Hi I have an old Sound City 50Plus that's started to make all these loud crackling and pop noises. It starts when I switch on both the mains and standby and lasts for about 30-60 minutes. Then the amp is dead silent like it should be. The power tubes are about 10 months old but I have no idea how old the pre amp tubes are. I would guess about 3 years. The sockets are probably a lot older... Any idea what the noise is all about? Thanks!