ellipser around input jack

[leo450450]

I am looking at a Ken Fisher inspired schematic where there is on diagram A1A. a line with a loop around the input jack connecting to the 100k plate resistor. Not sure what it means?
Anybody?

[sluckey]

That indicates a shielded cable between the input jack and the tube grid. The circle around that line indicates the shield is connected to the plate of the triode. This acts like a small cap between plate and grid, i.e., a small bit of local negative feedback.

[Helmholtz]

This acts like a small cap between plate and grid, i.e., a small bit of local negative feedback.

An example:
A 10cm shielded cable might have a capacitance of 10pF.
This adds to the tube's own Cag of around 2pF.
Both get multiplied by the gain of around 60 (Miller effect), so the effective input capacitance of the amp will be something like 700pF.
This adds to the capacitance of the guitar cable and significantly lowers PU resonance, resulting in treble loss.