Hi all, especially @LSchefman , perhaps... A couple of days ago I was jamming away, my HXDA made a weird high-hum sound for a few seconds, then cut out. I checked out the tubes on the back, and one of the power tubes wasn't glowing. Oh, OK, need to buy new tubes, first time I've had to do that, so I ordered some (not NOS, just regular "new" russian made) Mullard EL34s. Got them last night, swapped them in, went to bias them, and the bias voltage reads zero for both. I did go make sure my multimeter was reading properly by doing a bias test on my Archon, they came back good (34, 34, 34, and 35 mV). Testing with main pwr on, stby/pwr on, speaker plugged in, nothing plugged into guitar in jack, etc. The new tubes glow, so there is basic power to the amp - not a blown fuse. I looked briefly at the pre-amp tubes, they seem fine. So, what would be causing this? What generates the bias-voltage signal that I read on the back panel - i.e. what could have failed while main power to the power tubes themselves seems fine? I will continue to troubleshoot, maybe: the stby/on switch failed one of the pre-amp tubes is bad something more serious is dead But hints or tips of what else to check would be appreciated. I will also post this to the PTC section after a while, but thought I'd start here, since there has been essentially zero traffic there about HXDAs (just that one thread about manuals), whereas there is a fair bit here.