Inside the G-K MB112 Combo

[tstone]

I've been thinking lately about building a very light, compact 1x12 cabinet for amplified upright bass. A friend recently purchased a G-K MB112 combo , which is quite light (28 lbs) and sounds very good. With my friend's blessing, I took it apart this weekend to see how it was put together, and see if there were any ideas I could borrow.

The ceramic magnet speaker is no lightweight -- 10.5 lbs! If they'd used a Neo speaker they could have knocked off another 5 lbs. DC resistance Re is 4.2 ohms, in between what you'd normally expect from a 4 ohm or 8 ohm speaker. Presumably a custom speaker built to G-K's specs. No gasket between the front-mounted speaker and the baffle, but the baffle front is covered with Tolex, so it probably seals pretty well.

Cabinet is 12 mm plywood, birch or something similar. No cleats or bracing of any kind. Because of the copious glue squeeze-out, I couldn't tell if the joints were rabbeted or just butted. There are 1/4" thick strips applied in strategic locations so that from the outside, the cab appears to be made of 3/4" material.

I didn't remove the amp chassis, but looking at the comparable MB heads, I'm guessing it weighs around three or four pounds max. The perforated metal grille adds another pound.

Cabinet net inside volume is around 44 liters not including port volume or speaker displaced volume, which I estimate at around 2 liters.

There is one large shelf port, the full width of the cabinet less one 12 mm divider on the centerline. Port is 1.81" tall and 7.13" deep, for a port volume is 2.8 liters.

Here's the most interesting part: Using an external hi-fi amplifier, I measured the speaker impedance in the cabinet over a range of frequencies. I found the minimum impedance -- and thus the port resonant frequency -- to be at around 62 Hz. I am somewhat surprised that they chose to tune the port to such a high frequency, it's not like they didn't have room for a larger port. I don't know the detailed characteristics of the MB112 speaker, but I would guess there's quite a hump in the system frequency response plot with the port frequency that high. And presumably they are using some high pass filtering in the amp to protect the cone on the really low notes.

Can't argue with the final result in any case -- it's a fine-sounding little combo. I'll have it apart for another 24 hours or so if anyone has any particular measurements or details they'd like information on.

[robgrow]

The amp in the GK MB112 is basically the same as the GK MB200. Same ICEpower module, etc. Otherwise it's just a (GK made) Paragon 12-inch speaker in a ported box.

[gmarcus]

I took apart an MB212 and MBE410. They both seem to have the same kind of construction. I'm pretty sure the joints are rabbited. 62 Hz is the 1st harmonic of the low B string so that tuning makes sense for a small cabinet as long as they have a good high pass filter in there, which I'm sure they do.

[B-string]

The speaker is most def. made to G-K's specs, G-K makes 99% of their speakers themselves.

[AMJBASS]

There is a fairly prominent low mid bump in these combos. I actually find it difficult to take out the boominess with upright. It sounds great with electric though.