Combo to cab mod, but keeping the amp section also live

hairboy

Hi all

I have an old Marshal Valvestate 8080, which I've had since 90s, which I'm thinking of turning into a 1 x 12" cab, like this. 

But with this method, you lose the amp section? Does it keep the amp section active when powered on, and when it's turned off, it acts as a cab only?  

ICBM

hairboy said:

I have an old Marshal Valvestate 8080, which I've had since 90s, which I'm thinking of turning into a 1 x 12" cab, like this. 

But with this method, you lose the amp section? Does it keep the amp section active when powered on, and when it's turned off, it acts as a cab only?  

If you're using it as a cab you don't turn the amp section on.

A warning: make sure you use plastic-bodied jacks without a metal threaded section - or if you use metal ones, fit the one in the amp through a rubber grommet as he did - don't use a metal jack that contacts the amp chassis, that will cause trouble. He was lucky he did use the grommet, because he didn't appear to know that.

He has very little idea what he's doing in fact, he actually refers to a speaker cable as higher impedance than a guitar lead! ... but basically the idea is sound.

This type of jack is best:

hairboy

Brilliant. Thanks. But is that one meant to be stereo with 2 contact plates? Do I just solder on the 1 plate only for mono? Does it even matter which one?

ICBM

hairboy said:

But is that one meant to be stereo with 2 contact plates? Do I just solder on the 1 plate only for mono? Does it even matter which one?

That's a mono one. Stereo ones have three rows instead of two.

Positive (red wire on the Marshall) goes to the terminal furthest away from the nut, negative (black wire) to the one closest to the nut, both on the side you can see in the pic - the other two terminals are the switch contacts which you will see if you turn it round - you don't need to use those.

On that model of Marshall the wires come out through the floor of the chassis via a cable clamp - the hole should be about the right size. It can be difficult to remove the clamps if you're not familiar with them - they need squeezing hard with pliers to close the clamp part up tighter, then pushing through the hole.

hairboy

That's brilliant.Many thanks for the tip about the wire route, too! 

ICBM

I may have done this sort of thing a few times .

I mostly haven't used them as cabs, it's just to give the option of running through a different cab, so I've normally fitted a jack in the chassis hole and a plug on the speaker wires.

But my two workshop test cabs, both for guitar and bass, are ex-combos I acquired for scrap when the amp sections weren't economically repairable, and just fitted input jacks to.

hairboy

Nice.  I was going to sell the Marshall, and get a lunchbox head and a small 1x12" for the home practice.  Then remembered it has the G12T, so should work well with a range of heads, which led me to the idea.

I've now ordered the jacks. Hopefully I'll be tackling the job next weekend.