There's a YouTuber who goes by the name of Uncle Doug. He repairs and restores old tube (valve) amps, mainly the electric guitar variety but also the odd hi-fi unit too. Very knowledgeable guy, with a wry & dry sense of humor. Before he'll touch any piece of gear his black cat, Jack, must first give it the sniff test of approval. After the amp is in proper working order Doug tests it out via simple strumming on a cheap guitar before turning it over to "Jack" for a proper workout on a better instrument.
-Dave-