I'm looking to take out the 12ax7 preamp tubes in a Valvetech VAC and replace them (just for a test drive) with the 12ax7's I have in my Bogner Metropolis. Any risk in doing that without biasing? I only need to do it for a short while, to rule out preamp tubes as the source of a tone issue. The Valvetech tubes are Fender and Ei. The Bogner has Chinese tubes.