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Thread: Want to build an amp/cab switcher - advice please

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    Want to build an amp/cab switcher - advice please

    Hello all, I want to build this for my home studio:



    Instead of constantly reaching behind amps and cabs to switch things around, I think this little gizmo could make life much easier. The idea is to just leave everything connected all the time and just turn switches to select amps and cabs.

    Most of my amps are 50w and 100w heads. One of them is 22w.


    What kind of consideration do I need to give to the rotary switches? Voltage? Amp rating?

    What kind of dummy load can I maybe incorporate into this thing to safeguard against operator errors, and how to do it? (I will be the only one operating it).

    In my simpleton mind it seems very simple. That means I'm probably missing something.

    Any ideas or help is appreciated.

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    Supporting Member John_H's Avatar
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    I would suggest a patch bay. It would be much easier, especially when trying to protect the amplifiers by using switching jacks with a dummy load.

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    Quote Originally Posted by John_H View Post
    I would suggest a patch bay. It would be much easier, especially when trying to protect the amplifiers by using switching jacks with a dummy load.
    But then I'd have to manually plug and unplug cables. I wanna just twist the dials and leave everything permanently connected.

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    Don't forget the joker g1's Avatar
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    Are you talking about powering down to make changes? If not, it gets a lot more complicated.

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    Quote Originally Posted by g1 View Post
    Are you talking about powering down to make changes? If not, it gets a lot more complicated.
    Yes, definitely. I'm not leaving all the amps on at the same time. It's still just one amp and one cab at a time. I just don't want to keep reaching behind everything all the time to switch speaker cables around.

    I'm thinking keep all the amps plugged into the box, and all the cabs plugged into the other side of the box. One rotary switch to pick an amp, one rotary switch to pick a cab.

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    Supporting Member eschertron's Avatar
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    At a bare minimum, you'd want enough switching contacts to short out or add a resistive load to any amp not selected to the cabinets. Otherwise you run the risk of mistakenly running an amp into an 'open' load. Potentially fatal for the OT.

    Another way to approach it would be a set of contacts that allow power to the amp that is selected in the switcher. Requires relay control, but the individual components can be easily sourced.

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    Quote Originally Posted by eschertron View Post
    At a bare minimum, you'd want enough switching contacts to short out or add a resistive load to any amp not selected to the cabinets. Otherwise you run the risk of mistakenly running an amp into an 'open' load. Potentially fatal for the OT.

    Another way to approach it would be a set of contacts that allow power to the amp that is selected in the switcher. Requires relay control, but the individual components can be easily sourced.
    I'm not worried about the "open load" aspect. Mainly because it truly will be just me using it. I'm not saying I'm mistake-proof, it's just my risk I'm willing to take for simplicity of design.


    Can yall see my drawing in the first post? Since speaker cables will be connected to each jack at all times, I'm not sure how to wire in a safety dummy load. I really just want to be able to turn the knobs and select what amp I want going to what cab, and then power it up.

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    Supporting Member John_H's Avatar
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    Quote Originally Posted by Greg_L View Post
    But then I'd have to manually plug and unplug cables. I wanna just twist the dials and leave everything permanently connected.
    Another idea: Switch the amps with on/on toggle switches. One position going to the speaker cabinet switch, the other going to a dummy load.

    The patch bay that I envisioned would be a row of 1/4 switching jacks for the for the amps, and a row of jacks for the cabinets. One little speaker wire patch cable going from the amp you want to the cab you want.

    Doing this, you would be able to run two amps/cabs simultaneously if you wanted to jam with your friend John when he comes by, or plug any other amp in easily.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by John_H View Post
    One little speaker wire patch cable going from the amp you want to the cab you want.
    I definitely think the visual feedback of a cable plugged into the jack from amplifier "A" would be a reliable way of ensuring the desired amp was selected.

    Unless one envisions a panel space with a row of input jacks and a row of output jacks, and a couple 50s sci-fi set bakelite knob/indicators that can be seen from across a dimly lit room. That would be good visual confirmation too... and take up as much space if not more than the patchbay method.

    edit: I'm not being flippant. Thinking about my circumstances, I'd be hesitant to rely on a
    little gizmo
    if it weren't in plain sight and clearly readable while I was in the heat of artistic fervor.

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    Hmmm. I guess a patchbay is a pretty good idea. Less moving parts - no rotary switches. And the positive fool-proof verification that things are connected correctly. Good ideas, thanks.

    More jacks though. I'll probably need a bigger enclosure. Can all of those switching jacks just go to one dummy load? Or do I need a dummy load for each jack?

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    Supporting Member John_H's Avatar
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    Quote Originally Posted by Greg_L View Post
    Hmmm. I guess a patchbay is a pretty good idea. Less moving parts - no rotary switches. And the positive fool-proof verification that things are connected correctly. Good ideas, thanks.

    More jacks though. I'll probably need a bigger enclosure. Can all of those switching jacks just go to one dummy load? Or do I need a dummy load for each jack?
    I would wire the cables directly to the jacks. It's just speaker wire, and it would make for fewer mechanical connections to get dirty. The dummy loads would need to be isolated. One for each jack.

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    Quote Originally Posted by John_H View Post
    I would wire the cables directly to the jacks. It's just speaker wire, and it would make for fewer mechanical connections to get dirty. The dummy loads would need to be isolated. One for each jack.
    Oh yeah, okay. So I can make speaker cables and run them directly into the box through grommets or strain relief. The other ends will be connected to jacks for "patching". I'll have 7 on the amp side, 3 or 4 on the cab side.

