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

My experience was the same as Tommy583's.

Had a pair of Sovtek 7199 in my bone stock ST70. The sound was so-so, maybe a "C" or even "C-" on the traditional letter grade system. Put a pair of RCA 7199 and it was literally the biggest improvement I've been around when it comes to rolling tubes. No, it's still not the last word in power (though more than enough for my efficient speakers) or low end response, but the amp is now a solid "B+".

I get so much enjoyment out of this amp in my second system that I spend about 80% of my time listening there now. This amp has now caused me to question what exactly has improved over the past 50 years when it comes to tube amps.

Circuit topography hasn't changed much at all and the ST-70's came with excellent output transformers too(very important). The major differences in sound that can be appreciated with later designs comes from the improvements in passive components. Replace the old Black Cat coupling caps with some good polystyrene/polypropylene and foils(ie: auriCap, MIT, DynamiCap or WonderCaps) and the carbon composition resistors in your signal path with precision metal film resistors(Vishay or Caddock), and you'll understand SOME of the improvements. I won't even mention the cheesy RCAs, speaker terminals and power cord on the ST-70(OOPS- I guess I did after all).

Ok i put in a quad of the Svetlana EL-34 winged C, RCA 7199's and a GE GZ-34. The sound is alot smoother overall. I do think i lost a little bass when i installed the EL-34's. This leads me to belive the old tubes were KT-77's. The soundstage is a little wider and higher. The top end is more layed back and not harsh at all. I think i'm going to get the Mullard GZ-34 and see if that makes a differance. Over all i'm pretty happy with the sound, just looking for that last little bit.

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