I was looking into a Blues Jr. One popped locally, at a really good price. However, seller says all it does is hum, and his reason is the V1 preamp tube has turned white. I read this happens when the tube looses its vacuum. I may just contact him and bring a tube...it may work. Only issue would be a chronic problem. What else could cause such a thing, and if a tube swap on the spot seems to get the amp working...buy or pass? Buy anyway? Thanks!