Marshall 1936 cab conversion to semi-open?

funkadelic

Hi. I'm thinking of modifying my Marshall 1936 cab. 212 Cab with Celestion G12T-75 and Vintage speakers. I'm thinking of converting the cab to a semi-open one similar to the one in the picture below. Anyone ever done this? The reason is to get a bit more sound dispersion on stage without loosing too much of the "bigger sound"(best way I can describe it). The head is jvm410hjs. I have also acquired a laney lionheart 20w head recently. Style of play is clean (like in jazz clean), blues and fusion/rock.

Gearhead

Maybe not such a great idea unless you make the cutout a round shape in the center. The depth of said 1936 cab is only 300mm, so the distance that the soundwaves have to travel around the back to even out the waves fom the front is only about 0.6m. From a wavelength of 1.2m you get calcellation, which is everything underneath 285Hz. This will kill the 'big' in your sound quickly. Not a big deal if you want to take up less 'space' in the mix for rock, but the jazz will just not sound the same.
On top of that, the cab will sound less 'tight' and more 'loose' which may or may not be what you like.

Why don't you get some birch ply 600 x 750, cut it to size with a smallish hole in the center and just screw it in instead of the closed back, without tolexing? If you're happy with the sound, cut the hole bigger just to see what changes. If you're still okay, tolex and move on. If not, you haven't cut up the closed back.

funkadelic

Thanks for the detailed answer. Much appreciated. I found a picture of an Avatar cab with the rounded cut. I'll experiment and see what happens.

PS: One of the reasons was that I found that the jvm410hjs sounds softer to me with the 212 cab than the Fender Superspnic 60w 112 amp I used to have in the same hall. The Fender volume was dialed to 4 whereas the JVM is midway for the master volume and channel volume. Treble 4, mid 6, bass 4, resonance 0, presence 5, and gain on 4. I don't use pedals with the amp.

ezietsman

funkadelic wrote: Thanks for the detailed answer. Much appreciated. I found a picture of an Avatar cab with the rounded cut. I'll experiment and see what happens.

PS: One of the reasons was that I found that the jvm410hjs sounds softer to me with the 212 cab than the Fender Superspnic 60w 112 amp I used to have in the same hall. The Fender volume was dialed to 4 whereas the JVM is midway for the master volume and channel volume. Treble 4, mid 6, bass 4, resonance 0, presence 5, and gain on 4. I don't use pedals with the amp.

That's a 100 watt head. You shouldn't be having volume issues, regardless of the cabinet you are using.