How to Layout and Build a Guitar Amplifier Chassis

In this article we cover how to lay out and build the chassis for a vacuum tube guitar amplifier. While this article is intended primarily for the "homebrew" builder, it will also be useful background for the kit builder. With a few essential tools and proper procedures, you'll find that building a vacuum tube guitar amplifier on a metal chassis to be a relatively simple matter. If you're familiar with woodworking you are likely to know the adage "measure twice, cut once." Similarly, much trouble and energy will be saved by spending sufficient time in planning the chassis job. When all the details are worked out beforehand the actual construction will be greatly simplified.

Of critical importance in chassis layout is thinking ahead to the space required inside the chassis by the items mounted on the chassis top and side panels, so that all components and electronics have sufficient space. The steps shown here illustrate principles and techniques that can apply to any chassis layout. The specific needs of your project will likely differ from the project shown below, but you can apply these techniques to accomplish your project needs.

MATERIALS

Aluminum is the preferred chassis metal as it is easy to work and provides good chassis strength with low weight. It also does not conduct magnetic fields so it avoids causing any magnetic coupling between transformers and choke. The downside of aluminum is when it is not strong enough for heavy transformers or other components. In such cases steel may be an option, but then greater attention must be given to magnetic leakage, and the potential for oscillations due to feedback. Faceplate material is primarily an aesthetic consideration. Any material which is thin enough to allow the mounting of components and will hold the needed markings can work. The examples below use an aluminum chassis with a brass faceplate.

LAYOUT CONSIDERATIONS

There are many tried and true conventions for the layout of guitar amplifiers that have served the amp building community well. These are by no means the only way to successfully layout a design, but beginners are particularly encouraged to follow these conventions. All layout decisions are trade-offs and simultaneously achieving all of them can be a challenge.

• Power Supply components, particularly the power transformer (PT) and rectifier, are best placed at the far end of the chassis.
• Pre-Amp Section components are placed at the opposite far end of the chassis, away from the power supply. This is done to minimize the pickup of stray magnetic-induced hum in the small signals present in the preamplifier section.
• High Power Tubes for the audio output section and rectifiers are placed to be closest to the cabinet ventilation holes or cooling fans. This is to maximize the cooling and operating life of these tubes.
• Filter Capacitors are positioned away from sources of heat to extend their life and reliability.
• Audio Output Transformer and output tubes are positioned away from the pre-amplifier to prevent the stray magnetic fields from inducing positive feedback, which could cause ringing or oscillation. Further, the core of the output transformer (OT) needs to be oriented at 90 degrees (perpendicular) to the power transformer to minimize any magnetic interaction between the two. Similarly, the power supply choke, also a magnetic component, should be oriented at 90 degrees to the output transformer.
• Other Vacuum Tubes should be placed with ventilation and cooling in mind, and in order of signal flow from preamplifier to power section. This will minimize the length of wiring and the effects of stray signals and noise.

Valuable lessons can be learned by studying the layouts of successful amplifier designs by manufacturers such as Fender, Marshall and others. For example here is a Fender Bassman chassis using a layout that places the tubes on the large surface of the chassis. This is similar to what is used in a separate amplifier head, or in the case of the Bassman, the tubes are mounted upside down.

This next chassis example is of the type seen in a Fender Tweed Deluxe or other small combo amps. In this configuration the tubes are mounted on one of the narrow panels of the chassis. However the layout considerations are the same.

You'll see these principles reflected in the layout of our demonstration project below, which was the chassis for a Tweed 5F1/F2 style amplifier. OK, let's get started with the layout.

GATHER AND MEASURE COMPONENTS

First you'll want to gather all of the mounted components for measuring (Photo 1). Everything, including transformers, has to be considered so that you can lay this out without conflicting real estate.

Photo 1 - Gather and measure the components to be chassis mounted

Measure (Photo 2) and note the mounting hole required for each item. I usually use a digital caliper for this, but a ruler is also fine. The point is reasonable accuracy. Also note any non-round holes needed.

Photo 2 - Measuring with a caliper.

LAYOUT FRONT AND REAR CHASSIS PANELS

Next you'll draw your layout on grid paper in 1:1 sizing, meaning that one inch on the drawing equals one inch on the design (full-size). Use grid paper that is at least as large as the chassis panels so you can work in full size. Four squares to an inch is a good size, but it's up to you. Layout the holes and attachments to the chassis. Allow room for the size of the component beyond the mounting hole. For example, potentiometers take up much more panel space than just the mounting hole. Circle-drawing templates (Photo 3) come in handy here, but you can also use a drawing compass.

Photo 3 - Using a circle template

MAKING THE FRONT PANEL AND FACEPLATE

With our layout completed on grid paper we can begin work on the faceplate. The upper layout in Photo 4 is the design for the faceplate.

