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  • A Cleaner Clean Channel

    Hi guys, I'm new to this board,
    I have many many many amp projects on my runway and am very glad to be here. So thanks in advance!

    First up, an unusual mod Im interested in pursuing:
    Marshall AVT 50H (head on a 4x12 cab)
    Good little amp at 50 watts (just barely enough for one of my music projects)
    both channels work perfectly, but this clean channel just isn't clean enough it seems.

    Im calling it unusual because Im having a hard time finding anything on modifying an amp for LESS gain.
    Almost everything on youtube is how to get more or simply how to get it working at all.
    All my friend guitarists have all had mods for more gain of course but never for less.

    On the AVT 50H clean channel, to get any volume comparable to the overdrive channel, you have to open up the gain pot, even a little for any signal and then more for more volume. Problem is of course, the more you open the pot, the more gain I get. I would love to get this channel cleaner than clean as clean can be.

    My hunch is that a few simple cap changes or thereabouts can fix this.
    Wondering if anyone here has thoughts, opinions, or experience.

    BTW, I do have tools and experience, though I'm not a professional amp tech, I am a surgeon with a soldering iron and have done extensive work inside mixing consoles, recording equipment and wiring over the years.
    My goal for the 12 or 15 different amps I have that are either alive, sick or dead, is to enjoy the repair process and share a few tricks of my own here n there when possible...

    Thanks again!

  • #2
    Nice goal but letīs put it in perspective:
    in a simplified explanation:

    1) *all* amps clip, eventually.
    The level at which they start clipping is called roughly "maximum power / rated power / etc.".

    2) in your case,your amp is rated 50W because if you feed it a clean sinewave, it will be clean up to 49W and will be visibly clipping peaks at 51W. Ok, not so strict, but thatīs the general idea.

    3) so the trick for cleanliness is to use the amp way below clipping level, or well below maximum power. (same thing)

    4) BUT: you say that
    50 watts (just barely enough for one of my music projects)
    On the AVT 50H clean channel, to get any volume comparable to the overdrive channel, you have to open up the gain pot, even a little for any signal and then more for more volume. Problem is of course, the more you open the pot, the more gain I get. I would love to get this channel cleaner than clean as clean can be.
    problem is than now you have to open volume to, say, 5 to reach overdrive channel level. (and I guess itīs not that clean by then).
    IF you lower gain inside the preamp, amp will be cleaner, but sound weaker :O , to recover lost volume you now need to set it to 7 or 8 and again get dirty.

    Fact is, you do not have a gain problem but a power one.

    To throw some numbers on the table, just as an example:
    Your current amp is 50W RMS.
    The distortion channel can use all of that; even if it reaches the clipping point who cares or even notices? ... signal is already dirty !!!!!!!

    Now you switch to the clean channel, not only it can not reach the full 50W, but it will probably be usable only up to 25 or 30W ... because a guitar has dynamics, reacts to picking pressure, so "now" itīs a little louder, "now" a little smoother ... yet it must never clip, so your *average* level will be less than maximum.
    So in practice youīd need "a 100W amp. *used* at 50W" so clean never ever approaches clipping.

    OR: use clean as loud as you can, and *lower* dirty chnnel setting to match that, that way you can go back and forth without problem.
    If volume is not enough, you can mike and reamp your amp.

    Or get a louder head, say 100/150W, use distortion at about 50W and youīll have lots of headroom for clean sounds.

    Small mods wonīt change that, because they will not increase maximum power.
    Juan Manuel Fahey

    Comment


    • #3
      JM, thank you very much for your reply.

      I understand the concept of headroom. I have many other amps with much higher wattage that run on 2 and peel paint off walls.

      I guess I was just wondering if there was a mod that would bypass the gain stage on the clean channel or at least reduce its sensitivity.

      For example: in a moderate volume scenario, master around 30% (3-4)
      Tone pots all around middle
      Gain at 1 or 2, its already getting gainish.

      As an experiment: in my studio, the PA system is all hi end JBL, crest etc.
      I ran the direct straight into the board to see if it may be a cabinet or acoustic issue.
      Same deal. At any master setting, the gain on the clean channel still gets gainish by 2.

      Thanks for the interest.

      Comment


      • #4
        Some of what's going on here is a matter of perception. It's amazing how much power you can use up, even with efficient speakers, trying to get a satisfactory "clean" volume. I used to work with a rock star that had 4000 yes four thousand watts running EV & JBL speakers pointed at his head, monitors for his stage position for vocals and clean guitar. Never. Was. Enough.

