DIY guitar amp attenuator

I got a new Valve Jr. combo amp for Christmas and it turns out that 5W is really way too loud for my ears and probably won't make me popular in the neighboring apartments. I saw mention of a "radio shack attenuator" here and there and eventually went and picked up one of these:

That's an in-wall volume control from Radio Shack, part number 40-993. They go for $18. Inside is an autotransformer with a primary coil that measures 5.3 Ohms and a secondary that is variable from 5.3 Ohms down to 0. It's rated at 25 watts so it should be fine for a 5 watt amp.

Disclaimer

I am not an expert with guitar amps or analog electronics. I have only basic knowledge of electrical concepts. Using the below design with different impedance or higher wattage amps may damage your amp or start a fire. There's a finite possibility that I have made a mistake and this will even damage the amp it is designed for (the Epiphone Valve Jr. combo w/ 4 Ohm speaker). Use this information at your own risk.

Analysis

Ok, so the first thing I did was just insert this between the speaker and the amp's output. This works and sounds fine. But 5.3 Ohms is a bit higher than the 4 Ohm load the amp is expecting. Adding a 20 Ohm resistor in parallel with the primary winding brings that down to close to 4 Ohms. Here's a schematic.

R4 should be a big power resistor. I chose two 10 Ohm, 10 watt resistors connected in series. You may be able to get away with a 20 Ohm, 10 watt resistor but be careful.

I ran a SPICE sim to see what the input impedance for this circuit is.



Dividing those two we see it's right about 4.3 Ohms. Close enough. This is really a guess since the inductance of this transformer isn't known to me.

Construction

These transformers drop right into the wall breakout boxes you would put a light switch in. There's enough room inside for a couple jacks and the power resistors. When you're done you have the attractive white and gray enclosure shown below.

Ok, it's ugly but it works. And for $25 it sounds pretty good.

Sound Sample

I recorded this with a computer headset at very low volumes in my bedroom. I didn't alter the volumes of the samples with anything other than the attenuator.

To hear the sound clip click here. Apologies for my amateur playing.

Final Warnings

1. It's unlikely this would be safe to use with anything but a 5 watt, 4 Ohm amplifier. My understanding is tube amps aren't forgiving when it comes to driving lower-than-expected loads. You could blow the amp's tranformer or anything else in there.


2. Careful with the jacks. If you screw up and plug the speaker in where the amp should be and vice versa bad things could happen. The output side of this circuit has very little impedance when the attenuation is at its maximum. Make sure you label the input (from amp) and output (to speaker) clearly.


3. Keep an eye on it. If it gets hot or you smell melting plastic you probably let the smoke out of something.