Regardless of the other front end changes, which you can quantify using your other mics, the tube swap in the 99 WAS a significant change, so, not to worry.
regards,
David

I just finished my first session with a NOS Valvo from W. Germany into a Vipre. Made me remember how I felt when I first got the U99. The deterioration of the Russian stock tube was a slow process but eventually got to be very noticible. Next time, I'll probably try a Telefunken or Amperex, but the Valvo was a tad cheaper and sounds worth the money to me.

I'm bumping this old thread because I have just now become aware of my potential ability to change/improve the performance of my early U99 (S/N 880) by replacing the stock Russian-made EF86 valve. So I am hoping that some of those more experienced in this arcane art would care to comment on any new developments or further gained knowledge. For instance, after running a fruitless search for the British GEC CV4085, previously one the most highly praised replacements, I am wondering if it may have been hunted to virtual extinction.

On the other hand, I came across another replacement option, the NOS Tesla EF806S, that looks promising. Anybody tried this one?

Gonna try and replace mine as well.

Mullard used to make a different EF86 tube with a smooth silver coloured plate that was better performance than the expandamet style plate in the Mullard shown in this thread:

I use them in my 2 buss line amps after the Neve 33609 in this case, but they seem to sound slightly better to me than the other sort of Mullards.
Telefunken is always good to if you can get them.