Replace MONO/STEREO PCB jack plate on 1960a cab

FarleyUK

Hi all, I really want to replace the standard Marshall mono/stereo PCB monstrosity in my 1960a cab with just a single jack so I can wire it for 16ohms direct (plus, I've heard plenty of stories about the PCB failing). Has anyone done this, and do you know where I can get a replacement plate that will fit the cab...? All I can find are the circular jack plates.

ICBM

Don't bother - just remove the PCB by undoing the jack nuts, then fit two standard panel-mount 1/4" jacks, connected in parallel so you can daisy-chain the cab if you need to (sometimes useful) and hardwire the speakers to them.

You're right that they are a reliability hazard, and if they do fail they will leave the amp with no load, which I think is a contributing factor in the large number of modern Marshalls with blown OTs.

FarleyUK

ICBM said:

Don't bother - just remove the PCB by undoing the jack nuts, then fit two standard panel-mount 1/4" jacks, connected in parallel so you can daisy-chain the cab if you need to (sometimes useful) and hardwire the speakers to them.

You're right that they are a reliability hazard, and if they do fail they will leave the amp with no load, which I think is a contributing factor in the large number of modern Marshalls with blown OTs.

I'm kicking myself for not thinking of this.......!!

Thanks mate - will do later

ICBM

I forgot to say - while the jacks are out, peel off or put tape over the label so no-one thinks the "4-ohm" labelled jack is still 4 ohms and fries their amp by connecting to it.

I also forgot - when you rewire the cab, do it as two series pairs in parallel (like a vintage Marshall cab) and not two parallel pairs in series (as it is with the switch plate when using it at 16 ohms) - it sounds better.

FarleyUK

Just been down good ol' Maplins and got a load of hi-fi speaker cable to re-wire it all up:

http://www.maplin.co.uk/p/shark-loudspeaker-connection-cable-high-quality-ofc-s12-priced-per-metre-xs37s

Slight overkill on the thickness, I know. I'll be soldering them anyway.

This is how I was going to wire it all up:

But I also saw someone suggesting this method as well (same as above (series/parallel), but with a switch that can engage / disengage the white wire:

ICBM

It's the white wire that makes the difference between series-parallel and parallel-series. That might not look obvious from the drawing but it is if you look at a schematic. Leave it out for a guitar cabinet. (It's better for bass, usually.)

In both drawings the top and bottom pairs of speakers are in series, and then the pairs are in parallel, which is correct. Marshall always did it with left and right series pairs then wired in parallel, but it doesn't make any difference.

(The reason the extra wire makes a difference is because series-parallel has a much higher damping factor than parallel-series, which is why it sounds better for bass and worse for guitar.)

FarleyUK

Thanks - wired it up last night..... and there was no sound......!

Turns out the mono jack I had wasn't working. D'oh! Changed it and it's all fine now