Monthly Archives: March 2010

Drilling the chassis …again

The last post left off at the point where I had finished drilling all the 1/8 inch pilot holes in the chassis and my next task was to use my newly arrived step-bits to further enlarge the holes to the sizes indicated on the drill-chart. I decided to start with the larger of the step-bits, to enlarge the 9-pin tube socket holes. These needed to be 7/8 inch in diameter, so I took note of the appropriate markings on the step-bit. They weren’t exactly very clear as they were offset from what I knew to be the correct one (I measured with vernier calipers). Once I knew the depth at which I had to drill, I set the depth-stop on my drill. I did not want to accidentally make the holes too wide by going one step too much.

Clamped chassis, starting to drill.

Clamped chassis, starting to drill.

Drilling with a step bit is a noisy and violent act. Even with the chassis clamped down to the bed, and with lubricant, it was a disturbing affair. After referring to those in the know (the Hoffman Amplifiers forum) it would appear that I was either drilling to fast (rotational speed of the bit) or feeding too quickly (pulling down too hard on the press). With hindsight I was doing both, and probably just due to impatience. So, I committed two terrible sins here…I rushed the job, and didn’t really listen to advice. Nice move, Simon.

Step-bit drilling, done badly.

Step-bit drilling, done badly. The underside of the 8 and 9 pin tube socket holes.

The upshot was that I had some clean up to do. As you can see from some of the close-ups, there was an amazing amount of burring on the underside of the holes. I used my nifty Dremel mototool, with a cylindrical grinder bit, to get rid of the burring. While holding it almost like a pen/pencil I was able to ‘scrub’ the burrs away quite easily and quickly.

De-burred chassis

The chassis, after be-burring with the Dremel.

The next step is to finish cutting out the square hole for the AC power socket. I want to ensure that I measure a real one first before I start cutting into the chassis. To do this I need to procure one, so I’ve ordered some actual parts (potentiometers, fuse holder, light holder, tube sockets, switches etc). Once done cutting I can “offer up” parts and make sure the hole sizes are correct.  This  will also allow me to start amassing all the various fasteners that I’m going to need. Ace Hardware, which is 2 minutes walk from my house, will be seeing lots of me this weekend.

Good things:

The amp is really starting to take shape: The drill-chart implies there’s lots of complexity, but when you actually think of which parts go where it’s much much simpler. With the holes all drilled I can clearly see where everything goes.

Bad things:

Pilot holes: I drilled too many pilot holes. Have a look around the tube-socket locations. It wasn’t until I was sourcing the sockets themselves that I realised nobody actually makes a socket that requires 4 holes in the chassis. The drill-chart said 4, so I did 4. It is my fault for not checking, so that makes me an idiot.

Step-bits: I wasn’t impressed. Next time I will have to do two things 1) Feed slower 2) Slower drill rotation. This really wont be possible on my drill-press as it only goe as slow as 650 rpm. A formula posited by a Hoffman forum member goes as follows:

I use an empirical formula Rpm = 200 / Dia, so for 1/2″ => 200 / .5 = 400 rpm for HSS bits in standard steel. Better bits such as carbide = 800 rpm, lower quality = 200 rpm. Aluminum alloys: Rpm = 800 / Dia, 1/2″ => 800 / .5 = 1600 rpm.

This would be 200/1.0625 = 188 rpm (for the 1 and 1/16th holes). Impossible on my drill. And in fact, I don’t know how I’d do this by hand with a hand operated electric drill. If I am going to do this more regularly I’ll either get chassis punches, or use aluminium chassis’ (or both). I’m betting that an aluminium chassis WILL be far easier with step-bits.

The two step-bits hanging out before clocking in.

The two step-bits hanging out before clocking in. Note the largely unreadable sizing on the inside of the cutting face.

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Bias circuit capacitors

Bias circuit caps

Here's the new capacitors from the bias circuit. You can also see the 'pegged' bias pot. I'll be replacing that too.

Next on my list of ‘replace old components’ are the electrolytic capacitors present in the bias circuit. Doing this was an ENORMOUS pain in the arse as the PCB in the 2204 is by its very nature, soldered underneath the board. This meant I had to un-bolt the board from its 6 posts (luckily I had a nice screwdriver/wrench for this) and attempt to lift it away from the chassis. Clearly this had never ever been done before and I’m sure the board was first bolted in and then wired up to the tubes/pots/power. Marshall did not have the foresight to make the wires long enough to be able to lift the board and work on the underside. I made a note (just with a piece of paper) of the wires emanating from the edge of the board, and where they were soldered to. This meant I could de-solder a few of them, thus providing better access. It only took a few lengths of wire before I could turn the board over enough to get access to the bottom.

Once done, I hooked everything back up and fired her up. All was well, so I went ahead and checked the bias, which hadn’t changed at all. You can see in this photo the bias adjustment pot at the end of it’s track, so to speak. I will be checking the value of this in the future, and perhaps replacing it.

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.