Roughly 3 years ago, I purchased a VHT Special 6 Ultra (Combo) Valve Amp. It has recently been 'hissing' (not sure if this is the correct description), and this noise increases with higher wattage and higher volume. It also occurs without the guitar plugged in, and with the amp volume set to 0.
I have tried different mains sockets, to no avail, and from reading about a bit it looks like it might be a preamp tube that has gone.
Am I right, or should I be looking at another component? (Or this might be normal, I'm not entirely sure)
I have attached a video - sorry about the mobile phone quality