What tubes for ST-70

I would heed the advice of Stonedeaf....The 7199 is a medium mu triode / sharp cutoff pentode that is very prone to capacitance problems when it ages causing hum and premature clipping on one side of the waveform. That is, asymmetrical clipping and increased distortion. If you are into soldering I also would recommend replacing the mother board with and update that allows for 2 front end tubes instead of one per channel as the 7199/6AN8 was really not up to the task of properly driving the EL34 or the 6550 to clipping @ low distortion. G

Your going to find that the KT-77 has a faster, more aggressive bottom end vs. the EL-34. This is also true with the MK-3 amps when the 6550 is replaced with the KT-88. You will also get a bit more power output out of the KT-77 vs. the EL-34. When I had Mk-3's I would also keep them in Genelex KT-88's. That was a while ago when they were still available and reasonably priced. G

I have not tried them or tested them so I could not say...G