: Can anybody answer a question for me? I own a Marshall JCM 600 (60 watts all tube) with two 12 inch speakers. The amp has an extension speaker output. The owners manual says that when the internal speakers are connected, the extension cabinet must be 16 ohms. I own two sunn speaker cabinets - both with two 15" JBLs. Does anybody know what the ohms rating is of these old speaker cabinets. One cabinet is a 200S cabinet and the other is a Sentura II. Both have two 15" JBL's in them -- D130's or D140's. They date back to the late 1960's. I just don't want to risk damaging the Marshall amp I just bought. Or does the ohms really matter that much? Any advice would be appreciated.
Are the two speakers inside your Marshall combo 8 ohm models wired in series? If so, then the amp is seeing a 16 ohm load from the on-board speakers, and is set-up quite nicely to take an additional 16-ohm load from an extention cab(as suggested) for a resultant total load of 8 ohms...but I'm just assuming these values as I'm not a Marshall expert. Now, all of the Sunn 2x15" cabs(late 60's/early 70's)that I have had were two 8 ohm speakers wired in parallel, resulting in a 4 ohm load. If you were to add the 200s cab to your Marshall combo, once again assuming the 12's are putting out 16 ohms, the amp would see a 3.2 load, not the best thing for a tube amp. The solution, of course, would be to rewire the Sunn cab in series for a 16ohm load, then everything would be fine. Verify, though, that the two JBL's are indeed 8ohms individually. Are you setting this up for guitar, in order to get a deeper, bassier sound?
