I've been meaning to try the KT120's, but between work and my extended illness (cancer was just the latest part), I haven't had the chance. I have heard of them being run safely in a Marshall Major which has no where near the robust heater current of an SVT, but that's just anecdotal. I'm still a huge fan of the Sovtek KT88/Tungsol 6550 if you don't want to experiment.