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So I was thinking of replacing the speaker of my Marshall AVT50 Combo amp. I've had this amp for 14 years now and I love the thing, but it's dark sounding compared to my class 5. My class 5 is too small for stage use as it breaks up to early. so I read that open back cabs sound less bassy and more open. What would happen if I removed the back cover of my amp? Or should there only be a specific part removed, like the middle or whatnot? Would if needs be, I have to cut a new back, would the wood make a difference or can I just buy ply-wood and stick it in there? Could one, maybe cut the back in half and use hinges and variate between the two options?

Thoughts....?

What is the average time before one usually replace a speaker? This amp has been my work horse since I got it back in the day.

    Interesting - most folk complain of too much high end with those amps. What volume are you usually playing at?

    Cut a new back and experiment. When you find what you like, cut the original the same. A 6" hole in the back seems to be the usual recommendation. Celestion Vintage 30's are popular upgrades for those amps.
      Alan Ratcliffe wrote: Interesting - most folk complain of too much high end with those amps. What volume are you usually playing at?

      Cut a new back and experiment. When you find what you like, cut the original the same. A 6" hole in the back seems to be the usual recommendation. Celestion Vintage 30's are popular upgrades for those amps.
      Would any wood do, or should I look at a specific type? At home I play soft, like really soft, so I rather use the Class 5 for that, other wise at band practice and live I play really loud, Gain and Volume at about 12, gets a nice grit and cleans up nicely when volume is rolled back. I also have a complaint from my drummer that he can never hear me as the closed back might be to directional.

      A6" hole, where? I saw this reference on the web too, but no one seems to indicate where to make this hole, top bottom, middle?

      Maybe it all just sounds a bit dull since the speaker is so old?

      Thanks Alan.
        Jayhell wrote: Would any wood do, or should I look at a specific type?
        Needs to be fairly stiff, so go for a ply or even MDF (if it's just for testing).

        Sorry - something I meant to mention was the first thing you should do is remove the back completely and see if you like that. If you do, leave it off (or cut the backboard so it's just a support/cosmetic frame). If you think you'd like something in between, start experimenting with different port sizes, using your replacement back. Position isn't really important AFAIK, diameter is - the smaller the hole the more the air inside will compress and stiffen the cone response.

        Yes, opening up the back of the cab helps with sound dispersion, especially if there are walls nearby for the sound to bounce off. It also loosens up the bass and brings down the compression - which often leads to more mids and treble (as well as an amp that seems to "breathe" more). Closed back is great for throwing the sound further, but it is more directional. Some types of music also need a tighter bass response than an open back cab can supply.
          I would highly recommend opening it up, I used to play with a 4x12 with only the bottom two speakers installed, but now I play a 4x12 (all 4 are vintage 30's) with the top 3rd of the back panel removed.

          It doesn't work well for Low (C# or 7 string palm muting etc) tunings, but for clean -> breakup -> overdrive (foo fighters as my heaviest territory), it is a huge improvement in the presence, with less low mud.

          Cheers
            Thanks guys, I'm going to try it with the back completely removed and take it from there.
              Update, I am buying an Eminence Governor speaker, witch is apparently Eminence's version of the V30, some say it's even better, then I'm going to get three pieces of wood that together would cover the back of the amp. I will play around with that and see how much I need to open it up, 1/3, 2/3, none at all, all of it, and where to open it up, top, middle and bottom.

              I will post my results.
                Built a cajon recently.

                I tend to research stuff.
                After fishing around I realized that all resonating boxes with holes in them are Helmholz resonators.
                These include acoustic guitar bodies, djembes, kora's, bass reflex speakers, coke bottles, marimba resonators and speaker cabs.

                Some oke did some experiments:
                http://34iac.acoustics.sk/proceedings/Kicak.pdf

                The box resonates with a fundamental bass frequency.
                The hole size affects the frequency and the volume of this resonating fundamental note:
                - increasing the hole size increases the frequency; and increases the volume ... to a point
                - volume of resonance is maximised with holes in range of 150mm to 220mm (6"-8.5") depending on volume of box
                - open back is over the top ... and less loud than some optimal size

                All of this is in respect of the bass reflex effect.
                There's obviously the separate effect of hearing the speaker directly.

