01-10-13: Agathe79Do you have a clue what NOS 6550's go for if you can find them? Then which ones are you looking for? Solid Black Plates (best and my favorite) - Solid Gray Plates (close second to the black plates) or the 3 hole gray plate? I haven't heard the 3 hole ones but a friend has them and says they are not as good as even the solid gray plates but still very good. Here is a link to e-pray and if the quad of black plates is truly NOS that is an excellent price.
Does anyone know where I could get 8 NOS Tungsol 6550 tubes?, I know it's asking alot
http://www.ebay.com/itm/Tung-Sol-6550-KT88-Black-Plate-Tubes-NOS-Perfect-Matching-Quad-/160951867413?pt=Vintage_Electronics_R2&hash=item25797a9815
Finding USED singles and pairs and an occasional quad isn't that hard. But most important you need to hear them first. In my Octave they are like great EL34's on steroids. Warm and musical. EAT KT88's are the opposite sound. Very detailed - linear and accurate (kind of like solid state). The Vintage Gec KT88's are pretty close to the EAT KT88. Depends on your preference and again why you should listen to them first - if possible. Only the EAT's are currently in production - all the rest will never be made again. All are very expensive and because EAT is currently in production they are the only ones that will give a warranty.
That being said I'll also say get used to what you have first. Then decide if you want to change up the sound a bit which way do you want to go. Changing the input tube will make a big difference too and cost a lot less to experiment.