    Why can't I use just one dummy load though? I was thinking the shorting switch tabs on each switched jack can run to one dummy load for amp-side jacks that aren't being patched. Why does each jack need it's own dummy load?

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    Supporting Member John_H's Avatar
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    Why does each jack need it's own dummy load?
    If the dummy load was on a common bus, the result of turning on the wrong amplifier would feed current backwards into the other amps. They need to be isolated.

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    Quote Originally Posted by John_H View Post
    If the dummy load was on a common bus, the result of turning on the wrong amplifier would feed current backwards into the other amps. They need to be isolated.
    Okay, I see. In essence they'd all be parallel connected to each other through the single dummy load.

    So I'd need 7 dummy loads. Damn, that raises the cost and size and complexity. Good safety net though.

    So far a starter parts list, I'm thinking

    Some kind of enclosure
    ~ 100ft 16awg speaker cable/lamp cord
    At least 10 1/4" TS male jacks
    7 switched 1/4" jacks
    4 standard mono 1/4" jacks
    7 16 ohm 100w dummy loads
    Grommets and/or strain reliefs bushings
    And a partridge in a pear tree

    So this thing will kind of end up looking like a stage snake - a box with a bunch of cables sticking out of it.

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    Supporting Member eschertron's Avatar
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    If the 7 switched jacks short when the patch cable is pulled, the you have the same safety rating that a typical amp head has for its speaker output jack. You could consider not using load resistors in this case.

    The added bonus from making the patchbay is that (as pointed out earlier by John_H) if you have 3 speaker cabs, you could have up to three amps all plugged in and running simultaneously.

    I'm a little envious at this point. My music room is cramped enough that if I have more than one speaker cab pulled out from the equipment closet (literally, a repurposed clothes closet) then there's hardly room to swing a guitar. Sometimes I have to move a cabinet out of the room temporarily to swap stuff around. sigh.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    Quote Originally Posted by eschertron View Post
    If the 7 switched jacks short when the patch cable is pulled, the you have the same safety rating that a typical amp head has for its speaker output jack. You could consider not using load resistors in this case.

    The added bonus from making the patchbay is that (as pointed out earlier by John_H) if you have 3 speaker cabs, you could have up to three amps all plugged in and running simultaneously.

    I'm a little envious at this point. My music room is cramped enough that if I have more than one speaker cab pulled out from the equipment closet (literally, a repurposed clothes closet) then there's hardly room to swing a guitar. Sometimes I have to move a cabinet out of the room temporarily to swap stuff around. sigh.
    All of my amp heads are Marshalls....and one home-built AB763 clone as a head. All of my cabs are 16 ohm Marshall 4x12s. So yeah it takes up a lot of space, but I manage.

    I use each head and cab for various things. I play live alot, I record myself and others pretty regularly. I'm a stickler for real sound, real amp sounds, really loud amp sounds. I record blazing Marshalls all the time. There is no "little studio amp" shit happening here.

    As it stands right now, I use a bypassed Weber Mass 200 attenuator as a makeshift "patch bay". It's the go-between the amps and cabs. I have speaker cables connected to each head and each cab, and I just grab the amp cable and speaker cable I want and run them through the attenuator. It works, but it's a lot of reaching behind everything and....it's actually not that bad but I'd like it to be easier.

    I'd love to have everything connected to one box at all times and I could just just move a patch cable like a 1940s phone operator. Amp B into cab D....no problem.

    I could actually probably do without the dummy loads. I'm careful and I'm willing to take the risk. The way I do it now is also risky. I have cables everywhere. I could easily grab the wrong one and connect an amp that's not the one being used. But it doesn't happen. For all of my personality flaws, mistreating amps is not one of them. Knock on wood.

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    Quote Originally Posted by Greg_L View Post
    - - -So I'd need 7 dummy loads. Damn, that raises the cost and size and complexity. Good safety net though.- - -
    For those who know more about the tube amp output circuits do the dummy loads for this project really need to be 100 Watt capable if they are just there to protect the amp in case of an temporary inadvertent hookup? Also, do they even need to be 16 ohm values? I ask this because I have run across discussions of amps that had internal load resistors at the output that were much higher ohm values. Something like 150 Ohms. The stated explanation was that the high value would still protect the amp and did not need to be as high wattage because of the high resistance value.

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    If you power down the amps when not using, you don't need dummy loads. You want simplicity of it, you don't get fool proof.

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    Quote Originally Posted by Reader View Post
    For those who know more about the tube amp output circuits do the dummy loads for this project really need to be 100 Watt capable if they are just there to protect the amp in case of an temporary inadvertent hookup? Also, do they even need to be 16 ohm values? I ask this because I have run across discussions of amps that had internal load resistors at the output that were much higher ohm values. Something like 150 Ohms. The stated explanation was that the high value would still protect the amp and did not need to be as high wattage because of the high resistance value.
    They way I see it, 16 ohm would match what my cabs are and what my amps pretty much stay set to. Significantly higher resistance would be like an open circuit. Not that I'm too worried about it. I'm not going to crank amps into a no-load. I usually listen for the "hiss" to make sure I'm good to go before I go breaking eardrums.

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    Quote Originally Posted by mozz View Post
    If you power down the amps when not using, you don't need dummy loads. You want simplicity of it, you don't get fool proof.
    I agree. The goal is not to have all of the amps on at the same time. I will still be powering on/off one at a time.

    The dummy loads could be a good safety net if they're not too cumbersome or difficult to deal with. They're easy to find and fairly cheap, so what the hell.

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