                For my cajon I found if the hole was too small the bass response sounded "choked".
                Making it bigger made it "free-er" and louder.
                Until it got too big then you had a very different effect altogether.

                So ...
                No idea how this actually applies to cab speakers, but if I were you I would explore 3 options:
                a) closed (which is where you started)
                b) open (which is easy)
                c) holes ranging from say 6" - 9"

                The material doesn't make much difference providing it is rigid.
                The bass comes from the air resonating; not the walls.
                Absorbent walls can make a difference by absorbing sound ... but that's another completely different effect.

                If I were you I'd make a back out of some mdf or ply and cut a 9" hole in it.
                I'd then make a few squares that can be placed over the hole with smaller circles in them.
                Say 8", 7" and 6"

                Then you can play it and dynamically experiment by putting the "squares with smaller holes" over the big hole to see the effect.

                If you're in the mood for experimenting.
                  Wizard wrote: Built a cajon recently.

                  I tend to research stuff.
                  After fishing around I realized that all resonating boxes with holes in them are Helmholz resonators.
                  These include acoustic guitar bodies, djembes, kora's, bass reflex speakers, coke bottles, marimba resonators and speaker cabs.

                  Some oke did some experiments:
                  http://34iac.acoustics.sk/proceedings/Kicak.pdf

                  The box resonates with a fundamental bass frequency.
                  The hole size affects the frequency and the volume of this resonating fundamental note:
                  - increasing the hole size increases the frequency; and increases the volume ... to a point
                  - volume of resonance is maximised with holes in range of 150mm to 220mm (6"-8.5") depending on volume of box
                  - open back is over the top ... and less loud than some optimal size

                  All of this is in respect of the bass reflex effect.
                  There's obviously the separate effect of hearing the speaker directly.

                  For my cajon I found if the hole was too small the bass response sounded "choked".
                  Making it bigger made it "free-er" and louder.
                  Until it got too big then you had a very different effect altogether.

                  So ...
                  No idea how this actually applies to cab speakers, but if I were you I would explore 3 options:
                  a) closed (which is where you started)
                  b) open (which is easy)
                  c) holes ranging from say 6" - 9"

                  The material doesn't make much difference providing it is rigid.
                  The bass comes from the air resonating; not the walls.
                  Absorbent walls can make a difference by absorbing sound ... but that's another completely different effect.

                  If I were you I'd make a back out of some mdf or ply and cut a 9" hole in it.
                  I'd then make a few squares that can be placed over the hole with smaller circles in them.
                  Say 8", 7" and 6"

                  Then you can play it and dynamically experiment by putting the "squares with smaller holes" over the big hole to see the effect.

                  If you're in the mood for experimenting.
                  It's going to be a long experiment, because the only time I have to play loud is at band practice (once a week). So the first experiment will be with the new speaker, Followed by the new speaker and closed back, followed with open back, followed by 1/3, 2/3 and then closed again. Will take about 5 weeks or some patience from my fellow band members. Then I still need to try the hole experiment. Consensus around the web is a hole the size of a coke can bottom, but as I said earlier, no one seems to indicate where to make this hole. Then I think, maybe they made the cab the way they made it, because it is how it sounds best, but who knows?




                    Jayhell wrote: Then I think, maybe they made the cab the way they made it, because it is how it sounds best, but who knows?
                    Reminds me of a Top Gear program where the lads were given a budget to "upgrade" car.
                    Blew their money on new suspension and tires and brakes etc.
                    All their mods made the car go slower.
                    They concluded the designer had already optimised the design.
                    ?

                    But.
                    Fiddling around and concluding the original was the best design still leaves you with new insights.
                    And there's nothing bad about that.
                      Wizard wrote: They concluded the designer had already optimised the design.
                      I have but one objection to said conclusion: it was a Renault space wagon, and they were trying to optimise for speed.

                      Anyway, I would recommend trying to understand bass response in vented cabinets before going off on a six week experiment; even if it does not change the experiment it helps if one knows what to listen for. Recommended reading: Vance Dickason - Loudspeaker Design Cookbook.
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