I've completed my ST-70 restore, and the amp has about 20 hours on it using EL-34's. It's been rock solid, no hum, buzzing, and bias has stayed spot on. Last night I put KT-66's in the amp, and as soon as it warmed up, the KT-66's flashed bright light at or near the getter and a crackle sound came from the right channel. I instantly shut it down. I put the EL-34's back in and all was fine. I'm using the SDS power supply and VTA driver board. I have a 12AU7 in the center position and 12BH7's on the outside. The KT-66's I purchased are used, and I assumed were good.
I shut it down as soon as I saw the flash and heard the crackle, but I have no experence with this tube so I don't know if that's "normal", but I'm guessing it's not.
Can anyone shed some light on what I experenced, any thoughts on what is wrong (if anything), and possible solutions?