Anyone had experience of using them?

Problems or concerns?

Don't drop it. If it breaks you have an instant hazardous waste zone.

They sound delicious and they look delicious. They do radiate a bucketload of RF hash, though, so don't just plop one into a circuit that doesn't have adequate filtering to get rid of it.

What about the equivalent gas rectifiers eg 3B28 etc?

Any problems with mercury tubes arcing even if the heater has been 'on' (with no plate voltage) for the correct length of time?

Personally I prefer TV damper diodes such as the 12D4 or the 6CM3 if you need more power. I'd go with mercury & other gas vapour rectifiers only if I needed massive amounts of current which likely ain't gonna happen in a headphone amp. A couple 6CM3's will power both channels of a typical 300B SET amp, unless your amp uses more power than that or you just like the pretty glow, I don't see a reason to use gas vapour rectifiers.

What about the equivalent gas rectifiers eg 3B28 etc?

Any problems with mercury tubes arcing even if the heater has been 'on' (with no plate voltage) for the correct length of time?

Well xenon tubes aren't full of toxic material, so that's something, but they do lack that hard to decribe magic that MV diodes have.

Yes, MV diodes will arc over sometimes, especially when opperated at the edges of their SOA.

Personally I prefer TV damper diodes such as the 12D4 or the 6CM3 if you need more power. I'd go with mercury & other gas vapour rectifiers only if I needed massive amounts of current which likely ain't gonna happen in a headphone amp. A couple 6CM3's will power both channels of a typical 300B SET amp, unless your amp uses more power than that or you just like the pretty glow, I don't see a reason to use gas vapour rectifiers.

Yeah, damper diodes are usually the most sensible option. I'm planning on using some MVs for a cathode supply, though, as you can't get 5A from any damper diode that I'm aware of.

I bought an 872A MV rectifier from the local surplus shop a couple months ago just because. I can't even fire it up to see the pretty glow but it was just so cool I had to have one.