Hi,
I've tried posting this on guitar amp forums but no luck so far. Hope
I have it in the right section here.

I have a Marshall amp (VS100), which has one tube in the preamp.

There is a LOUD buzzing noise from it that only started the last day.
Even when no guitar is plugged in and the volume is turned down to 0.

Could this just be a tube that needs replacing?
If not, is it definitely a problem with the power amp and not the
preamp? Considering the volume is turned down to zero (volume controls
preamp doesn't it???).

Schematic here: http://www.schematicheaven.com/marshallamps/vs100r_100w.pdf

Thanks for any help you can offer