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Speaker Selection Explained

Standard mounting dimensions provided by most guitar speaker manufacturers are: overall diameter, baffle hole diameter, mounting hole diameter, mounting holes B.C.D. and depth.

Can you explain mounting dimensions and how to install a new speaker in my combo amp?

Standard mounting dimensions provided by most guitar speaker manufacturers are: overall diameter, baffle hole diameter, mounting hole diameter, mounting holes B.C.D. and depth.

Overall diameter is the dimension from opposite sides of the speaker frame. It is used to determine if the speaker frame will fit on the baffle. The baffle hole diameter is the dimension from the inside of the gaskets on two opposite sides. This is the area that projects the sound. Ideally, you don’t want any of this area to be covered. If you are building a new cab, use this dimension to make the speaker hole in the baffle.

The mounting holes diameter is the width of the mounting hole. This helps determine the size of bolt or screw to use for mounting the speaker. The mounting holes B.C.D. dimension is measured from the center of one mounting hole to the center of the opposite mounting hole. “B.C.D.” stands for bolt circle diameter. This measurement helps determine if existing mounting holes line up with the new speaker. You can use the mounting holes diameter as a tolerance in relation to the mounting holes B.C.D. For example, if your mounting hole diameter and the B.C.D. dimension isn’t equivalent, the speaker may still fit. The mounting holes diameter may provide you some extra room; the bolt doesn’t necessarily have to be centered in the mounting hole.

The depth is simply the total distance from top to bottom of the speaker. If you’re front-loading the speaker, you can subtract about 1/4” from the speaker depth for the basket flange. Then subtract your baffle thickness from that to determine how much of the speaker will be sticking out past the baffle. Certain applications may require the need for other dimensions to avoid hitting transformers or tubes. Don’t be afraid to call or email the speaker manufacturer for this info.

For the sake of this discussion, I’ll assume this is the most common, rear-loaded application. To install your new speaker, you obviously need to remove the old one. Some applications require removing the amp chassis. Typically, the screws to remove the amp chassis are located in the top of the cabinet, possibly in the handles. Unhook the wiring from the old speaker (be sure to remember how it was). This may require soldering. Carefully remove the amp chassis, and make sure you set it somewhere it will remain level. You don’t want to harm the tubes or any circuitry.

Remove the speaker mounting screws or bolts, and pull out the old speaker. Assuming you’ve chosen a direct fit replacement, install your new speaker. Be careful not to shove a bolt or drill through your new speaker’s cone. Been there, done that, not good! As you tighten the new speaker down, work from opposite sides; in other words, tighten in one spot and then go to the opposite side and do the same thing.

Try turning the nuts about the same amount. How much to tighten is the topic of much debate; you’ll just have to use good judgment and common sense. If you get too tight, you could potentially create problems, like distorting the speaker frame. I think a few good turns with a nut driver/ screwdriver without causing a hernia is sufficient. Carefully re-install the amp chassis and wire the new speaker back just as you found it (be sure your new speaker is the appropriate impedance).

We hear a lot about how wiring affects your sound. What size wire should I use on my guitar speakers?

I recommend using 16 or 18 AWG speaker wire for guitar applications. You won’t gain any advantage using larger wire unless you are dealing with a long length. You don’t want to use any smaller wire because it may not carry the current of the amp.

I can’t find a 4-ohm speaker replacement. Any suggestions?

You can use a higher impedance speaker, but you will sacrifice output. You lose ½ the perceived output (-3dB) for every doubling of impedance. It is not recommended to chose a speaker lower in impedance than that stated on your amp. It can potentially damage your amp.

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Anthony Lucas is a senior lab technician at Eminence Speaker LLC, where he specializes in guitar-speaker design and customer support. He has been with the company for more than a decade.