Old Marshall Speaker Settings

[Betos]

I have this old Marshall head, a Major, and there's a plug to change the resistence level on the output to the speakers. I know it can be 4, 8, or 16 ohms, but the numbers are scratched off and I can't tell which is which.

Any help?

Thanks,

Betos

[John Phillips]

If it's one of the original pull-out 'bean' types with three selection holes at the top of the 'circle', it should be 4-8-16 left to right.

If it's the type with the 'window' plug, and the three selection holes equally spaced, 8-ohm is the hole vertically down in the center (so the 'window' is horizontal), 4 ohm counterclockwise from this, 16-ohm clockwise.

Sometimes they are inverted, but I don't think I've ever seen one where 8-ohm is not the 'center' setting, 4-ohm CCW, and 16-ohm CW from this.


Almost forgot - which I can't quite believe! - do NOT trust these selectors (especially the 'window' type). They are extremely unreliable and prone to making poor contact. This is a serious cause of blown output transformers on old Marshalls; I would definitely not rely on it if you're running the amp. There are two solutions: if you know you're only going to use one impedance setting, you can hardwire it inside the amp by moving the center-pin wire to the correct tap; if you do want to have the option of more than one impedance, replace the whole thing with a modern rotary-switch selector. You can do the same with the supply-voltage selector too, although if that goes bad all it's likely to do is stop the amp working, not cause damage.

This really is important - don't consider it as a 'mod', or making the amp 'non-original'. If you blow the OT, the amp will be irreversibly non-original.

[Betos]

UQ, thanks. Just the perfect information I needed to hear. Except the part about the output transformer blowing, of course.


I've been running the amp for about 10 years now, without any problem. I had changed the setting a few times because I used different cabinets, until I finally came across an early 70s Marshall 4x12 bottom. Now I never change the setting, but I wanted to know what the others were in case I ever did.

Now mind you, I don't gig with this amp often, but it's already been modded anyway, so I'm not worried about the bone stock original aspect of it.

I'm actually thinking of selling the amp anyway (because I use it so infrequently) but if I don't I'll likely make the change to the rotary switch.

Thanks again for the info,
Betos


PS, it is the window variety.