BNB had 2 matched quads to my house within 3 days.
I installed them and biased to .400VDC like Bob said. I let the amps warm up for 1/2 an hour or so, checked the bias and it had risen slightly to .420-.430 per tube. They sit solidly in that range with the bias pots full clockwise. I've been playing music for about 30 hours straight now with no change.
The difference in sound quality is night and day compared to EH EL34s. The EL34s sound thin and weak. I think I understand now why they are so cheap. They were a bargain to get my amps up and running, but I'm now hearing the full potential of what I've put together. Music sounds somehow more full bodied. The soundstage seems to be projected in front of the speakers as opposed to a seemingly recessed (behind the speaker) presention with EL34s. THe EL34s don't sound bad at all for the money, but the KT66s make the amps sound richer, fuller and overall more powerful. Instruments are much better defined and seem to sort of hang in their own space. Both male and female vocals sound much more natural and powerful. It's almost as if I put different amps in the chain as opposed to simply swapping tubes. What has really impressed me is distorted electric guitar. Heavier music has a lot more definition and cohesiveness to it now. With the EL34s, it was almost as if some artificial compression had been added to certain tracks, but that is almost completely gone with the KT66s. My system will absolutely rock with either tube set, but my friend commented last night that it now sounds loud without sounding loud? I beleive he's just hearing better definition overall. I guess we'll see what happens after a few hundred hours of play, but right off the bat, I am one happy camper.
I've never experienced this level of sound quality in my own home. Over the years, I've had the pleasure of playing with some VERY good demo equipment in a local shop. VTL, Rogue, Mac, Cary and the like driving all sorts of speakers. For the first time, I feel like I own a system that is at least on par with some of the combos I've heard that cost more than I could imagine paying.
I dare anyone to take one of these VTA modded, GL KT66 tubed 70s home along with a Mac 240 or the like and play with both for a few weeks. I'd bet the Mac would soon be back at the store sitting on a shelf collecting dust.
I'll tell you folks a little secret... Back in the mid 90's, I had the opportunity to meet Luke Manley of VTL at a little siminar in the above mentioned shop. He was touting the sonic benefits of tubed equipment in general, not just his stuff. He mentioned the st-70 and told us that it made for an excellent platform to build on if one wished. He was a very candid guy and basically told us that he undertood that not everyone was able to afford his gear and if we wanted to hear tubes at an affordable price, a modded 70 was the way to go. He was met with sneers and a little laughter from the " I have more money than God" crowd that was there, but he seemed like he was just trying to be honest. I think I see where he was coming from now...