UTILITY: Speaker Cab Impedance / Speaker Switch

When I set out to build a proper test cabinet for amp building, I came across a great switch wiring setup that perfectly suited my needs. With the parts I had on hand, I was able to put together a versatile cab without risking damage to my favourite speaker cabs.  There are of course commercial products that achieve the same thing, and sometimes more.  But this is a DIY blog right.  

Why a test cab?  I already own several cabinets that I really like, however, I was a bit concerned about accidentally destroying a vintage speaker.  A dedicated test cab allows me to safely experiment with different speaker loads without worrying.

What You’ll Need

• A twin-speaker cabinet with 8-ohm speakers
• A 3-pole, 4-position rotary switch (Make sure it's a sturdy one, ideally rated at a couple of amps)

There are four switch positions, covering the most common loads:

• The first two switch positions isolate the individual speakers, providing an 8 ohm load.  Handy if you have two different speakers in a cab that you want to isolate.
• Position three is both speakers in series, providing a 4 ohm load
• Position four is both speakers in parallel, proving a 16 ohm load

I kind of rushed into throwing this together, and now I want this to rework this to put it in sequential order...  4, 8, 8, 16.    

A Word of Caution

WARNING: If you wire this incorrectly, you could fry your amp’s output transformer - a costly mistake.  Always triple-check your wiring with a multimeter before plugging in your amp.  If you're unsure or don’t have the tools to verify your work, do not attempt this.

4-WAY SPEAKER / IMPEDANCE SELECTOR

The wiring diagram I used was posted by YellowBoots on the Tele forum

TEST CAB

The cabinet I’m using is a late '70s Peavey Deuce combo, minus the amp.  It houses two 12-inch CTS speakers, which I think are rated at 50W to 75W each.

It's quite a well built twin 12 combo cab, proper ply etc instead of the awful stuff they use these days in most amps.  I have it sitting on wheels, so it's easy to drag around the workshop.

Yes, I will eventually label it properly, but for now, this will do.  

I gave the new test cab a run with this little AWA PA 774 amplifier.  I modded it to have a Fender style preamp with bass and treble controls, cause I only had three knobs to work with.  It's a cool little amp and sounded great through the cab.  As I'm not a fan of the look of the old peavy cabs, I pulled the aluminium strip and logo off the front.