That's awesome! You've got to love a good bargain..
edit: regarding advice on fixing it up:
-I would first remove all of the tubes, then check if the power cable is 3 core and that the earth is connected to the chassis and the earth pin on the plug - if not, rewire it so that it is.
-Then I would plug it in with all the tubes removed, switch it on, and measure the voltages across the power transformer windings with the multimeter set on AC.
>you should measure around about 500-700V between the two rectifier HT pins (pins 4 and 6, there should be numbers on the socket. if not, here's what the arrangement looks like:
>you should measure around 5V across the heater pins {2 and 8}
>you should measure around 6.3V across the heater pins of a power tube {2 and 7}
if you don't measure any voltages, it might be the switch or fuse - either that or your power transformer is probably stuffed.
-then you can check for continuity all the way along your 6.3V heater line pair (on each of the power tube and pre-tube sockets).
-after that you can measure the output transformer primaries and secondaries (primary is the side that sees the tubes, secondary sees the speaker). you should see resistance on both, probably between 1 and a couple of hundred ohms, higher resistance on the primary side.
-then measure the resistance across the speaker, should be around 7 ohms
(these resistances need to be measured with the leads disconnected from the circuit, to avoid parallel misreads).
if all of those check in reasonably, then (and be VERY careful from here on. you should have been careful up there ^, but from here on you can die harder):
-plug in the rectifier, switch on again, watch for smoke and things. if all seems fine, measure the voltage across the chassis and the rectifier heater pins. you should see high DC to the order of around 300V.
-measure the voltage between each power supply node (the positive terminals of the big electrolytic speaker capacitors) and the chassis. you should see between the high DC you measured before and probably around 100V on each of the nodes.
if all of that checks:
-turn off, plug in the rest of the tubes and turn it on. watch for smoke or anything strange again.
-if all seems well, plug in an instrument and measure the voltage across the strings and the chassis. you should have 0V here.
if thats all good, strum away and see what happens ?
-if nothing happens, you should probably start by rolling some tubes or testing yours in another amp that is known to be working, and replacing caps. if you get no joy out of that, then you can start debugging the circuit, starting at the input.
here's a schem:
http://www.gibson.com/Files/schematics/ga-20.gif
bear in mind that all of the above attaches EXTREMELY stringent safety warnings. unless you really know what you're doing, do not touch ANYTHING inside the amp when it is plugged in with anything other than a long stick of wood or other non-conductive material. also bear in mind that after turned off, the power supply capacitors store charge at the same potential as the DC HT, which can and will not hesitate to remove your soul from your body. you need to make a discharge contraption for these before you can safely touch anything with the amp off - which consists of an insulated resistor and wire that you can connect across the capacitors and the chassis without having to touch any metal parts yourself.
google safety and tube amps before touching anything. any terms you don't understand, google them, or ask here. there is LOADs of info on this tube stuff on the net, so google is your friend.
another thing, those 6SL7's are probably one of the best kept secrets of the vintage glowy bottle world ?.
oh, and in the odd case that you decide you don't want it anymore, let me know ?
let us know how it goes!
