Due to lockdown etc. I haven't switched on some of my amps for say 12 months. I have a 1987 from 1970, a 1959 from 1975 & a Hiwatt DR103 1974. I have read people using variacs presumably to introduce voltage 'gently' so as not to damage components. I don't have a variac whatever that is, any suggestions or do I just hit the on switches ?