Next up, the Mullard EL33.
Another nice pair, good condition. Same envelope as the EL37.
Okay so in terms of biasing, these bias really nicely in this circuit. With a GZ34, they are right near their maximum ratings, not quite there, but close. So you could technically run them with a GZ34. For longevity reasons, I would probably run them with just about anything BUT the GZ34 to get a little more voltage drop. So even GZ32 / 5V4 will be fine. If you want to treat them very nicely, GZ37, 5U4, 5R4, or 5Y3 types.
These sound really nice, better than I expected. Tonally, they remind me a lot of the EL37, probably not surprising since they likely use the same materials. A little less airy and a little less body than the EL37, but still very great tone and air, still very smooth. I think these could make for a nice budget alternative to EL37.