Slow recovery -- no good for flyback/resonant supplies, for the most part. Need a big old mains-frequency transformer, or audio frequency anyway. Not really enough for much sparky high voltage stuff, unless you have a bunch to put in series, and an old school neon sign transformer to power it.
Would be fine for tube amps yeah, but be extremely careful around that much DC plus beefy caps backing it up. And, not a cheap project, tubes and sockets aren't cheap unless you already have some, plus the power transformer, output transformer(s) for audio, or coils and caps for RF, plus, well, the license you need to actually put the thing on the air (not so much expensive, as something you need to get).
Tim