who mfg valvo tubes?

It depends on the tube. From what I gather many were produced by Philips owned companies. Which means that they could be Philips or Mullard (maybe Amperex Holland, but I am fishing here). Again depending on the tube though they might even be Siemens from Germany (labeled Valvo) or even others that I am not aware of.

I think Mazda produced some Valvo tubes, too. The 5AR4 if I am not mistaken. Like Dekay points out, Valvo production (and much of the other tube production from the past) was all over the place. This is why it is important to develop relationships with knowledgable, reputable sources for older tubes.

Phillips bought Valvo, so a lot of Valvo tubes, were made by Phillips, to Phillips specs. The only true Valvos, I've seen, were distinguished by their tall bottle. But Phillips, made some great sounding tubes.

Yes, some Valvo were made by it's own factories while some were made by other Philips' plants.

I have genuine Valvo EL84, ECC83, E188CC, E88CC, ECF80 and E80CF tubes.