ST70 makes a very brief but startling sound

I have an ST 70 built by Will Vincent.  I turned it on and maybe a minute or so later, back to my speakers an hear a very loud sound I can only describe as a large box of books being dumped on the floor from considerable height.  I look at the amp and the bias meter is pegged.  The sound was brief and I powered off immediately after.  I am not a technical person.  Where should I start to diagnose this?

To clarify, this amp has Tung Sol KT66 output tubes and is tube rectified. I do have a spare rectifier. The fuse appears to be intact. My memory is that all the tubes were glowing when I shut it off but that was a pretty brief observation as I was startled and a little panicked. There are no visual issues with the tubes. One person suggested changing the rectifier but I'm hesitant to throw another tube in there, turn it on and cause further damage or unnecessarily burn up another tube. Any input would be much appreciated.

kayvee wrote:I have an ST 70 built by Will Vincent.  I turned it on and maybe a minute or so later, back to my speakers an hear a very loud sound I can only describe as a large box of books being dumped on the floor from considerable height.  I look at the amp and the bias meter is pegged.  The sound was brief and I powered off immediately after.  I am not a technical person.  Where should I start to diagnose this?

To clarify, this amp has Tung Sol KT66 output tubes and is tube rectified.  I do have a spare rectifier.  The fuse appears to be intact.  My memory is that all the tubes were glowing when I shut it off but that was a pretty brief observation as I was startled and a little panicked.  There are no visual issues with the tubes.  One person suggested changing the rectifier but I'm hesitant to throw another tube in there, turn it on and cause further damage or unnecessarily burn up another tube.  Any input would be much appreciated.

If this is an original ST70 then KT66 is unsuitable, they need lower value of grid resistor to be stable.
EL34 is recommended ( and has lowest distortion ) for this amp.

You might have encountered just the kind of problem ( flash over) that could be caused by too large grid resistor, a tube encounters grid currrent avalanche.

Thank you for the reply. This model has been modified by the builder and running the tubes supplied by him since it was built 2-3 years ago. Will knows his stuff and doubt this to be the case.