WEM Custom 15

[Mike Watterson]

- earth to heater centre taps at the start of the circuit as well as the end

Things ought to be earthed at only one place. Also with decent layout earthing the centre tap of the heaters makes little difference, or no difference.

There are three main earth schemes:
1) Star wiring to a common point at the main electrolytic.
2) bus wiring, either an actual bus bar (US sets with an isolated chassis, but otherwise "live"), or point to point with no loops, progressing from main electrolytic and OV of transformer, past power stage, stage by stage to the input. So no output or smoothing currents are flowing on input earth wiring.
3) The "good" Aluminium chassis. No earth wires at all. Each set of tags or earthed part is earthed by the chassis at the nearest point.

What you can't have is just connecting points that are all OV/Earth in any order.

Single sided PCBs are usually laid out using type (2) philosophy. Double sided (or more layers), especially if for RF, will use philosophy (3), one layer will be earth. A four layer board may use an entire layer for Earth/OV and and one for +5V. More complex designs might have a logic Earth layer and an analogue signal earth layer.