DC plug in back of Mark III?

[fome]

What is the power jack in the back of the Mark III for? Can I use it to power other things, like pedals?

 

[MrMarkIII]

What is the power jack in the back of the Mark III for? Can I use it to power other things, like pedals?

Yep, but it's A/C, and don't go over the amperage/wattage rating (3 A/300 watts on a Mk III?). So you can't plug the bass player and the P.A. into it. It's what amp makers used to call an "accessory outlet". Almost all old blackface Fenders had 'em.
Ahh, the good old days... buggy whips, rotary-dial phones, pointy bras...