The options that are avaliable to you are either the 6550, or the KT-88. Getting Old Genelex/MO Valve KT-88s will be QUITE expensive, and they will probably be used. Sylvania 6550s could be had for some folding money, otherwise you are looking at mostly Chinese tubes. There is still some black art to making tubes, so eventhough some chinese KT-88s are 6550s in a bigger bottle, there is a difference. As to the driver tubes in this amp, stick with what the lettering says, or the industrial equiv.
I would strongly urge you to stay away from the KT-90, it is an unfinished product with a lot of hysteresis.
Happy listening.
I would strongly urge you to stay away from the KT-90, it is an unfinished product with a lot of hysteresis.
Happy listening.