I did some searching around the Internet and have not found whether it is safe/possible to use a 6N3C in a VTA-70. (Without the PT/OT upgraded.) I ran some 6N3Cs and they were fine but one of the tubes started to red plate. I purchased some new ones and whammo, two of the tubes red plated. To make sure that I didn't screw up the bias resistors and the caps, I always made sure that the amp was immediately turned off. When I go back to standard EL34s or KT66, no red plate and the bias is fine and the amp loves it. I'm thinking it might be these tubes.
1. Can they be used in the VTA-70?
2. Has anyone run them?