal as having it's own master volume:

    Guitar signal ---> pedal distortion--> pedal MV---> amplifier preamp section

    It gets complicated then by where and how various eq ("tone stacks") are placed in the preamp circuit.

    Go here for some general info on various classic tube amp preamp topographies (Fender vs Marshall vs Boogie etc.)
    http://www.harmony-central.com/Guitar/guitar-amp-evolution.html

    PREAMP STYLES:

    1. in----stage1--------volume----stage2----EQ------------------->
    2. in----stage1---EQ---volume----stage2------------------------->
    3. in----stage1---EQ---volume----stage2--RESISTOR--stage3------->
    4.in----stage1---EQ---volume----stage2------------stage3----stage4---------------------------master volume--->
    5.in----stage1---EQ---volume----stage2------------stage3----stage4----stage5-----------------master volume--->
    6.in----stage1--------volume----stage2------------stage3--------------------------------EQ---master volume--->
    7.in----stage1---EQ---gain------stage2---DIODES---volume----stage3----loop---Phase Inverter--master volume--->
    8.in----stage1--------volume----stage2---DIODES---stage3--------------------------------EQ---master volume--->
    9.in----stage1--------volume----stage2------------stage3----stage4----------------------EQ---master volume--->
    10.in----stage1--------volume----stage2------------stage3----stage4----stage5------------EQ---master volume--->
    11.in----stage1--------volume----stage2------------stage3----stage4----stage5---stage6---EQ---master volume--->

    1. 1950's Fender "tweeds", 1960's Marshalls
    in----G1---------volume----G2----EQ-------------------->

    2. 1960's Fender "blackface" normal channel
    in----G1---EQ---volume----G2--------------------------->

    3. 1+2: 1960's Fender "blackface" reverb channel
    in----G1---EQ---volume----G2--RESISTOR--G3----->

    4. 1970's and 1980's Mesa-Boogie lead channel
    in----G1---EQ---volume----G2--------------G3----G4-----------------------master volume--->
    in----G1---EQ---volume----G2--------------G3----G4----G5----------------master volume--->

    6. 1970's Marshall Master Volume
    in----G1---------volume----G2----------------G3---------------------EQ---master volume--->

    7. Early JCM-800's with diode clipping
    in----G1---EQ---gain---G2---DIODES---volume--G3--loop--Phase Inverter---master volume--->

    8. Later JCM-800's, and JCM-900's with diode clipping (diodes in rectifier circuit)
    in----G1---------volume----G2---DIODES----G3-------------------------EQ---master volume--->

    9. Bogner "FISH" preamp (Brown Channel)
    Marshall 30th Anniversary (Lead Channel)
    in----G1---------volume----G2--------------G3----G4----------------EQ---master volume--->

    10.Soldano lead channel
    in---G1---------volume----G2--------------G3----G4----G5---------EQ---master volume--->

    11.Peavey EVH 5150
    in---G1---------volume----G2--------------G3----G4-----G5---G6---EQ---master volume--->


    No matter how it's arranged, though, the Master Volume is always used to control the level of preamp tube distortion and the level of signal sent to the power tubes.

    Power Attenuator between power tubes/output transformer and speakers

    A Power Attenuator like a THD Hotplate or Dr.Z Airbrake etc comes AFTER the output transformer. It therefore allows you to drive the power tubes harder (and transformer), get distortion and compression from them, then cut down the current level going to the speakers in order to cut down on the final volume.

    Guitar signal--> preamp--> power section distortion/compression--> power attenuator-->speakers.

    Now, the key point here is this: preamp tube distortion/compression sounds and feels differently than power tube/transformer distortion. (And there is also speaker distortion to consider, but that's another topic).

    Why that is so is something I'm really not qualified to explain in technical terms. I will note that unlike the signal's voltage increase in the preamp, the power section has non-linear fluctuations due to electronic (apart from acoustic) FEEDBACK from the speakers to the power section.
    ---------------------------------------------------------------------------------
    Additional comments on master volume circuits:

    Here's a quote from an article by Ken Fischer (Trainwreck)


    "Most people have the idea I'm not in favor of master volume
    controls because I don't use them in Trainwreck amplifiers. The
    fact is there are many sounds that only a master volume will
    provide, particularly for many hard rock, metal and grunge tones.
    The Master Volumes in Marhalls and the Sovtek Midget are good
    examples of this kind type of use. MESA BOOGIE and DUMBLE amplifiers
    offer not only high crunch master sound, but also a very smooth master
    volume tone which is useful for fusion, jazz rock, and other styles.
    There are of course, many other brands of amps that have master volumes
    that give them their special sound. To clear things up a bit, my
    personal favorite sounds come from a non-master volume amplifier
    cranked up. I do really like many master volume sounds and own master volume

    I know of about sixty different amps for my own use. master volume circuits, some much
    better than others. Master Volumes can be installed at any point
    in an amp's circuit, which will influence many factors. Also, you
    can have more than one master volume in an amp. The most common
    master volume circuits are used in the preamp stage. There are
    also post inverter and output stage M.V. circuits...

