what is this?

[otto]

In your opinion what is in this schematic the line going from grid of V2a to general input for?
http://www.drtube.com/schematics/marshall/4210.gif

[Enzo]

That is a mute.

Note that it does not go to the input, it goes to the sleeve contact of the input jack, which is grounded, and the arrow means it is a cutout contact, as opposed to the actual sleeve contact. This is a contact that opens when a plug is inserted.

the net effect is that when nothing is plugged into the jack, that line is grounded. That grounds the grid of V2A, so no noises generated before that point will come out the speaker. The initial stages are where the gain is, and so also where the noise comes from. The grounding contact for the input jack tip grounds off only the very input to the circuit, but does not silence the gain stages.

If you never have the amp sitting there without a guitar plugged in, it will never matter to you.

it is just an extra step to keep the amp from making excess noise.

Marshall does that on a lot of models, including solid state ones.

[otto]

Great answer Enzo, thank you very much!

[Doug H]

awesome, I was wondering about this as well. I can scratch one more question off of the list.