Thanks Billy for getting back to me.
I think it's more of a hum than a buzz, it's usually when I get to about 3 oclock at lower settings but kicks in earlier if I turn up the K-master and gain. I turned the reverb tray 180 and also in different positions whilst on and it didn't make any difference
Could you let me know how I'd go about adding metallic shielding - what to use, how to do it etc? Sorry if this is something quite basic but I have no idea - hopefully this is quite easy to do? It is in a tolex bag screwed to the bottom of the cab.
I've also noticed that in between the RCA cables (at both ends), there is a plastic wire with what looks like copper or something inside. It rests into a tiny hole next to the input in the input tray and at the other end it's just floating next to the inputs but not going anywhere. Is this some sort of ground, I'm not really sure if it serves a purpose but even in the input tray it's not really connected, just balanced in a small hole. You can see the the two holes next to the input on the actual reverb tray here http://www.mojotone.com/amp-parts/ReverbTanks_1/4AB2C1B-Reverb-Tank-Fender-Marshall-Ampeg-2-Spring