        Meanwhile distortion fools the ear into thinking it's hearing something powerful. A 50 watt Marshall head blaring thru a 2x12 will just about set your ears on fire. Perception, it's a weird thing.
        Enjoy. Every. Sandwich.

        Comment


        • #5
          the last stage should distort first and the first stage the last. That will be a progression guarantee nothing cliping before reach the max power. Don.t think is a good receipe for a guitar amp. Even for a clean sound it need a moderate amount of harmonics and some compression to sound decent. You example is unconcludent cause no matter by gain pot position you dont.t know which stage distort first in you amp. More, some volume pots are allready frequency compensated,bypassed to recover hights, means it will pass max amplitude for certain frequencies from first click
          For a proper designed guitar amp you should get clean just roll off the guitar pot
          Last edited by catalin gramada; 03-30-2017, 01:31 AM.
          "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

          Comment


          • #6
            Originally posted by NickDFX View Post
            For example: in a moderate volume scenario, master around 30% (3-4)
            You do have the clean volume set at 10 and then bring up the clean gain? That's the only way to get full power clean.
            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

            Comment


            • #7
              It is a C68 bypass cap over VR1 gain pot in AVT50 schematic.It not shows its value but have some doubts. Take it off and see if it help.
              Master volume on 10 as g1 sugested
              Last edited by catalin gramada; 03-30-2017, 02:28 AM.
              "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

              Comment


              • #8
                Thank you all for the interest. Yes of course I have tried with master at 10 and then ease up the gain pot. This channel passes no signal unless the gain is up at least a hair. And yes, i can get cleaner by rolling back my guitar volune. This just doesnt seem like an optimal way to acheive a cleaner clean channel. But as I said, its a great little amp, i have many, I do like the tone in general, just always looking for new improved ways of screwing things up.

                Comment


                • #9
                  AVT 50 schematic:

                  AVT 50.zip

                  Comment


                  • #10
                    Thank Jazz P!

                    Comment


                    • #11
                      Originally posted by NickDFX View Post
                      Thank you all for the interest. Yes of course I have tried with master at 10 and then ease up the gain pot. This channel passes no signal unless the gain is up at least a hair. And yes, i can get cleaner by rolling back my guitar volune. This just doesnt seem like an optimal way to acheive a cleaner clean channel. But as I said, its a great little amp, i have many, I do like the tone in general, just always looking for new improved ways of screwing things up.
                      If you're using it with the master on 10 as above and then turn up the gain pot until is just starts to distort then that's the max clean volume it can have but if that happens before the gain gets to 1 then it's uncontrollable. There's just too much voltage gain (not distortion) for a clean channel. To reduce the clean channel voltage gain (sensitivity) without affecting the O/D channel you could try lifting one end of R47 and/or reducing the value of R64 by tacking a lower value (10k?) resistor across it. That shouldn't screw things up too badly but it will change the tone because of C53 and C70. You never know it could sound better

                      Edit:
                      What is the function of LED5 and LED6? They look like they could be clipping diodes but why have clipping diodes in the clean channel signal path after the master volume control?
                      Last edited by Dave H; 03-30-2017, 10:15 AM.

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                      • #12
                        Thank you Dave,

                        This is all purely for interest and experimentation. This amp was gifted to me and im in no rush.
                        Im going to pull the chassis and try your ideas and others as scientifically as possible, (one at a time) to document the results and see what does the trick. Where in the circuit do you reccomend I get a base line test voltage? Just for documentation sake. I will mark and note exact settings, do audible test, and go from there.
                        Thanks again.

                        Comment


                        • #13
                          I made a mistake in the post above. Lifting one end of R47 will affect both channels not just the clean channel, sorry about that. Try reducing the value of R64 as it will only affect the only clean channel.

                          Comment


                          • #14
                            There's an additional mod Im planning, this amp has no reverb footswitch.
                            I think I can Insert just ahead of R56. Then either change the current jack to TRS or install another 1/4" mono. I have a nice 2 switch box laying around somewhere, so if TRS doesnt fit, a custom cable should do the trick.

                            What ya think? R56 look like a good spot? Or maybe ahead of R53?

                            Comment


                            • #15
                              Originally posted by Jazz P Bass View Post
                              AVT 50 schematic:

                              [ATTACH]42978[/ATTACH]
                              Jazz P, do you also have a schematic for Marshall AVT 150hx ?

                              I have 2 of these amps that are both dead.
                              Funny thing is the mr. Obvious check shows identical smoked resistors JR 110 and JR 112.
                              So burnt, i cant even tell the values.
                              Probably spills straight down the heat vent onto the heat syncs. They were in a public rehearsal room, also gifted to me for scrap.
                              These are a down the road project for sure but im really enjoying exploring schematics.

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