Preamp Valve gone in 'hissing' amp?

Question

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

Answer 1

If the hissing gets louder as the volume control is increased, the source of the hiss will be before the volume control. I would start looking at the input lead(s) and sound source (guitar). Swap the guitar and connecting lead with a known good one or even two. It is cheaper than buying a replacement set of valves you might not need.

If the hiss is not less with the new source, lead and plug you know the problem is in the amp's input circuitry. Start with the socket making sure the soldered joints are not cracked or "dry". Then work your way towards the volume control. How far you go will depend on your electronic knowledge. But my first guess would be a failed capacitor somewhere. Wiggling components can sometimes reveal a dry or cracked soldered joint.

As this is a Music Stack Exchange and not one for electronics or roadies, don't forget to hum a tune and to curse in a musical manner.