aluminium plate at the top of the amp what function?

[sunn drone]

The alluminium plate above the tubes is for what function?

[R.G.]

Depends on the amplifier, and since you didn't post a model number or a picture, I can only guess it's there as a shield to keep RF and hum out of the amplifier. I know it does this in some amps.

[sunn drone]

seen it in my randalls ,in layfayette tube heads and univox pa head.Thought it was a common feature but it looks like if tubes are facing down instead of up they don't need it. I remember taking apart a randall pro tubes and seeing the shielding.

[R.G.]

It could also be a heat shield to keep the rising hot air from the tubes from charring the (usually wooden) enclosure. That might account for the difference between tubes-up and tubes-down.

[Chuck H]

It really does depend on the application. Tubes do a lot of things. So does aluminum. Including both in a vague post doesn't mean much.

As R.G. has said, it's either there for shielding or as a heat sink. And as far as I'm concerned tubes up or down isn't the issue. A picture would really help.