    The first type...is simple and effective. Depending on the preamp
    design, it gives a good range of popular master volume sounds. This
    circuit uses a single potentiometer at the very end of the preamp,
    but before the phase inverter. It's simple and useful in a multi-channel
    amplifier as it does not cut back the power stages, making it easier
    to switch to a clean sound from another channel. This is the type
    used most by Marshall, Boogie, and Fender. In preamps that end in
    a tone stage it will give lots of crunch and grunge. In a preamp that
    ends in a gain stage it will have a smoother sound. A JCM800 vs a
    MKIIB Boogie, for example....

    Another type of MV is one that is used in the phase inverter
    section. This one is most useful in a single channel amp because
    as you turn it down the amp gets dirty and stays dirty. If you
    need to switch between clean and dirty with the same amplifier
    a simple phase inverter master is not the way to go.

    The phase inverter master volume has its own advantages for certain
    amplifiers. The first advantage is, with a lower gain amplifier,
    you can get the additional gain of the phase inverter as part of your
    total gain structure. Secondly, all the phase inverter master circuits
    have their own unique sound. If you don't like the sound of preamp
    masters, then maybe a phase inverter master will be more to your
    liking...

    Years ago, I invented the phase inverter master I'm about to discuss,
    to address two nagging problems I had with other phase inverter masters
    of the time. In fact, now I've come to appreciate the sounds of some
    of those early designs, so I count my circuit as just one more useful
    design.

    The problem I had with the older circuits were: 1) when turned to 10,
    these older circuits stole some signal, so the amplifier never had
    a 100 percent non- master sound when fully turned up. 2) I was just
    looking for a less compressed sound with more of an edge than say,
    a dual 1-meg pot-type phase inverter master...

    (Article continues...)
     
  6. chrisjw5

    chrisjw5 Supporting Member

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    Softpaw and everyone,

    Thanks, that makes a ton of sense. I'm following along rather well. If I can, I'd like to ask a follow-up:

    It's my impression that "modern" amps in the Mesa vein use a lot of pre-amp gain. Correct? Am I wrong in thinking that older "vintage" non-MV amps then get a lot of their signature sound from power tube distortion, which is why they need to be so loud to hit the sweet spots?

    If I prefer the sound of a Marshall Plexi to the more modern types of amps, is it fair to say that I should be hunting for more of a power tube type of distortion?
     
  7. john b

    john b Supporting Member

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    Wow softpaw. Thanks..I can't think of any other questions after that response.
     
  8. electronpirate

    electronpirate Member

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    Yes. PT distortion is what you're looking for. Even attenuated, it sounds much better to me than almost any MV circuit (havn't tried the power-scaling tho).

    The beauty of the Plexi is that it's not at all only about the preamp tubes (when it sounds right). I think most people find that alot of Plexi's don't get louder after a certain point, they just get RICHER.

    These days I see alot of people going with 18 watter's, which you can crank on the power tube side to get *that tone*.
     
  9. cnardone

    cnardone Supporting Member

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    Doesn't phase inverted saturation get into this too? Also go read on Randal Aikens site. I believe his Master volume is between the PT and the transformer. which will might work better for you than attenuation
     
  10. Tom Gross

    Tom Gross Supporting Member

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    I think you're on the right track Chris.
    I don't know much, but to me it's fairly simple:
    Pre-amp type distortion - Mesa, Dumble-style, etc., go Master volume.
    PT type distortion - Marshall, Vox, etc. - go attenuator or small wattage.
     
  11. Tone_Terrific

    Tone_Terrific Member

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    Wait! Before you attenuate!:D

    Does your amp sound good at full volume?
    Is it stable and with low noise?
    Do you play in a band or style that requires more than one level of OD at any given volume level?

    You may not be a good candidate to attenuate.

    Try OD pedals. More choice, more versatility, cheaper, better colors.
    Not so popular in the amp forum and unduly deified in the effects forum but a practical solution to good tone at differing levels.

    (There is no substitute for db's.)
     
  12. r9player

    r9player Silver Supporting Member

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    to throw more monkey wrenches there are technologies now called Power Scaling and Wattage control
     
  13. LaXu

    LaXu Member

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    Yup. Power Scaling works really well, I wish it was part of every amp that aims for a vintage tone.

    The problem with attenuators is that the more you attenuate, the more you lose high end and the more compressed the tone gets. Big minus in my book since I love playing with good dynamics where the tone cleans up nicely when you pick softly and really wails when you hit the strings hard.
     
  14. chrisjw5

    chrisjw5 Supporting Member

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    Thanks so much. This is such a useful discussion to me. This all came about, really, because I bought an OD pedal as a novelty idea. I like the artist and thought I'd grab one just to pull out in 20 years when he's gone.

    But I yanked it out of the box and put it in front of the green and yellow channels of my Mesa and that amp sounded better than it ever has. However, now that I hear dthe closest approximation yet of "that tone", it felt like cheating to get it out of a pedal, which I know is stupid. But I also thought I could get a "purer" form of that by switching amps.

    I guess in retrospect, I shouldn't worry so much about using that pedal.

    ONE LAST QUESTION: any good books out there on amp construction where I can try to learn some of these vairables that go into producing tone? I don't necessarily need to know how to build one as much as how they work. Thanks.

    Oh, and BTW: if you've read this far, the pedal was the Digitech Clapton Crossroads on the "Sunshine" setting. Holy schnikes! Slap me in the forehead and call me "Aunt Ruth"!

    Really, check it out. It's a little noisy but it oozes woman tone. I can put on the neck p/u of my Singlecut, turn down the tone knob and it NEVER muds up.
     

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