Tune (Hybrid) preamp Making Loud "Burping" Sounds

My VTL 7.5iii is a hybrid tube/solid state preamp. Lately, it makes loud burping sounds,  like what a backfire in a car would sound. I suspect one or both my Amperex Bugle Boy 12 AU7 tubes may be drawing near its useful life or is defective.

Any of my colleagues who could diagnose? I can open the lid and play with the tubes but its a bit of a pain. Thank you. Neal


if i heard any piece of my equipment "backfire", i'd chuck it right out the window!

In tubed equipment, if I hear something strange I always swap out the tubes. Then depending on the age of tubes and configuration (one per channel… two, three, or four) start swapping the old back in until I find the culprit. It has always been a tube.


I always keep an extra set of tubes around. I also listen to my systems a lot daily.