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 inpedance 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, 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 get nervous when trying to state power handling. 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, somethings 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 voltage, but if you go lower in volume with an 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, I suppose I should, like that old joke about how do they know the weight limit of bridge overpasses? They drive heavier and heavier trucks over it until the bridge collapses. In this case I would keep running the attenuator with a bigger and bigger amp until it burns up.
I have a variety of amps from about 10 watts rms to 100 watts rms, maybe I'll test it. In 4 ohm mode, I would put the top end RMS wattage at about 35 watts, and in 8 ohm mode, about 25 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.
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 JTM-45
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.
Maybe, but I doubt it:
Vox AC-30
Any 40 watt RMS head or combo
Fender Pro Reverb
Fender Vibrolux
Fender Vibroverb
These amps are safe to use with this attenuator:
Marshall 18 watt or equivalent
Vox AC-15
Any amp with a single power tube
Fender Tweed Champ
Fender Blackface Champ
Fender Deluxe Reverb
Fender Princeton
Fender Tweed Deluxe
Fender Tweed Bandmaster
Epiphone Valve Junior
Blackheart Little Giant
Gibson GA-5
Build notes:
The impedance switch calls for a DPST switch, but a DPDT switch can be used.
A hole saw cut the fan hole, and a Dremel tool shaped the jackplate hole.
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.
