Impedance Switch

[ab3640au]

I understand that the original 18watters had no impedance switch .. and .. strictly speaking, it's not necessary to have one (assuming that the impedance set is correct for the speaker used, of course!). However, I'm interested in your collective thoughts on where would be the best place to put one in the chassis. I'm drilling out my chassis (for a combo) now and thought I'd establish what others have done. I've got a modern Marshall impedance switch (round flat dial variety) - it might fit just below the component board close to the power supply (about level with V5). Any thoughts, or is it stupid to put it in at all (I'm only doing it to allow easy changeover of different speaker types with different impedances, should I want to do it ..). Thanks.

Barry Marie

[markh]

hi Barry,

Having the switch in there is a good idea if you plan to experment with speakers. As long as you keep it away from the board inputs, you can place it anywhere it'll fit conveniently.

--mark

[ab3640au]

Thanks Mark ... I'll probably put it where I was planning to put it ... below the board & trannie (1/2 way between them) - about level with V5. It's not the sort of thing that I'm likely to change much, therefore I'm happy enough to do so from inside the chassis.

Barry Marie

[Wayne_Alexander]

Barry, you have to be careful about the placement of the wires from the OT to the selector and the wires from that to the speaker jack(s). If they pass over the middle of the board, or anywhere under/over the wires from the input jacks to the preamp tubes, it can induce quite a bit of noise. On my first one I put the speaker jacks and impedance selector way over past V1, but I had to route the wires all the way around the edges of the chassis, avoiding passing them over the board itself, to keep it from being very noisy. My advice is to splice some long wires onto the OT output leads, and move them around the amp with the amp on and the speaker(s) connected before picking a final location for the switch and the jacks. It'll be quite evident which routing causes noise. It won't be subtle. Don't just pick a location without trying that.

[ab3640au]

Thanks Wayne ... I'll leave the actual "placement" until last, then punch the hole for the impedance switch.

Barry Marie