[OnlySoMany]

First question: what do all the letters and numbers in a tube mean?

Second question: I have heard that these two tubes (5u4g and 275b) are interchangeable. Is this true, or do the extra letters and numbers mean something that would change that?

Any harm in doing so?

Tube veterans please help!

For reference, I am considering switching out the basic 2x EH5U4GB with 2x Takatsuki 274B on my Viva http://vivaaudio.com/en/products/egoista-845/

 

[tomb]

I see both opinions in the ether. Here are at least three different takes on it:
https://forums.stevehoffman.tv/thre...irectly-replace-a-5u4g-rectifier-tube.880263/
https://www.diyaudio.com/community/threads/5u4g-power-supply-to-274b.338095/
https://www.thetubestore.com/preferred-series-274b-5u4g

I googled for those - you could have, too. Although I confess that I used TDSL first, but then found info on the 274B severely lacking. The modern 274B (Western Electric makes a version that sells for $thousands!) appears to be a Chinese knock-off that is an equivalent to the 5U4G. It looks like you will run into problems if you buy a NOS 274B like Western Electric's. Those may really be different.

Sorry it's not more definitive, but you run into that sometimes with obscure tubes.

The very best thing to do is contact the mfr of your amp. They know the circuit and should be able to tell you better than anyone else.