During the last five years I've wound up retrofitting/restoring many old cabs. One of the many problems I've had to address are the casters being bad, and replacing them. So here's what I do.
Caster hardware needed:http://www.tubesandmore.com
These are what I use to retrofit old cabs. 4.95 per, so $20 for a whole set.
P-HW01S 2007 Catalog page 80
GUITAR CASTER, WITH SOCKET, PUSH-IN, 2", FENDER TYPE
Heavy duty swivel castor for use on Fender and other amps. 2" diameter wheel supports 44 lbs. Can be purchased as individual component
Pic here: http://www.southbayampworks.com/69spec/dm1&2.jpg
Caster base mounting hardware needed: Sixteen #8 x 1" (up to 1.5") long wood screws, zinc plated so they don't rust.
Tools needed: Electric drill, measuring tape, 2" masking tape, 9/16" drill bit, Sharpie, philips screwdriver bit, cardboard if you want to make a template for exact placement.
Tools & parts pic here: http://www.southbayampworks.com/casteri ... -parts.jpgDrilling the new caster base hole:For Vintage cabs, pre 1975
1) You need to drill out the hole they have in the bottom of the cab (under each caster base) with a 1/2" or 9/16" drill the same depth as the new caster base, which is 1 1/2" deep. You should measure that on your drill bit and tape it off with masking tape at 1.5" from the tip. For 75 and later cabs
you probably have the plastic base casters, or something equivalent.
1) You need to measure out the hole to be drilled in the bottom of the cab (also under each caster base) with a 1/2" or 9/16" drill the same depth as the caster base, which is 1 1/2" deep. You should measure that on your drill bit and tape it off with masking tape at 1.5" from the tip.
2) The exact location to drill the 9/16" hole is 1.75" in from the front (or back) of the cab edge, and 3.75" in from the side. This will align the caster base to fit on top of a regular (1968-2000 era) Marshall straight cab, and allow the cabs to align on the sides when stacked. The casters will need to face IN towards the center, not out towards the sides, in the straight cab caster bases if you want them to align correctly.
3) You'll also need #8 x 1 - 1.5 inch long wood screws. You'll need 16 total to mount all four caster bases. Select the appropriate drill bit (1/8" or less) for the #8 screw, and use the caster base to align where to drill. Then screw in the new wood screws into the caster base.
4) Slide in your new casters and they should "pop-in" or lock within the last 1/4/-3/8" so they don't fall out.
Now start rolling your cab around without spending $80 on the Marshall replacement casters.
Caster socket base installed. Orient the base like this in case you use longer screws, you don't want the screws coming out in front of your grill cloth piping! If you do use longer screws, these will drill into your baffle board, making it more solid, and connected to the bottom of the cab, too.
Caster base pics inside cab:
Rear caster base sticks up in wood support for back panel.
Front caster base is almost hidden in this pic. That's because the 69 spec cab I do has support triangles for the caster base to make the cab more solid. When you have a later baffle board without these triangles, you'll see the metal part of the caster base on the back side, but partly drilled into the baffle board. You can just see the tip of the caster base in the edge of the triangular brace if you look in the lower left portion of the triangle.