Blackstar HT Studio 20 finished.
So after rebiasing, ruling out all tubes as being related to the buzzing issue the root cause was eventually found to be a soldering issue on a power supply filter capacitor leg. It would have been possible to reflow without removing the pcb but because it wasn’t possible to see the state of the rest of the soldering I chose to remove the board. Looking at the other high voltage component legs it was apparent that they too would benefit from additional solder.
Following reassembly of the chassis the amp was put back on soak before a PAT test.
No more buzzing 😁
For those who are interested, the insertion of the jack plug on this amp is effectively the standby switch, the power switch toggle on the front panel being the heater enable just like the old days when there were two switches on the panel.
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