Speaker cabinet gurus, I need your advice

Ok, guys, I finally got my GM40 and when I first plugged it in, I gotta say, it sounded nasally and very thin. I could dial in the feel of a decent tone, but there was no bass or resonance to it at all. I honestly thought I had made HUGE mistake. However, after fiddling with the connections, I realized the issue: the impedance to the speaker cabinet was all kinds of wrong.

I have an old (late 80's - early 90's) Jackson J412 4x12 speaker cabinet that has no switching for the impedance. You can run it in (2x input) stereo at 8 Ohms per input or (2x input) mono at only 4 Ohms per input. I've always run my heads in stereo so there's never been an issue. However, the GM40 only has the one output, and the mono stage of the cabinet, plugged in to one of the two mono inputs, I only get straight AM radio tone. When I plug into one of the 8 Ohm inputs I get thick, heavy, warm tone for days. But, as should be obvious, the amp is only driving two speakers.

So, though I've been playing for 37 years now, I have what I am sure is the ultimate newb question: How do I run this speaker cabinet in mono with all 4 speakers going? Do I need some kind of adapter or maybe a Cab-Link? Help would be appreciated.

Picture of cabinet inputs:
h-t-t-p://jereth.dyndns.org/Linked/Jackson412SL.jpg

Hi Jereth,
Easiest way to solve your problem is to use Palmer Cab M merger or some other brands cab merger.
The other way to solve this out is rewiring the cabinet.

If you are running your cab at 8ohms per input then all you need to do is to run both Stereo inputs in series and they will add up to 16ohms with all speakers sharing the power equally.  It is easy with just a cheap standard jack socket, a couple of standard jack plugs and a bit of cable.  If you can solder that is!

I have attached a pic of the setup.  This only needs to be a short adapter piece so it needs only to be 3 or 4 inches long.  If you prefer you could replace the socket for your speaker cable with a longer length and another standard plug to go straight into the amp output.

You need to make sure you get the correct terminals for each plug to ensure the speakers are all kept in phase.  The +ve (tip) of the cable from the amp should start to the +ve (tip) of the first cab input.  The -ve (sleeve) of that plug is wired to the +ve (tip) of the second cab input.  The -ve (sleeve) of the second cab input goes back to the -ve (sleeve) of the amp.

You have now wired your two stereo inputs in series making 16ohms.  I would recommend if you are making a permanent version that you get your hands on some proper metal cased oversized speaker jacks.  Look at Neutrik Rean NYS225 type to see what I mean.  Also just use a piece of thick mains cable, there are no special "oxygen free" or "audio grade" cables.  Don't fall for any of that BS, it's just marketing hype!

I have a Marshall cab with 2x 16ohm modern G12Ms and 2x 16ohm original 1967 G12H which I rewired keeping its original single input and using a pair of slide switches mounted in a new modern handle (still have the original untouched of course).  I can set these to give me mono 4x at 4ohms, mono 4x at 16ohms, and either left or right pair of speakers at 8ohms.  Suits any amp I have.  If it would help I can show you the wiring for that too.

MAKE SURE TO SET YOUR CABINET TO STEREO IF YOU USE THIS APPROACH!

Gentlemen,

Thank you for the info. Your comments were very helpful. While I was waiting for an answer I ended up getting a Radial Cab-Link Speaker Cabinet Combiner from Amazon, which worked perfectly. I'm glad you both came to the same conclusion. Hooked up using the series circuit she screams like a banshee, wails like a widow, and, unlike when connected directly, growls like a lioness in heat. There's a good amount of grit, as there was before, but the warmth is there as well.

Much obliged.

UPDATE:

Ended up getting a B52 LG-412V 4x12 cab to add to the tower. The beauty of this model is that there is an input and an output, so I can run both in series.

B52 LG-412V (8Ω impedance), straight out to the Radial Cab-Link Speaker Cabinet Combiner, hooked up through the parallel circuit and out to both stereo inputs of the jackson speaker box (8Ω x 2 in parallel = 4Ω) for a total combined impedance of +/- 12Ω load at the H&K GM Deluxe 40. Sounds great and, despite what I was told would happen, I don't notice any obviously discernible difference in volume between the two. Together they compliment each other and give my tone a bit more depth than either gave, alone.