4/8 ohm Guitar Amp Attenuator
Here is another guitar amp attenuator, it is the same circuit as my Havanatone cigar box attenuator. I had a can of metallic copper paint in my shop which I used to finish it and thus dubbed it the Coppertone.
This attenuator is not as unique looking as others I've built, but is probably the most practical. Unlike my other attenuator projects the switches here are on the front, and only the in and out jacks are in the back.
The case is an old computer power supply case I had salvaged. These are of great use to DIY hobbyists, they contain all kinds of stuff we can use: Lots of wire, heat sinks, capacitors, 12 volt fans, and the case itself. I have several of these lying around from computers I've junked through the years. You could probably go to a computer repair shop and they might give you any burned up ones they have for free.
The case has a built in fan mount and vents so I didn't have to cut holes for them. The other holes weren't usable for the jacks and switches needed for the project so I cut pieces of scrap aluminum to install over the old holes in front and back and re-drilled for what I needed.
I also tried with success something I've wanted to do in the past, and that is ventilate the inner gang on the L-Pad. I've been hesitant in the past to do this because you have to disassemble the L-Pad completely to modify the metal cover and that's a tricky operation. You can't permanently remove the cover because it's needed to hold the L-Pad together. I carefully took it apart, bent the tabs and removed the cover, then used a cutting wheel on my Dremel tool to cut out a section from the top and bottom of the it to allow air from the fan to wash over the inner coils and keep them cool. After reassembling I put some superglue gel on the tabs to counteract any weakness cause by the removal of those sections of the metal casing. Venting it like this will keep it cooler, allowing the use of a higher power amp then in previous builds when using the 4 ohm setting. As in my Havanatone, this unit uses a dual ganged L-Pad that has two 8 ohm sections. One of them is switched in and out to achieve 4 or 8 ohms, and consequently the 4 ohm setting will take more power because it is using both sections in parallel.
I went back to using a switch for the bright cap rather than hard wiring it. It is a slight difference but still noticeable and I had room for the it, so what the heck, I added a switch for it.
Other than those changes and the layout differences it is the same as
In keeping with the Coppertone theme I labeled the switches thus:
- 4/8 switchable impedance
- Bright switch
- True bypass
- Fan cooled
- Shorting jack for the speaker output to protect against amp damage
- DC jack for powering the fan
- UV4: 4 ohm impedance
- UV8: 8 ohm impedance
- Lighter: Bright cap engaged
- Darker: Bright cap disabled
- Shade: Attenuator engaged
- Sun: Attenuator bypassed
Here are some top and bottom shots of two L-Pad's. In each picture, on the left is one I modded to remove some of the casing for ventilation, on the right is an un-modded one.
Here are a couple more shots of my Coppertone Attenuator.
I want to emphasize that if you build one of these you should put a fan in it! It makes a huge difference with how cool they run!!
With small amps 10 watts or less you could probably get by without it but even so it's a good idea to keep things cool with a fan.
Other guitar amp attenuators I have built:
The Ampwell House:
Built in a coffee can, good to the last watt!
With details about how L-Pads work.
Built in a cigar box, with smokin tone!
Schematic here, same circuit as the Coppertone!
A high octane guitar amp attenuator!
Built in an Altoids tin, a curiously small attenuator!
The Bohemian Rhapsodyne:
A 16 ohm, stone cold crazy attenuator for the AC30!