The back plate on the K-55M has been known to get loose and vibrate. This can sound like distortion at times. A little silicone caulk will hold it in place. Quick and easy fix (IF that is the problem). Worth a try.
I have drawing but on a drive I cannot access. This is a simple design to copy you can see the approximate dimensions the front plenum is the same siv=ze as the outer edge of the woofer suspension the mounting plate is over sized. You have the outside physical dimensions from the Peavey data and the cabinet material is either 5/8 or 3/4 inch material so you can scale this up on a piece of paper. The exact dimensions are not critical as this is to begin with not a horn but rather a crude wave guide, it is too short in length and the mouth too small to be critical in any way but as you can see it it works well enough and is still a popular sounding horn with lots still in use today. Hope this helps. Perhaps one of our members with an MB2 would be kind enough to post the dimension so an exact copy can be made but I assure a good approximation will do just as good a job. So far as I can recall the vertical walls of the plenum are parallel.
just so others reading have the same info as you do (to help make this thread more useful)
here is a video with a look inside
There are no diaphragms available for the K-55M. I have managed to modify K-55V diaphragm to use in the K-55M, but that is a tricky procedure and probably not a good thing to do as long as yours are working.
Thanks guys. They will fit inside the far left/right cabinet slots in this photo. As I mentioned, we had planned to have cabinet doors with grill cloth but maybe not anymore! I have yet to hear these particular speakers but will crank them up when our new house is finished in a couple of months.
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Does the existing amplifier work?
If it works and sounds good, maybe build a copy of the same amplifier.
Maybe the output xformer on the existing amplifier could be turned 90 degrees in relation to the choke and PS xformer, but it may not matter.
Stick with orange drops until you get things going, and then maybe upgrade coupling caps down the road. For appearance, two similar looking chokes would look best, but maybe you can mount the chokes under the chassis. You could go with carbon comp resistors like the existing amp, or go with carbon film or metal film. Maybe metal oxide for the larger wattage.
Worry about a good strong 6C4 before rolling them.
R6 in the schematic must do the signal inversion...not too sure...