[turretslug]

It's worth connecting the new reservoir negative directly to the transformer secondary HT centre-tap (assuming a bi-phase, full-wave rectifier circuit) if it is a type with insulating sleeve as this completes the charging pulse circuit without sending ripple current through the chassis. The traditional layout would have a short wire link between centre-tap and chassis with the reservoir can clamped to chassis possibly quite a few inches away, thus ripple current flows through the chassis and can modulate the signal circuits with hum. The smoother capacitor negative can generally be connected to the chassis, as its ripple current will be of much lower amplitude than that of the reservoir. In the case of a bridge rectifier, the reservoir negative would go straight onto the bridge's negative tag, again with the aim of completing the ripple current circuit without a chassis path.

Of course, there are 101 things that can cause loudspeaker hum, but this is one of the quickest, cheapest and easiest to get right from the outset- it's puzzling that so many manufacturers omitted this precaution for so long.