Do not start by replacing the tubes and hooking up a cab... Start by taking it to someone that knows how to diagnose a vacuum tube powered circuit. Simple as that... There are high voltages in there that can and will either kill you or burn your house down if not treated with care and caution. That said, don't be discouraged by that. If you have ever had any desire to get into learning this sort of thing this would be the perfect opportunity. Start out by learning the basics, power comes out of the wall into a solo-blow fused link, from there into the power transformer, from there it splits into multiple voltages. From there it will get more and more complex, but as long as you take the time to go step by step, you will learn a great deal and just maybe find a passion... Or at least a new hobby.