If you buy an older tube amp I strongly urge you to have it inspected by a qualified technician as part of the deal. Many of these older units have capacitors, and tubes often, that require replacement. It can be disastrous to use one with such capacitors; it may sound OK for some time and then fail.
Assume you will spend some money to bring a used tube (if older) up to date when you budget for your purchase. The only exception to this would be if the unit has been serviced recently and it is documented. It is also good to find out if the unit can still be serviced. Sometimes they use parts that no longe exist and cannot be found anywhere; I have such an anchor that I (foolishly and had not yet learned my lesson) have under a couch waiting for the parts to pop up used; they are no longer made. That was a year ago and it still sits under the couch.
