I thought the Foxbat used vacuum tubes to improve avionics endurance, and boost its radar output.
It did. And it was an impressive adaptation of old tech to new uses, but it also had the virtue of necessity. The old CCCP simply didn't have the tech base to make the modern avionics used in virtually every western aircraft, in sufficient reliable quantities to equip even their most sophisticated front-line aircraft. Nor could they buy or steal sufficient quantities. Western built aircraft got the same level of performance (or better) using modern avionics. Think about it. If vacuum tubes had been the way to go, we would have been using them as well.
And this is where the comparison between the MiG-25 and the PAK-FA break down.
It is far much easier for the Russians to buy the tech needed on the open market than it used to be, so I do not expect the PAK-FA to be the paper tiger the Foxbat was. But, and as Lando says, this is a big but, when moving from prototype to production aircraft, the devil is in the details.
This is going to be massively expensive. Factories are going to have to be built, raw materials are going to have to be secured, etc., etc., etc. Just gathering the trained workers necessary is at least a two or three year evolution because many will have to be educated from scratch.
It's a bluff. A typical Russian growl, make faces, thump chest, bluff.