When you want to design a prototype for a custom amp, it is almost impossible to find a pre-punched chassis that can be easily adapted to match your control layout. To get around this, you?ll need to drill and punch your control layout on a blank chassis. You can obtain a blank chassis in a wide variety of sizes from a number of online amplifier parts merchants like Watts Tube audio and Tube Amp Doctor. When I was doing my first prototype amp, I decided to get an aluminum blank chassis in the same size and shape as a traditional Marshall 100 watt Super Lead. I figured I could easily have a cab made for it and that size would make a good platform for the amp I had in mind.
Prior to receiving the chassis, I used Microsoft PowerPoint to create a rough (not to scale obviously) front and back channel chassis layout. You could actually use just about any graphics or CAD program to create your layout, just use whatever you are familiar with.
To custom punch/cut the chassis, I recommend you get the right tools. The first thing you?ll need is a drill press. You don?t need a super expensive one, I think I paid about $60 for mine. You?ll also need some proper drill bits of various sizes and some specific cutting tools for the larger holes. I ended up getting these tools that work great on a drill press and cut very clean holes that did not require any de-burring. I also built some plexiglass templates to use to mark the chassis. I tried using them as guides and attached them to the chassis when cutting, but that step was unnecessary and actually made things more difficult. It would have been better to just use the template to mark the holes on the chassis and cut it directly.
As I mentioned and you can see in the pictures, I'm using the template I cut out of plexiglass to cut the chassis. This is actually a complete waste of time as I found out. All you really need to do is mark the hole centers with a marker directly on the chassis and then cut the right size hole on each center mark.
There are still larger holes for the chassis mount filter caps, etc, that I could not cut with the Rotacut hole cutters. For those holes, I used Greenlee punches. These are essential tools if you are going to be doing a lot of custom chassis layouts. They are pretty expensive, but you can find used kits on eBay fairly reasonably priced. Here is what the Greenlee hand punch kit looks like:
Here is how you use it to cut the holes. You need to cut a 3/8" inch guide hole first so you can insert the Greenlee punch to cut the larger hole required. Then, you simply insert the ball bearing threaded screw (with the die attached) through the 3/8" guide hole you cut, screw on the punch and hand turn the top bolt to punch a very clean hole. It is pretty easy and required little effort (at least with aluminum). I also included a picture of the top of the chassis which has a bunch of punches for the large filter caps:
To do the rectangular hole for the AC Inlet, I cut two 3/4" round holes using my cutting tools and then used a flat file to square off the edges. I've attached some pictures of the chassis in various stages of the punching/drilling process. There are also some final shots where I test fit all the components and hardware, as well as some custom plexi panels I had made for the amp:
It?s a fairly simple process, although time consuming. An alternative is to draw up a fab file and take it to a metal shop in your area. Unfortunately, unless you are willing to commit to a pretty good number of chassis, it will probably be more economical to cut the chassis yourself as I?ve shown.
In my next entry, we?ll talk about schematic and layout design. If you?ve got suggestions for future topics you?d like to hear about, just drop me an email at firstname.lastname@example.org and I?ll see what I can do!