The issue is this. When plucking a string, hard, there is no sound from the amplifier at first but then as the string vibration reduces the amp produces the sound. It is like a delay, pluck the string, nothing, then sound. It is similar to a compression, I guess, like the sound is limited at first. This occurs on both channels, with any of the effects on or off. It is not the guitar or the lead. These have been changed and tested elsewhere. As I say it doesn't occur all the time but once it starts then it carries on for the rest of the session. Switching on and off again doesn't help. It also doesn't seen to matter if the amp has been on for a while before it starts to happen. ie sometimes it happens straight away and sometimes after its been on a while. Does any one have any suggestions, It doesn't sound like it needs new valves from the symptoms I have read.?