Bad rectifier or...?

[plumcrazyfx]

So, I have a new Dr Z DB4 that seems to work fine.  I wanted to upgrade the Chinese 5AR4 to a Mullard 5AR4 so I tested with my Aplitrex AT1000 and threw it in a Champ build with a single EL34 and played for an hour before chucking it in the DB4.  Put it in the DB4 and it blows a fuse on standby.  Not immediately when I turn it on but after about a minute, did it twice.  Since it's on standby, don't know if it matters but it's on the full 4 tube setting.  Amp has a 2a slo blo.  Put the Chinese back in for a few minutes of playing and seems fine.  Any ideas?

[92Volts]

There are a few ways to use a standby switch with a rectifier tube. Neither is perfect and I’m not sure I’d put an expensive rectifier in that amp... the effect on sound is minimal anyways.

If the rectifier doesn’t see high voltage on standby, that’s very hard on the tube. When you leave standby, it’s warmed up and ready to conduct a surge of current to charge the power supply.

That’s bad, but the only way it could blow a fuse on standby is a shorted heater which seems unlikely 

If the rectifier does see HV and charge the power supply during standby, this isn’t as hard on it (though it can be hard on the standby switch itself).

However, it’s more likely that the tube is already bad and responsible for the blown fuses if the amp is set up that way. Any HV short or arc, like plate to plate, plate to cathode, etc, could conduct plenty of current even in standby mode, and blow a fuse.