So do this at your own risk.
You will need:
Jacks £1.79 x 2 = £3.58 Matrix board £1.69 7w resistor 0.59 3w resistor 0.55 Altoids tin 0.90 Gym membership card - stolen Tools - already have some
£7.31 total, mostly from Maplin, easy to find stuff, that’s around 10 bucks. You could spend 100 dollars on something that does the same thing, but that’s no fun. It's a simple project, you'll make 10 solder joints, which isn’t too much work. Leaving out the chassis ground saves you 2 if you’re in a hurry.
The whole project took me a morning to build. Drilling was the slowest part for me, it helps if you have good tools. I had a small drill, a soldering iron, some wire, a multi-meter and a pair of pliers.
According to the attenuator calculator we need a 5.1 ohm 7w resistor in series with the speaker, and a 4.4 ohm 3w resistor in parallel for a 9db output cut to 8 ohm speakers. The values I ended up using were 5.6 and 4.7 ohms, the closest I could find at Malins. For a bigger amp, you’d need bigger resistors with the same vaules, a bigger tin and some way of getting rid of the heat. For a smaller amp, the extra beefy resistors won’t make a difference. A 5w amp like the Princeton will put out about a watt through this attenuator. If you're working with something like a Champ or an Orange AD5 find out the speaker impedance and do your own calculations.
Step one. Take mints and dispose of. Feed to dog.
You’ll need to drill holes in the tin for the input jacks. The Altoids tin is curiously strong, I ended up enlarging the hole by filling it away. Took forever. You’ll also need to make space so the lid can close, but this is easier to do once you have the jacks in.
Next, cut the matrix board to fit the tin and populate it with the resistors. See the annotated layout picture, this should give you the idea. I originally had a capacitor in the circuit to add a bit of treble, but it didn't help the sound audibly so I took it out.
Wire up the board to the jacks and the chassis ground. The wires should hold the board in place.
Plug it in, and turn up the amp. Enjoy.
Oh, one last thing. Label your handiwork, so you always plug the right end into the right place, and no one picks up your box and mistakes it for a tin of sweets.