Following the evidence
A substantial 100 Hz ripple was traced to the B+ reservoir stage. Around 126 V peak-to-peak was recorded at the C72/R120 supply node, compared with around 2 V peak-to-peak at the following filtered node.
With C72 removed, inspection found electrolyte leakage, a damaged positive termination and an internal break at the positive lead.
Checking the valves
All seven valves were tested individually and found serviceable. The three ECC83 preamp valves also passed the microphonics check. The findings supported repairing the power-supply fault without replacing serviceable valves.
Replacing the failed component
C72 was replaced with a 47 µF, 500 V electrolytic capacitor. The replacement was checked on an ESR meter before fitting, soldered in place and mechanically secured. Accumulated dust was removed from the chassis and internal assembly.
Testing the result
The supply was raised in stages from 25 V to 230 V using the workshop current limiter and variac. Operation remained stable and the original power-supply buzz was resolved. A final play test confirmed normal audio operation.
The recorded Class I electrical safety test also passed: earth continuity 0.07 Ω, insulation resistance 19.99 MΩ and substitute leakage 0.10 mA.