Drilling a chassis

I am starting to build a simple (very very simple) tube amplifier based on the AX84 Project’s P1 eXtreme. It’s a ‘single-ended’ design, thus a simple output section. And it uses both halves of one 12AX7A for the pre-amp section. As my time is limited I’ll be dribbling this out over the coming months, which is fine with me. At this stage I need to learn and appreciate each stage of the journey.

I chose the P1 eXtreme over the regular P1 as it has more choices in the output stage. i.e. 8 pin tubes (EL34 etc) rather than just the nine pin (EL84). I know i’m going to want to tweak the amp once I’ve built the stock design from the plans, so options are good From the introduction:

One of the great features of this amp is the way the sound can be varied on the Gain control between clean to edge of breakup, mild bluesy P1-style overdrive, and on to full classic M@rsh@ll-style crunch. With hot humbuckers or a boost pedal and the gain control cranked, you can even get metal sounds.

I decided that my first step would be the chassis. Nice and easy…just some drilling. I’d also be getting to know the layout, and the parts are cheap so far (mostly tube sockets and grommets). The plans call for a 16″x8″x2″ aluminium chassis. These are freely available in many different places, so naturally I bought a 16″x8″x3″ steel chassis. I don’t know what happened, but between my brain reading the bill of materials, and actually placing the order, ‘the Pentium error’ kicked in.

Top of chassis with plan attached

Top of chassis with plan attached. You can see the highlighting of certain holes and not others.

I went to Kinko’s and had the drilling plans printed on 11×17 at exactly 100% (no scaling) and they came out beautifully. All I had to do then was to cut them out and tape them to the bare chassis. Although it was covered in clear plastic for protection, I chose to take that off so I could tape the plan more sturdily. As I’d bought a taller chassis, I had to make sure the front/rear holes were in the right place. So careful measurements ensured they were on-center.

By now I had been staring at the drill plan for a while and realised that it had WAY too many holes than I needed. So I went back to the build guide, and some pictures of other people’s P1eX amps to decide which ones I really needed. I marked the drill plan with a highlighter so I’d do the right ones. The drawing with all the holes seems very complex, but when it comes down to it there’s only a few components there: the tube sockets, the transformers, potentiometers, jacks and power. Also, the P1eX requires one each of an 8 pin and 9 pin. I will be drilling more 9 pin sockets in case I need more pre-amp tubes, for a reverb circuit or effects looping, in the future. (Yeah, I know….ambitious). With all parts of the plan taped on it was easy to use a spring-loaded punch to mark all the holes.

Pulley speed chart

The pulley speed chart from my drill shows which speeds to use depending on material and drill size.

I used my trusty Craftsman 9″ drill press, with an 1/8th inch drill in the chuck, for all the pilot holes. I made sure the speed was set according to the handy chart inside the drill’s pulley cover.

When I could, I used the small chuck that came with the drill; mostly when drilling the front/rear panels.  The chuck helped with any unwanted rotation, or ‘grabbing’ of the piece as the drill bit made it through. When drilling the top of the panel I realised that the drill presses bed was too small. I found a couple of short lengths of 2×4, which added up to 3 inches in height. This meant they’d fill the void underneath the chassis, thus making it much easier to secure it with a clamp.

So far I have all the 1/8th inch holes drilled. I am waiting for a step-bit to arrive (should be this week) from Harbor Freight. Shipping from them has proved to be terrible/slow/undetermined. That’s probably the price to pay for cheap tools. Hopefully I will not pay a further price by the step-bits turning out to be fragile/wrong/useless. I’ve never used a step-bit before, let alone on steel, so for all I know this could be disastrous.

2x4 spacers

2x4 spacers helped hold the chassis when flat.


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