Matisse Atom Problem

Hi C-Hing,

First of all, I'm not the DIY person.
I'm using the Matisse Atom(original version).

I hope someone can give me some advices/answer of my problem below:

I bought a GEC A2900 to replace my old tube of Mullard M8162 recently. When it runs around 20-30 mins, the 2 tubes(A2900(tubes ok) & 12AX7(CV4004)) will be turned off(looks like no power supply go through to the tubes, but the Atom is still on). I'm quite sure that the 2 tubes is working properly.
I heard that the voltage of A2900 is much high than the other 12AT7, may be that's the reason why the tubes go off.
someone say that the grid voltage can be adjusted, is that true?

Many tks.

Peter