Advice on Pot for Marshall valvestate 8100

[jonty]

Hello all.

I must admit to being a relative beginner when it comes to delving under the hood , so to speak of guitar amps, so please exuse any naivety that I may exhibit!

I recently got a lovely second hand Marshall valvestate 8100 head and vintage marshall cab [mid to late 70`s I suspect - the cab that is - the valvestate is circa 1994] .
It sounds fantastic, but there are a few minor problems associated with leading a pretty active life over the years.
I replaced the chinese valve [looked like it had been there for quite a while] with a JJ AX7 today - nicer tone and an immediate improvement once warmed nicely...

But, the potentiometer on the effects loop is a non original unit and isnt working properly.
I want to order a replacement , but am unsure of the value to order.
I`m also not sure if the current model is correctly wired and being somewhat rusty when it comes to looking at wiring diagrams [think building your own radio transmitter when aged 15 , [now 40] as the last time...] so, I want to ensure the wiring is reconnected appropriately.

As I say, excuse the newbee stupidity, but I am keen to learn, and this place seems to be a fount of exceptional knowledge.
Thanks in advance.

[Enzo]

22k linear.

WHat is wrong with the way it works? It should act as a "balance" control between the dry signal and the signal coming through the loop.

[jonty]

It ...er doesnt work...that is the problem!

It was working intermittently but isnt now.


The pot is a replacement - changed in a previous life.
I need to put a new pot and simply wondered if anyone knew the correct value.


Thanks for the info.

As said - the head has had a pretty tough life , but needed only a new valve, the dodgy pot replaced and a new plug for the footswitch as the original has a broken wire somewhere in the plug.
I got the rig at a good price and it is now my 13 year old son`s pride and joy.
It will be used in anger soon at his band`s next gig, so I was keen to fix the little niggles before then.


The band are here-

www.myspace.com/circadianmetal

[jonty]

Thanks Enzo

your advice was invaluable.
The pot arrived from Hotroxuk this morning and duly fitted.
The old one was completely the wrong resistance and size.
The effects circuit now works beautifully.

One final question - is there a resource where I could find the specs for the remaining pots on the amp , in case replacement of others is required at a later date.

Thanks

[Enzo]

The schematic.