Drilling the Chassis, Part I.
Pix here:
http://www.diycustomamps.com/images/18W_lite/chassis/
Let me start by saying that chassis work is my least favorite part. It can be a royal PITA to get all the holes in the right places and lined up correctly. It is also very easy to put holes in the wrong places and completely ruin a chassis. For this project, the chassis is one of the most expensive items.
This is why I recommend kits to beginning builders. The nice thing about kits is that a lot of the tedious work is already done. I'm no machinist, so it has taken me many years to get to the point where I can get consistent results when fabricating chassis.
I use the Bud Industries AC-409 for many of the amps I build. It measures 2" tall, 7" wide and 13" long. It is about .040" thick. It is made of aluminum and is easy to drill/punch. It can be used for just about any amp from 1-20 watts depending on the number of controls on the face. I can get about 8 controls on the front of one of these: on off switch, standby, pilot light (small), TMB tone controls (small knobs), volume control (small knob) and an input jack. For more than that I would move up to the next size chassis which measures 2" x 7" 17".
I like to make a 75% sized drawing of the chassis and the various holes that will be made with their measurments. I print this out on 8.5" x 11" paper as a reference.
Once I have made the board, I start by drilling and punching the chassis. "Punching" refers to using a chassis punch. This is a device that has a draw bolt, a die and a cutter. These are great for tube amp work because they make nice round holes.
The first step that I do is to mount the board. I used the unstuffed board itself to mark the locations for the standoffs. These keep the board components from touching the chassis. I trace inside the holes with a fine-point Sharpie then use a spring loaded center punch to mark the location of the hole. Then I drill out the hole using an appropriately sized bit. Then I mount the board to make sure that everything lines up correctly.
Next I drill/punch the holes for the tube sockets and in this case the chassis-mounted cap can. I use three methods here. I need a 1.375" hole for the cap can, but don't have that size chassis punch. So I use the biggest punch I have which is 1.25". To make a hole I measure and mark the location. Then I use center punch to put a small dimple where the hole goes. This dimple keeps the bit from wandering. Since this is a large hole I then use a stepped drill bit to make a hole that is just the diameter of the draw bolt on the chassis punch. Next I assemble the punch, and tighten it down on the chassis until it cuts out the hole. To enlarge the hole to accept the 1.375" diameter cap can, I take a template made of .75" MDF with a 1.375 hole bored in it and tape it down over the existing hole using double-sided carpet tape. I try to center the jig as best as I can. Then I use a laminate trimmer with a flush cutting bit to follow the template and cut the hole to its required size. This is okay to do with Aluminum, but don't try this on steel: you will damag the bit, the work, and/or yourself. Since most of the drilling punching operations leave a sharp edge, I then use a deburring tool (sometimes called a "chaser") to deburr the sharp edge.
Next I cut out the rest of the tube socket holes. The octal socket requires a 1.06" hole, the two power tube sockets require .75" holes and the two preamp tube sockets require .875" holes. I use my chassis punches to cut out the octal and power tube socket holes, but I don't have a .875" punch, so I go to the stepped drill bit. The last step on the bit is .875" so I just drill through. There is a sharp burr left that needs to be removed so I use the deburring tool to scrape it off.
A word on stepped drill bits: the best ones I've found are Irwin brand Unibits. They have a single flute and make nice round holes. Several of the cheaper bits make triangular shaped holes.
Next I flip the chassis over and proceed to fit the transformers. Starting with the power transformer I measure the location and cut the feed-through hole for the various wires. I then debure the hole using the deburring tool. Since I did not have a mounting pattern, I carfully positioned the transformer and marked the inside of the mounting brackets with a Sharpie. I then remove the PT and center punch the hole locations and then drill them to size with an appropriately sized bit.
Mounting the output transformer was a little easier as I had a footprint drawing on a clear sheet to overlay on top of the chassis. Once I had marked the center line of the mounting holes, I placed the sheet on top and then marked the hole locations with the center punch. I use sheet metal screws to attach the OT to the chassis as the board covers up one of the mounting locations on the inside. Getting a nut off the machine screw when covered by a board is very frustrating, and can't readily be put back on.
Finally, I locate and drill the location for the feed-through hole for the OT's wires. This hole needs to have a rubber grommet to protect the wires. (The PT doesn't need a rubber grommet; the wires do not come into contact with the chassis) The OT I'm using for this project has both the primary and secondary wires on one side. Generally, the primary wires are on one side and the secondary wires are on the other. Having both on one side does not seem to affect the sound.
That's it for part I.
All that needs to be done is to drill out the holes in the front and back of the chassis.
tung
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Originally Posted by duhvoodooman