Here is another guitar amp attenuator in a unique enclosure. This one is for my friend Geoff for use with his Victoria Tweed Bandmaster, and his Silverface Princeton. Geoff used to be a cigar smoker and gave me the box. The circuit is a slightly more complex version of my Ampwell House Attenuator. Instead of a single impedance it is switchable between 4/8 ohms. Instead of a single variable L-Pad, this one uses an 8 ohm dual ganged stereo L-Pad. For an 8 ohm load, one section is switched out and it acts just like the Ampwell House. For a 4 ohm load, both sections are used, connected in parallel. (See my Ampwell House Attenuator page for more information about L-Pads.) It's very easy to build, see the links below for a diagram, schematic, and parts list. Of course, you don't have to use a cigar box, use your imagination to come up with something interesting, or just use a commercial enclosure.
In the Ampwell House, the treble boost cap was wired to a switch, but the treble boost was very slight. In this version, I just wired the cap across the terminals of the bypass switch. That reduced the number of switches cluttering up the jack panel. The power jack is only used for the fan. When in use you just open the lid to let the air exhaust out.
Regarding power handling: I am very conservative when trying to state power handling for my attenuators. The best way to define it is this: The attenuator should be able to handle a dimed amp when dialed all the way down to whisper quiet volume. In any attenuator, the excess voltage is dissipated as heat. If you can't dissipate the heat quickly enough, something is going to burn up. You have to keep it cool to maintain the power rating, thus the fan. With small single ended amps, the fan probably isn't needed, but I use it anyway. With larger amps, it might work if you just keep it dialed down a bit to shave off some of the horsepower, because you aren't dissipating as much power, but at lower attenuator settings with a larger amp like that, you are asking for trouble. I wouldn't do it.
I've never really pushed one of these DIY attenuators to the limit. In 4 ohm mode, I would put the top end RMS wattage at about 40 watts RMS, and in 8 ohm mode, around 30 watts RMS. The 4 ohm mode will take a little more power because you are using both sections of the L-Pad in parallel, dissipating more heat. At 8 ohms it is only using one section. I run my AC30CC (32 watts RMS) dimed into a single section attenuator, and it works just fine. This is assuming you are running a fan at all times for all of this.
Don't be fooled by the 100 watt rating these Parts Express L-Pads have, that is for when you are using them in a hi fi system with a solid state amp, after a crossover.
If you build one of theseI strongly recommend putting a fan in it.
Do not use these amps with this device:
Any Fender Bassman head or combo.
Fender Twin Reverb
Fender Super Reverb (Wrong impedance)
Marshall 50 watt Plexi
Marshall 100 watt Plexi
Any 50 watt RMS head or combo
Any 100 watt RMS head or combo
Anything similar to the above.
These can be used with a fan running:
Vox AC-30 (8 or 16 ohms)
Any 40 watt RMS head or combo (4 ohms)
Fender Pro Reverb (4 ohms)
Fender Vibrolux (4 ohms)
These amps are safe to use with this attenuator:
Marshall 18 watt or equivalent
Any amp with a single power tube
Any amp with two 6V6 power tubes
Any amp with two EL84 power tubes
Fender Tweed Champ
Fender Blackface Champ
Fender Deluxe Reverb
Fender Tweed Deluxe
Fender Tweed Bandmaster
Epiphone Valve Junior
Blackheart Little Giant
Marshall Class 5
The metal covering of the rear section of the L-Pad was removed to help cooling.
The metal covering of the front section cannot be removed.Yes it can!