Photo 4 - Completed design of faceplate and rear panel

As shown in Photo 5, I use the chassis to outline the faceplate size on my piece of brass stock.

Photo 5 - Outline chassis on brass stock for faceplate sizing

I then cut (Photo 6) the brass faceplate to size with a jig saw.

Photo 6 - Cutting the faceplate to size

With the faceplate cut to size, I then wrap it (Photo 7) with my grid paper design for the faceplate, attaching the layout sheet to the faceplate blank. A brief side note here - You may notice the wire in my left hand, which goes to the 35mm camera I used to shoot these photos. I did all of the photography in this essay myself including the ones showing one or both of my hands, so the wire was a necessity.

Photo 7 - Layout sheet attached to the faceplate

I apply a layer of clear packing tape (Photo 8) over any paper I am going to drill through. It prevents shredding of the the paper template.

Photo 8 - Applying packing tape to the faceplate layout

Attach the faceplate to the front of the chassis (Photo 9). This allows us to drill both at the same time and have them match perfectly. Check to ensure that faceplate and chassis are properly aligned and then you are ready for drilling.

Photo 9 - Attach faceplate with layout to chassis before drilling

Drill small pilot holes first (Photo 10), through both the faceplate and the chassis. I always use a smaller bit to make pilot holes to keep the larger or stepper bits from "walking".

Photo 10 - Drill pilot holes

With the pilot holes completed (Photo 11) you can see how our earlier step of covering with clear packing tape has kept the layout intact.

Photo 11 - Pilot holes completed

By being drilled together, of course the faceplate and chassis are a perfect match (Photo 12). Normally I keep them layout and faceplate on the chassis until all of the holes are completed. However in this project I removed the faceplate at this point because I expected to make some changes and didn't want to waste the faceplate.

Photo 12 - The faceplate and chassis are a perfect match

I like to use stepper bits (Photo 13) because they're the quickest way to enlarge the front panel holes. If you don't have stepper bits you can use individually sized drill bits.

Photo 13 - Stepper bits

Now drill out all holes to the full size (Photo 14) needed for each component using the pilot holes as the centers. From time to time I wipe the stepper bits light machine oil on a cloth to make the drilling a little easier.

Photo 14 - Drill out each hole to full size

Check for fit as you go (Photos 15, 16 & 17). Keep all of the parts handy to check. If you drill the faceplate separately it may give you a second chance should you "goof" with the chassis itself at this point.

Photo 15 - Checking holes for fit

Photo 16 - Hole size may vary among manufacturers

Photo 17 - Checking pilot light fit

MAKING THE BACK PANEL

To start the back panel I attach (Photo 18) a 1:1 copy of the rear panel sheet to the rear of the chassis, and then drill my pilot holes.

Photo 18 - Attach layout and drill pilot holes

Using the stepper bits again, enlarge the holes to size (Photo 19).

Photo 19 - Enlarging the fuse and AC cord holes

Some holes, such as for the power cord strain relief, may need a non-round hole. A common plastic strain relief (Photo 20) requires a D-shaped hole.

Photo 20 - The power cord strain relief is D-shaped

To turn our round hole into a "D" shape we use a nibbler metal cutting tool (Photo 21). The round hole is nibbled out to form the "D". You can find a nibbling tool at suppliers such as Radio Shack.

Photo 21 - Using the nibbler tool

With two of the hole corners squared we are left with a D-shaped hole (Photo 22). Check for fit with the strain relief.

Photo 22 - Finished "D-shaped" hole

The tube sockets (Photo 23) require larger holes than can be made with common drill bits.

Photo 23 - Tube sockets require larger holes

A Greenlee chassis punch (Photo 24) makes a large, burr-free hole. Shown here is the large 1.0625" punch for the octal sockets.

Photo 24 - Greenlee chassis punch

Place the cutting edge inside (Photo 25) the chassis with the die on the opposite side. Feed the stud through the die and chassis hole and thread into the die. Tighten the bolt head on the stud until the die has been pulled clear through and cut the metal chassis.

Photo 25 - Chassis punch in place to cut

When the hole is completed just pull out the punch and you should be left with a precise, burr-free hole.

Photo 26 - Hole completed

Now use a 3/4" punch for the smaller 9-pin tube sockets.

Photo 27 - A 3/4" punch is used for the smaller tube sockets.

Tighten the stud (Photo 28) until the hole is cut clean through.

Photo 28 - Cutting the 3/4" hole

Now test fit all of the components for the punched holes.

Photo 29 - Test fitting the tube sockets

CHASSIS INTERIOR

Next you'll mount any components going inside the chassis, usually a circuit board. First, Hard Money Property recommends you mark the location (Photo 30) for the board mounting holes on the inside of the chassis, taking care to allow enough room around the board for any components or wiring to come.

