The shop amp is back on the bench for repairs after an event during use which was a loud pop followed by red plating of a finals tube. The amp was shut down and the tube replaced but to no avail. There was nothing more to be heard from it.
I focus my troubleshooting with a visual inspection in the vicinity of the finals and immediately locate a melted screen resistor. This is not unknown. Sadly, the original tube that did this was not delivered with the amp and I wish it was, because it would have easily identified the mechanical failure in the tube that caused the current through that resistor to flow to ground.
The owner gave me all new (NOS) tubes to replace the finals with and match by request so after repairs are made I move the project to a matching and biasing event.
This is the first project to use the Hickok 6000!!! A full tube testing event is accomplished with the existing tubes first. We further see that many of the re-label tubes are not what they seem.
Just to be safe, voltages are taken to ensure good operation on the B+ before the finals are added.
This brings the project to the bias and matching event, noting the B+ improved with the 6V6's added. I dial it back the bias to 35ma for matching per tube. In the end the closest match came to .3ma match
A few hours of testing shows all functions working and everything stable.
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