Peavey PAG 120 power stage diodes - what are they for?

[marvintm]

On my PAG, there are diodes on the power tube inputs that I don't find on other amp schematics of any make. What are they for, and would they affect tone or attack appreciably?

[Enzo]

They keep your grids from swinging positive and drawing current.

Remember those grids and the resistors thereto are sitting at -55VDC or thereabouts. Those diodes will only conduct if the voltage at the anode end is over maybe half a volt positive. SO they will never do anything in the circuit until a signal peak exceeds 55v. Any part of the waveform that crosses into positive will be clamped off by the diodes. So unless you are feeding the bejesus out of those power tubes, they should remain invisible to the sound.