Purpose of diodes, cap and res between chassis and power ground?

[Fast Edward]

Does anyone know the purpose of 2 diodes, 1 cap and a 10 ohm resistor between chassis ground and the power supply ground in some Marshall amps?

[tubeswell]

Ground-return elevation.

Fender does it in their RI amps as well. The resistor places a few (handful of) ohms of resistance between audio ground and chassis/earth ground. This minimizes hum by eliminating ground loops internally (and externally). It is usually a Flame Proof/Fusible resistor, which will not burn, but merely open.
The diodes provide a safety feature by providing an alternate path to earth ground if the resistor opens.

[Enzo]

The diodes are there to clamp the voltage across the resistor to about half a volt - whatever the junction drop of the diodes is. They are protecting the resistor.