Hi,
I wonder if someone with this knowledge can help me? I’ve noticed on my tube preamplifier the designer has taken a wire from the casing of the small power transformer to one of the adjacent reservoir capacitor clamps. Presumably a handy place to pick up an Earth. Would this be for transformer screening or electrical protection as the transformer is suspended on rubber for anti-vibration purposes. Also that reservoir capacitor has half of its plastic insulation removed so that the clamp can earth its body. Its fellow reservoir capacitor also has half the plastic removed where it is clamped to the chassis. Is that for screening which I would have thought a little unusual? Any advice about this design feature would be much appreciated as I’m wondering if I need to remove the plastic coating when I replace the electrolytic? Thanks.
Sounds to me like, at some point (in production?), (in use?) the preamplifier developed a "hum" and this is how it was chased and eliminated.
The body of the power supply capacitors should be grounded through the lead to ground.
"I’m wondering if I need to remove the plastic coating when I replace the electrolytic?"
Depending on the age of your preamplifier, very likely new capacitors will be a different physical size and not fit in the existing clamps. Personally I would install the capacitors as normal (unstripped) and only revisit if a hum develops.