would putting a 15" speaker in a Valve Junior be possible without any modifications to the chasis of the cabinet?
Youll have to remove the baffle and make a new one. Also you cant put any speaker in. I reckon the volume of a 12 cab would be too small for most 15" speakers.
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Its open backed so the volume shouldnt be such an issue. But its unlikely there will be enough room in the cab for a speaker 3" bigger than the original.
Quote by willwelsh816
will 3 inches make that much of a difference?

Yes actually. You're talking about a speaker that is 25% bigger in diamiter than the one your amp is designed for. that's quite a considerable difference. You'd need to bore out the speaker hole to be bigger than what it is now, that's if it'll fit in the cab which i doubt it will.

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guys, the VJ has an 8" speaker. We're talking nearly doubling the speaker size.
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Quote by willwelsh816
would putting a 15" speaker in a Valve Junior be possible without any modifications to the chasis of the cabinet?
probably not.

The exterior dimensions of a VJ extension cab are 18.25" square for the front.
subract the thickness of the materials and you're down to about 16" for interior dimensions. But there are cleats that the front baffle attaches to.
Measure between these cleats. I doubt you'll have 15".

In addition to ripping off the grill cloth, tolex and logo, making a larger opening, and drilling new holes to mount the speaker, then putting on new grille cloth, you'll also need to modify the cleats. It can be done. It will be messy.
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It is a tight fit to get a 12" speaker in there. You can not get any bigger than that. Instead you need to think about an extension cab or building a custom cab
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