Marshall JCM900 Red Plating and Bias Issue

[The Dude]

C15 is indeed in series. It couples some AC from the transformer to the rectifier, which makes the DC bias voltage.

[g1]

For noise suppression? The problem with the X2 is that it is only 250VAC rated and seems at least a bit prone to failure in this application. If you go with class X, I would suggest at least upping to X1 which is 400VAC rated.

That's not right.

X Class has ratings up to 660Vac. 47nF at 660Vac capacitors are readily available.

I know the schematic calls for Class X, but does anyone think they are really necessary?

You are correct, not all X2 are 250V. The ones Marshall used were though. I've edited post accordingly.
I don't think class X are really necessary. Stipulation for X is that in case of failure, there must be no chance of shock. So it can not be a safety feature.
I asked about the possibility it is used for noise suppression, but I agree with The Dude's suggestion that they are using them because they supposedly fail short, rather than open. But all the ones that fail in this series of amps fail open. And they seem more prone to failure than any other series of amps that uses cap coupled bias feed.
So as far as I'm concerned, I will continue to replace them with good quality, approx. 600V rated caps. Like all the other manufacturers use in similar circuits that are not failure prone.