Continuing with the Ampeg SVT Classic the issue now appears resolved. Initially believed to be a bad tube the root cause was a met. Poly 400v capacitor (C10) in the bias circuit. Failed soldering on one leg, not visible, even with my magnifying goggles 😂. Tapping the chassis induced the noise so a probe around the main board with a chopstick identified the problem component, although it is accessible through the top plate after removing the power tube board C10 is still partially obscured. A bit of a task when you have to do this with the amplifier powered up, stood on end & split into three sections 🙄.
Re flowing the legs of C10 has restored noise free operation.
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