Always use a soft-start circuit to protect those precious 1930s filaments.
The most coveted versions are "mesh plate" tubes, where the internal structure is a fine wire grid rather than a solid piece of metal. This is widely believed to provide a more detailed, airy soundstage. Why "Prima Donna"? Prima Donnas: 2x56
High-quality, matched pairs of vintage 56 mesh plate tubes are increasingly rare and expensive. Always use a soft-start circuit to protect those