Marshall 1960a jack assembly

Glaw3434

Hey there everyone, I have a Marshall 1960a that was modded by the previous owner to be mono only (he added a Neutrik Speakon socket and put the PCB in the bin), the problem is that I would like to use the same cab with two amps when in the house for convenience, I understand you can do this if you have the PCB by setting it to stereo which means that by plugging one amp into each input then each amp essentially has its own 8 ohm 212, am I correct? I would just buy a new plate but I have heard that the PCBs are a nightmare when it comes to reliability a friend of mine managed to kill his during a recording and I don't like the idea of my amp running with no load, basically what I'm asking is, is there a way to wire the jackplate for the same functionality as the PCB without the reliability issues? any help you can give me would be greatly appreciated.

ICBM

Yes.

You need two standard switched sockets - the black plastic type like Marshall use. For some reason they seem bizarrely hard to find in the exact combination you want - plastic nut, mono, switched contacts, solder tags - but these appear to be correct (bad pic, but the type number is right):

https://www.amazon.co.uk/6-35MM-SOCKET-CL1160A-ELECTRONIC-COMPONENTS/dp/B0118KCH3S

Connect each pair of speakers to the normal contacts on each jack as if they were two separate cabinets. Then connect the switched contacts on one jack to the speaker contacts on the other. You need to do both, sleeve as well as tip, in order to avoid a possible ground loop issue between the two amps.

The jack without the switching is now the mono input, and you use both for stereo.

(If you want to be really clever you can connect the second jack in the same way, to the speaker contacts on the first, which will make *either* jack a mono input when there's nothing in the other one.)

Glaw3434

That's great thanks, will this function the same way as it would unmodified?

ICBM

Glaw3434 said:

That's great thanks, will this function the same way as it would unmodified?

No - unmodified you get the choice of 4 or 16 ohm inputs as well. If it still has the panel sticker it would be best to cover it or remove it to avoid confusion…

It's best to avoid that scheme since it introduces too much unreliability, mostly because the complex switching using the jacks makes it possible for a bad contact in one of them to leave the amp with no load. (Often blowing the OT.) With the simple hardwired scheme I described, the worst that happens is that half the speakers stop working and the amp sees double the impedance it's expecting, which is still safe.

CasperCaster

@ICBM I assume I could do exactly the same with a 2x12 cab which is currently mono only i.e. replace the jack plate with one made for two sockets and wire as you described in your first post, with the appropriate components?

Glaw3434

That's great, thanks a lot mate, you've been a massive help as usual