TAKE IT TO THE TECHNICIAN!!
For two reasons. The first is the most
important - it might save your life. Old amplifiers which haven't had
attention for years can be in a dangerous and even lethal condition.
I've had several "tingles" and one belt which I wouldn't like to risk
getting again from messing around with things I don't really know much
about. Don't plug straight into an old amplifier you know nothing about
- if you just can't resist the temptation, at least use an RCCD
(residual current cut-off device), a few pounds from the local hardware
store. That will cut off the current in micro-seconds if anything goes
wrong. Ordinary fuses take milliseconds, slo-blo ones longer - they are
designed to protect the equipment, not you, and by the time they blow
you can be brain-dead, which is a bit of a barrier to enjoying your
Selmer.
Reason 2. An old amplifier which has been stored for 30 years in
someone's loft, or had 30 years of abuse by a succession of teenage
guitarists, is unlikely to be putting out it's best performance. Apart
from a safety check, it will probably need new valves (though don't
throw out the old original Mullards or Brimars as a collector will
value them), new electrolytic capacitors ("caps" or "filter caps" which
dry our over a period of time), and probably lots of the other little
red, yellow & green bits inside aren't doing now what they did
30 years ago. Find a good tech recommended to you as experienced with
vintage valve amps - the less hair, the better !! It'll be worth the
money.