Speaker/Transformer how much seperation?

Jeff Watson · Post by **Jeff Watson** » 30 Sep 2006 11:26 am

I want to put a Bassman chassis in a combo cabinet with a 15" speaker and try to keep the height of the cab at 20". This would bring the rear cover of the JBL I'm using within an inch or two of the center transformer. I seem to remember reading about too close to each other being a problem and was hoping someone could explain the issue a little bit. Thanks

Mike Fried · Post by **Mike Fried** » 30 Sep 2006 7:59 pm

Jeff, the only problem I'm aware of would be the speaker's magnet being physically in the way of the output (center) transformer. Any problem from the proximity of the speaker magnet's field to the transformer would be more theoretical than practical - plenty of amps over the years have made them very close together without ill effect. I would, however, allow at least a couple inches clearance (by offsetting the speaker on the baffle board) to avoid bumping the chassis and the speaker when removing one or the other from the cabinet.[This message was edited by Mike Fried on 30 September 2006 at 09:09 PM.]

Mike Fried · Post by **Mike Fried** » 1 Oct 2006 2:09 pm

There are other considerations I didn't mention in last night's response, mainly that of impedance matching of the speaker to the output transformer (very important) and the probable lack of bass response from using a 15" speaker in such a small cabinet.

Your original post didn't say which type of Bassman chassis you're using - the tweed 4-10 combo type or the later-design head type. If it's the combo type with the original output transformer, it'll be looking for a 2-ohm speaker load, which no single JBL (or any other single speaker) will provide. Using a 4 or 8-ohm (or, even worse, a 16-ohm) speaker will result in a significant loss of output power. If it's a head-type chassis, the original output transformer is looking for a 4-ohm load, so again, anything higher than that will lose output power. Most JBLs are 8-ohm, so if yours is, you'd better replace the output transformer to one with an 8-ohm output tap.

As for the small cabinet issue, I think you'd be disappointed with the bass reponse using a 15" speaker in a cabinet that small (20" height). The bigger the speaker, the more baffle board and cabinet volume (even if open-backed) it needs to produce a given fundamental frequency. Back in the late '90s, Evans tried using their 1-12" SE cabinet with a 15" speaker in their "Parlor Amp" and the results were very disappointing, to the extent that the amp actually had better bass using a 12" speaker.

I can appreciate what you're going for here, I just wanted to make sure you're covering your bases...
[This message was edited by Mike Fried on 01 October 2006 at 03:12 PM.]

Jeff Watson · Post by **Jeff Watson** » 1 Oct 2006 3:42 pm

Hey Mike, thanks much for the responses. I'm aware of the issues you raised in your 2nd post, just trying to put together a small club rig in the smallest package possible. I want to keep the height at 20" so I can use the amp as a seat when necessary! Guitar case in 1 hand, amp in the other and accessories in the backpack.

Robert Leaman · Post by **Robert Leaman** » 1 Oct 2006 6:44 pm

Old fashioned tube amplifiers had their power and output transformers mounted so that their magnetic cores were at 90 degrees to each other. The power transformer was able to induce mains hum in the output transformer secondary winding which was heard in the speaker. A somewhat obscure but important design considertion.