Dimensional tolerances for a back panel?

[Axtman]

I am going to build a back panel for a Fender Showman amplifier. I am going to build one because the place I was going to order from has a long lead time and is rather expensive. Plus I have scrap hardboard and tolex just lying around my workshop.

How short should I cut the wood panel to allow for tolex and fitting tolerance?

For example if the panel is 20-1/2" by 6-1/2", what size wood panel should I cut?

[Chuck H]

What is the thickness of the Tolex? So it's the space width + Tolex thickness + a fraction to allow for a gap. You'll want to allow for a small gap between the Tolexed panel and the side wall just to insure against any audible vibration. I use thin shims when I set cabinet panels to evenly space a tiny gap for this reason. You can secure it tightly at the faces, but the edges are a crap shoot if you don't set a space or fit it tightly. A space is easier to do since it can be off a little without much consequence.