Catastrophic failure of my VTA ST70

The problem: Heard a loud whomp type sound and went in the other room to find my power amp dead and a distinct smell of something burning.

What I can see: Fuse is blown and I wasn't able to see anything clearly burned/melted/scorched. There is some residue around the choke inside the unit that may be burn/heat related.

History (Why I'm an idiot). Since the build in early 2020, there has been a loud electric zap sound through the speakers infrequently. I posted back then and Bob suggested replacing the rectifier which I did. The problem persisted. The zap would somehow result in a system shutdown and initially I'd wait 10 minutes before starting it back up. But being impatient and not so bright I eventually started just flipping the switch down and back up and I'd be back in the music in seconds. Some time in the last year I'd also checked the bias and found I was unable to get the bias low enough on any of the tubes. All of this went to the bottom of a to do list and lived there until now.

Any and all suggestions are welcome of course. I'm assuming there are just some things worth replacing that will hopefully resolve the issue?

Thanks in advance

OUCH!....Hope it all works out for you. Following to see what happened.

On a side note and just as a suggestion from a very happy VTA ST-70 builder. I have my preamp managing all my inputs having the output going to my ST-70. in powering up or down, I always have the preamp muted soas no signal to the amp. Zero speaker "pop"

I’m quick to point my finger at rectifiers. ANY ST-70 ‘type’ amp is generally over-taxing the rectifier as originally designed. This can often lead to early rectifier failure, which can manifest in multiple forms. Today’s music is NOT the Lawrence Welk and Doris Day if yesteryear!! We push amps a LOT harder than most HiFi users did back in the ‘50s, ‘60s, and even into the early ‘70s. I’d take a good hard look at your rectifier, make sure you have a good quality unit in there, and do as Bob suggested and just look over all your build’s connection points. Beyond that, some deeper diagnosis may be necessary.

of course ignoring the bias is bound to get to the point where the bias keeps creeping up enough that the tubes are drawing more current than the rectifier tube can handle,
and then BOOM !! Simple common sense. And at least two ways to prevent this.
One - fuse the rectifier output. (and also the output tubes) Two - switch to dual rectifiers or bite the bullet and use an SSR (and then you can worry about the instantaeous 500+v on all the tubes)
Do yourself a favor and buy at least two meters to keep tabs on the bias EVERY WEEK !
AND - do not turn on a hot amplifier, wait at least 3-4 minutes to turn on after it's turned off, or . . . POOF !!

Safe to assume I should replace the rectifier tube as part of the process? I'd already planned to order a new power switch, speaker posts, and those digital bias meters. Sounds like the fuse kit is a good idea as well.

Anything else like the choke (original) or the quad cap (new upgraded in 2020) that make sense to replace? If I'm going to be under the hood meticulously checking every solder, I figure I might as well swap out any of the other more likely culprits.

And one more thing? does the fact that all tubes had the same bias issue tell me anything about where the issue may be? Am I safe to focus less on the tube sockets?

Any practical advice is greatly appreciated as I'm woefully ignorant of how exactly this piece of technology works. I'll try to get some photos up soon as Bob suggested.

Thanks

many times when a rectifier tube blows out, it can short out the quad cap.
With the power off and the cap discharged, you can check it with an ohmmeter.
All sections should measure greater than 2M ohm

Solved (at least so far). I replaced the switch, added digital bias meters, added fuses on each output tube, and probably most significantly installed the dual rectifier pcb setup[. All of this, maybe short of the switch, was from tubes4hifi. I needed some guidance on the dual rectifier install, but for someone who is generously called a hobbyist, this was very manageable.

Would the solid state rectifier prevent most of this issue?

I saw some posts about solid state rectifiers, but like I said, I'm a hobbyist and sorting out how specifically to replace the quad cap and single rectifier tube was a headache I wasn't interested in. The tubes4hifi upgrades are specific to the st-70 and there's great support, which I fully took advantage of.