There is a question, does one size fits all? In tube world – it’s not all but you might find treasures in places where you don’t expect.
Let’s consider a very simple case of 6BQ5 or EL84.
The tube is searched and loved by many due to it’s very delicate sound. And as a result, it costs quite a lot. But if move to higher voltage tubes, say 8BQ5, now a fairly expensive tube, could become not very expensive – price drops from say $50 for RCA NOS 6bq5 to about $15!!!! That’s a huge difference!!! But at the same time, you would expense a huge difference in sound to the point where a tube wouldn’t work at all. Well, it’s not the case. Putting 8v tube into 6v circuit would decrease emissions of the tube, but who cares! I tried it in multiple amps, and I couldn’t detect any difference in sound!!! Testing 8v tube on Amplitrex with 6v setting shows exactly why all is still good. Mathematically, you would expect 25% reduction in trans-conductance and emissions – the tester shows exactly the same thing in fixed bias, but in auto bias it actually brings the tube to performance of 6v tube!!!!! Now here is your untold secret – going up in voltage just a bit would save you money and would extend life of the tube – really can’t see any issues with that 🙂
Preamp tubes like 6DJ8 to 8DJ8, ECC88 to PCC88, etc just works.
If your tube amp is a modern one with auto biasing, the change in voltage becomes even less relevant!
Now, when it comes going to lower voltage, things are not as simple. Amount of emissions in a tube starts to stress out a tube and harmonic distortions in a lot of cases go through the roof. So going down is not advisable, but up is not that problematic. Even using 9v tubes in 6v circuit produced fantastic results. That’s 33% difference!!!!
Hope that would help you to find that fantastic tube for your amp for much less. Happy listening!!!