What are these tubes

The Tung-Sol 6SL7GT is probably just that, and, if it is still a good tube, it's probably a really nice one. The two Raytheon's are probably 6SN7 GT's since that is what the tube complement information calls for. The power tubes should be four KT 88's. If you wanted to replace these tubes, you should be looking for one 6SL7 and two 6sn7gt's. I don't know if your unit is self biasing or not. It probably isn't. If the power tubes need replacing, it should probably be done by a qualified technician. Do not remove the power tubes without noting which socket each tube should return to. The other tubes (6sl7,and 6sn7's) can be removed and replaced at will, with the 6sl7 going in the 6sl7 position and the two 6sn7's in their respective positions. Happy listening.

The Tung Sol 6sl7 and the two Raytheon 6SN7GT's do not require biasing. Therefore, test and replace at will! I say test because the Tung Sol 6SL7 is likely a VERY nice tube indeed. A good source of 6SN7GT, can be found at Andy Bowman of Vintage Tube Services, but Russian, and Chinese varities are plentiful and not bad at all. Try the Electro Harmonix versions, cheap and servicable. Ralph Karsten at Atma-Sphere.com can hook you up with Chinese versions, if you really can't find them anywhere else on the net (Tubeworld, Tube DepotVacuum Tubes, Inc., etc., etc.,). Just Google 6sn7gt and see what comes up. I hope that you had the KT 88's biased when you put them in. If not, get thee to a qualified technician (should not be too hard to find in the Yellow Pages of your area) Electronics professors at local technical colleges may also be able to help you. If you really can't find a good pair of 6SN7gt's, I can sell you a pair or two of mine.