Mark I "Mystery Control"

[zebpedersen]

I had my Mark I chassis out of the cabinet earlier on, purely out of geekery and general interest, and I noticed something on the inside of the amp. Now, my Mark is a 15" model, modded at some stage to have an FX loop added, but other than that completely stock.

The two things I noticed:

The first can probably be answered easily and is likely general knowledge, but there's a 1/4" jack socket on the underside of the chassis - anyone know what it does?

And now onto the aforementioned 'Mystery Control'

Now, on the inside of the amp I saw this control was wired toward the end of the tone controls on the preamp. Obviously I don't use the 'Slave' control and never have done, however just purely through fiddling I turned it, and it seems to affect the sound dramatically, producing noticeably more distortion and adding a pleasing bottom end to the sound. Now I for one am very pleased with my newly acquired dirty bottom end, but does anyone know if this will do any damage to the amp? Or is it just 'one of those things' like with the volume pedal and a Mark II?

Thanks

 

[Restless Rocks]

Can't tell much without actual internal circuit shots but just guessing it sounds like the "gain boost" function has had a pot wired in series with the switch to allow you to adjust the amount instead of simply off or on. The "GB" normally operates by lifting the Midrange pot from ground which "deactivates" the tone stack.

 

[zebpedersen]

I'll post this shot then - the only camera I have is my iPhone so it may not be that detailed. The fact that it's labelled 'slave' from the factory is just what makes me think I could be rinsing through my preamp tubes a little quicker than desired.

It's the blue cable going from the second-to-last control on the front panel (Mids, the last one is Reverb) to the jack plug and pot on the back.

 

[jonarobb]

Looks like someone put a variable pot in series with the Boost circuit. If you can take close-up shots of the back of both pots showing the lugs I can confirm....

That's a novel idea nonetheless. With the addition of the effects loop send and return you theoretically wouldn't need the factory slave anymore so seems cool to take advantage of that pot just sitting there. The boost on the Mark I's can be pretty extreme. Being able to dial it in so the boost isn't overkill is a great idea. Making the boost footswtchable is another great non-offensive mod for Mark I's. Give's you the feel of a 2 channel switching amp.

The jack on the bottom should be your footswitch control for reverb/eq, though on some Mark I's it's wired strictly as a preamp out/power amp in tap. It's really amp to amp depending on who ordered it at the time.