Spiral wound T 315mA went back in there
One of those balance of probabilities fault deduction scenarios. There
was a bust valve but as zero current for zero bias probably failed spot
weld and failed months ago unnoticed as only used at low level.
Bad Hi socket meaning bad Lo input also, due to the cross-coupling
giving cracks and bangs. But also a bad spade at the speakers so
probably interpreted as the input problem and repeated moderate back-emf
surges straining the HT fuse that eventually gave up the ghost at a
normal switch on surge