Photo 30 - Mark the circuit board location

Drill the mounting holes to size (Photo 31). Rest the chassis on some scrap wood so that the chassis top doesn't dent due to the drill's pressure.

Photo 31 - Drilling where marked

We'll use standoffs (Photo 32) to mount the board to the chassis. The standoffs provide the space to keep the circuit connections from touching the chassis and shorting out. Standoffs are readily available from Radio Shack and other sources.

Photo 32 - Standoffs

Insert screws from the chassis top through to the inside and thread into the standoffs until tight (Photo 33).

Photo 33 - Standoffs in place

Test fit the circuit board on the standoffs (Photo 34). This is where the board will go AFTER you stuff the board with the components.

Photo 34 - Board in place

OUTPUT TRANSFORMER

Select the location for the output transformer making sure there is no conflict of space with other components and in accordance with the layout considerations given at the beginning of this article. Hand Made Mind Made recommends that you mark holes (Photo 35) for the transformer mounting screws and for the wire pass-through hole that will be grommeted.

Photo 35 - Marking holes for output transformer

Drill the transformer mounting holes and for the transformer wires (Photo 36).

Photo 36 - Drilling transformer mounting and pass-through holes

Install the rubber grommet in the pass-through hole. Test fit the transformer and wires (Photo 37).

Photo 37 - Test fit the output transformer

POWER TRANSFORMER

Hopefully your initial planning addressed the location of the power transformer and you have a space on your chassis where there is enough room and you are consistent with the layout considerations above. Depending on the type of transformer used, the steps may be the same as above for the output transformer. However if you're using a "laydown" type of power transformer, as I do in this project, there are some extra steps required. The laydown refers to the transformer being mounted on its side, instead of "standing up." To make this work a rectangular hole is needed in the chassis to allow space for part of the transformer windings to fit down into the chassis. You'll want to start with a template of the transformer as shown in Photo 38. This includes the mounting hole and the cutout for the transformer windings. As before, I mount the template to the chassis and use clear packing tape to hold it in place and not shred when I cut or drill into it.

Photo 38 - Power transformer template

First we'll work on the cutout. Drill small pilot holes for guides, and then drill larger holes (Photo 39) in each corner of the cutout. These holes need to be large enough for our jig saw blade to fit.

Photo 39 - Drilling corner holes for the cutout

Next I use a jig saw (Photo 40) with a metal cutting blade to connect the holes along the straight template lines.

Photo 40 - Using a jig saw to connect the holes

With the cutout finished (Photo 41) you'll want to test fit the transformer.

Photo 41 - Power transformer cutout finished

Now drill the power transformer mounting holes (Photo 42)

Photo 42 - Drilling the power transformer mounting holes

Use a metal file or sand paper to remove any rough edges or burrs, and you're done (Photo 43).

Photo 43 - The completed power transformer mount.

TUBE SOCKETS

Test fit a tube socket, make marks for the mounting holes, and drill to size (Photo 44).

Photo 44 - Drilling the tube socket mounting holes

To attach the tube sockets I prefer to use 1/8" pop rivets (Photo 45) instead of nuts and bolts. The rivets are quick, clean and won't loosen over time. The rivets and tool are widely available at hardware stores.

Photo 45 - 1/8" pop rivets

I like to dry fit both rivets (Photo 46) before attaching either, so the second hole doesn't become misaligned during compression of the first rivet. Go ahead and complete all rivets.

Photo 46 - Riveting a tube socket

The finished sockets look neat and professional (Photo 47). By the way, you can also see in this photo that I've attached ground straps that I'll use during the build for an easy way to solder to ground. Depending on your project, you may also want to do this. The location is not critical.

Photo 47 - Tube sockets in place

WIRING AND COMPLETION

When all of the chassis mechanical work has been completed you'll want to do your electronics work and wire everything together. Wiring to the switches and potentiometers inside the chassis can be a bit of a challenge, as the access angle is a bit awkward. A handy tip here is to mount your front panel parts temporarily to the faceplate, without the chassis. Then do all of your front panel wiring with the easy access of just the template. When you're done, dismount the components from the faceplate and move them in one group inside the chassis and insert them in the corresponding chassis holes.

Then place the faceplate in position and thread and tighten the mounting bolts. NOTE: this should only be done for those components that mount from the rear. A typical pilot lamp, for example, mounts from the front. If you attach it to the faceplate and connect the wires you won't be able to remove it. So leave any front mounted components until final assembly.

With everything mounted in place the project looks very neat, organized and professional in both the front (Photo 48) and rear (Photo 49) views. The faceplate was still at the engraver when took these photos so it is not shown.

Photo 48 - Completed project, except for faceplate - front view.

Photo 49 - Completed - The extra hole was a "goof"!