I like to experiment with "Do It Yourself" projects so even though I have three Sholtz Power Soaks I wanted to build a guitar amp attenuator from scratch. What got me thinking was a page on Ted Webers site about building an L-Pad attenuator with high power sand cast fixed resistors. I was going to go that route (Which I did, check out my Ampoline Attenuator!) when I saw a 100 watt variable L-Pad in my Parts Express catalog. With a few caveats it would work for a low powered amp. Since my intention was to use this with my Kay 720, a 20-22 watt amp, I went for it.
I always look for ways to do things cheaply and thought why not keep it cheap and have some fun too? So, I built the attenuator in a coffee can and called in the Ampwell House Attenuator. More about the can later, but let's talk about L-Pads.
L-Pads are primarily used in the Hi Fi speaker world and are most commonly used to balance a tweeters output to the rest of the speaker system. An L-Pad is a passive device which lets you control the output level of speakers without changing the impedance seen by the amplifier. Meaning, if you have an amp with an 8 ohm output the amp always thinks it sees an 8 ohm load, but the L-Pad is still attenuating the power getting to the speaker, thus reducing volume.
The L-Pad I use here is a variable resistor but as mentioned above you can build them using high power sand cast resistors. The Sholtz Power soak does this, and Ted Weber has some models that are like that as well.
The attenuated wattage is dissipated as heat. As long as the L-pad dissipates as heat the wattage it is absorbing, it will work ok. This L-pad I used is rated at 100 watts, but just looking at it it's obvious that if you plugged a dimed Super Lead into it and dialed it down to bedroom levels, mayhem, carnage, and sadness would result. Testing proves that the most power this attenuator can handle safely is 35 watts, maybe a little more.
The Expresso/Decaf switch is a bright switch which is useful because it adds a little crunch back in to the tone. Right now I am using a 4mf cap for it, in later versions I used up to 15mf. The treble bypass works like a bright switch on an amp: The less attenuation you have, the less it is effective, and the reverse: The more attenuation you have, the more effective. Even so, the difference is very subtle. You could eliminate this switch and hardwire the cap across the Light/Strong bypass switch to have it on all the time.
The Light/Strong switch is a full true bypass switch, it completely removes the attenuator from the signal path. I used a shorting jack for the output jack to protect the OT in case I forget to plug in the speaker. In order to keep things as cool as possible I mounted a fan in the bottom and drilled vent holes in the top to supply a constant air flow. I removed the casing from the L-Pad , exposing the coils to the airflow of the fan. It's just a matter of bending the tabs of the casing and pulling it off. The power jack is for the fan, it's a 12v fan but I drive it with an old 9v power supply I had in my junk box.
Preparing the can took a lot more time that the wiring portion of the project. I went through a couple of cans before being sure about where to mount the jacks, and drilling that thin metal was a drag because the metal tore rather than drilled clean. I had to do a lot of cleanup on the holes with my Dremel tool. I was fortunate to have a hole saw that cut a hole exactly the size of the can to make the bottom where the fan mounts, and used the Dremel tool again to shape the insert for the fan. Making the label took some thinking, I ended up designing it with my webpage software and used a lot of blue ink getting it right.
If you want to build one of these, you don't need to use a coffee can, you can use a commercial project box. Or, use your imagination and come up with something cool from around the house. Just make sure there is plenty of ventilation and a fan to provide cooling.
How does it sound? I have to say a tube amp sounds best without any attenuation at all, but I think as attenuators go it sounds good. Like all attenuators it sounds best when used sparingly, just shaving a little off the top so to speak. Dialed down low it sounds good too. It doesn't sound the same as it does un-attenuated, but it still sounds good. I don't think any attenuator will let your amp sound the same dialed low as it does full bore, whether it's this DIY Ampwell House of mine, or some expensive commercial product.
Below I've posted some good links on L-Pads, attenuators, and building your own, as well as the schematic and some more pics of my Ampwell House attenuator.