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Making a Tube Amplifier Chassis That Survives the Heat

September 24, 2026

A valve amplifier chassis has a set of problems a solid state chassis does not: real heat over a long period, real weight in specific places, exposed mains and high tension voltages, and a top plate that is both the structure and the thing people look at.

Layout drives the metalwork

Before any of the machining questions, the layout decides most of the cost.

The usual arrangement, for good reasons:

Power transformer at one end, as far as possible from the input valves. Its field is the loudest thing in the box.
Output transformers near the output valves, keeping the high current secondary wiring short.
Input valves at the opposite end, away from the power supply.
The rectifier and smoothing near the power transformer.

That layout puts the three heaviest objects in the design at three corners of the top plate, which is why the top plate is a structural component rather than a lid.

Material and thickness

Part	Material	Typical thickness	Why
Top plate	Aluminum 6061	3 to 5 mm	Carries transformers and sockets, stays flat
Top plate	Steel	1.5 to 2 mm	Stiffer per millimetre, some magnetic shielding
Chassis sides or apron	Aluminum 5052 folded	1.5 to 2 mm	Cheap, light, not structural
Bottom cover	Perforated steel or aluminum	1 to 1.5 mm	Ventilation plus safety barrier
Transformer bells or covers	Steel	1 mm	Appearance and magnetic containment

A frequent mistake is a 2 mm aluminum top plate on an amp with two transformers and four valves. It will sag visibly between the mounting points within a year, and it will already flex when somebody changes a valve.

Socket cutouts

Valve socket holes are the signature feature of the part and the one most likely to be drawn badly.

Practical rules:

Dimension every socket cutout and its screw holes from a single datum, usually one corner or the plate centre, not chained from one socket to the next. Chaining accumulates error and leaves the last socket visibly out of line.
The cutout is clearance, the screws are position. Give the cutout a generous tolerance and the screw holes a tight one.
Decide the socket orientation on the drawing. Sockets have a keyway, and if the orientation is not specified, the assembler picks it, and the wiring dress changes between units.
Deburr both sides. The underside burr on a socket hole is where a wiring loom gets cut during assembly, and nobody sees it until the amp is finished.

Common cutout sizes, which vary by manufacturer and must be checked against the actual socket drawing:

Socket	Typical panel cutout
Noval, 9 pin (B9A)	22 mm diameter, chassis mount
Octal, 8 pin	30 mm diameter, chassis mount
Small 7 pin (B7G)	18 mm diameter

Those are starting figures. Ceramic and PCB mount sockets differ, and the mounting screw spacing varies more than the hole does. Always confirm against the drawing for the exact part you are buying.

Heat

Valves dissipate a lot and they do it continuously. The metalwork has to let that out without the chassis becoming the heatsink.

Ventilation above and below. Convection needs an inlet and an outlet. Holes only in the bottom cover do nothing.
Keep the pattern as round holes rather than long slots where EMI matters, and keep the open area meaningful. A decorative perforation that is 5 percent open area is decoration, not ventilation.
Keep electrolytic capacitors away from the valves. This is a layout decision with a metalwork consequence: the capacitor mounting holes go at the cool end.
Expect the top plate to run hot enough to be uncomfortable to touch. That is normal for valve gear and it changes the finish decision, because a hot anodized plate is fine and a hot painted plate can discolour over years.

Safety, which is a manufacturing requirement

Mains and high tension in an open topped chassis makes some of this non-negotiable rather than a design preference.

An earth point that is a dedicated stud or bolt, not a random screw shared with something else. Bare metal landing around it, masked before anodizing.
A bottom cover that requires a tool to remove. Not magnets, not a slide fit.
Adequate creepage and clearance around mains terminals and the transformer primaries, which affects where you can put mounting holes and how close a folded edge can come.
No sharp edges anywhere a hand goes during assembly or servicing. Break every edge, including the inside of cutouts.
Valve guards or cages if the product is sold where exposed hot glass is a concern. That is a separate metal part with its own drawing.

If the product is going to market rather than being a one off, the applicable safety standard will set specific numbers, and those numbers change the metalwork. Get them before the chassis is drawn, not after.

Finish

Top plates take heat and get touched. In order of durability for a valve amp:

Anodize on aluminum. Best combination of heat tolerance, hardness and appearance. Black shows fingerprints and dust from the ventilation, so a bead blasted mid tone often wears better in real use.
Powder coat on steel. Durable, thick, hides the sheet surface. Softens machined detail, which rarely matters on a valve chassis.
Plated steel, nickel or chrome. The traditional look. Shows every fingerprint and needs the surface prepared properly before plating.
Paint. Cheapest, least durable, and the one most likely to change colour above a hot valve over several years.

Drawing checklist

Top plate material, thickness, and flatness over the full length
Every cutout and screw hole dimensioned from one datum
Socket orientation
Transformer mounting hole positions and the bolt size, with the weight noted
Ventilation pattern with individual hole size and total open area
Earth stud location, thread and the masked bare landing
Edge breaks on every edge including inside cutouts, both sides
Which faces are cosmetic
Masking before finishing, for the earth point and any joint that must conduct

Frequently asked questions

Steel or aluminum for a tube amp chassis?

Steel for the top plate if the amp is heavy and uses an EI power transformer, because it carries the weight without flexing and gives some magnetic shielding. Aluminum for everything else, because it is lighter, machines faster and anodizes. Plenty of good amps are all aluminum with a thicker top plate, typically 3 mm or more.

How thick does the top plate need to be?

It carries the transformers and the valve sockets, so it is a structural part. In aluminum, 3 mm is a practical minimum for a small amp and 4 to 5 mm is normal once a mains transformer and an output transformer are bolted to it. In steel, 1.5 to 2 mm does the same job.

Do socket cutouts need to be that accurate?

The hole itself is forgiving, usually a clearance around the socket body. The mounting screw positions are not, because a socket that sits at an angle puts side load on the pins every time a valve is changed. Position the screw holes relative to the cutout centre, not to a panel edge.

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This guide is part of Audio Chassis Manufacturing: The Whole Picture, one of four sections in the guide library.