It's stated in the original Dyna ST70 manual that KT66 can be used in place of the EL34 output tubes.
I did plenty of research, asked questions on audiokarma(didn't get much help) and some more research. It seems like there's quite a few people out there using KT66 in their EL34 based amp. Some Marantz 8B owners prefer KT66 over EL34 in their amps.
Looking at the specs, KT66 are similar to EL34's. EL34's can handle more plate voltage and KT66 have less current draw for the heaters. The KT66 are being used at close to their max operating limits in my ST70. No part is overdriven.
Well, I decided to pull the trigger on Sun night and ordered a quad of Sovtek KT66 just to see what kind of a change I'll get.
They came this morning and I've been listeing to them for almost 4 hours.
Only thing I can say is WOW! I think I prefer this tube over any EL34 in my amp. OK, first I'll talk about what's missing compared to EL34. The warmth that I hear in EL34's is not present. The KT66 also sounds less tubey than EL34. Bass extension is about the same.
Now the good part... Dynamics in my rig is now INCREDIBLE! I still can't believe it. I'm talking "live" kind of dynamics. Snare drums are making the floor shake and reach up to my couch. Dynamic range in vocals seems to have no limit (on good recordings, of course). I'm getting slam like never before.
I wasn't expecting this at all because I knew prior to listening that KT66 have less power output than EL34. But wtf, these tubes sound powerful as hell in my amp. I've tried raising the volume to insane levels and I still can't make them clip. My mind is still boggled. How can this less powerful tube sound like this? My guess so far is that because they're being operated close to their limit in my amp. Another guess is that because they're easier to drive than EL34's, the 7199 driver tubes in my amp have more space to work with dynamics now. Maybe my Klipsch like them more.
Now, on to their best qualities... Mids, highs and detail (I've already mentioned dynamics/slam). The mids have an incredible purity to it and sounds very lively and lifelike. The EL34 sounds more mellow with less dynamics. The high frequencies just shimmer and is better than any of the EL34 I have. And the low level detail is now much more audible. I love it. Everything sounds to real with these tubes. Can't wait for them to break in.
They're not cheap. At close to $100 a quad, they're much more expensive than EL34 but worth every penny IMO.
I wanted to take a pic but my brother have the digi camera. The tubes are so sexy looking. Big voluptuous bottle with a metal base.
So if you own an amp that use EL34, check the specs to see if they can use KT66. If they can, give them a try.