On the page of American Classics 5F6-A Bassman Parts of Ceriatone we can see the description of tubes including the power tubes : 5881*2. And, on the other hand, on the layout, we can see that the power tubes are 6L6GC. Where is the truth? This tubes are different, no?

On thetubestore.com, we can read for 5F6-A Bassman : " Power tubes are a pair of 6L6 (5881s were originally specified). Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current TAD 6L6WGC-STR and re-issue Tung-Sol 5881 are the best option in place of NOS tubes.... DO NOT use other types."

Which tubes can i really put?
Thanks.
 

I built a Weber 5F6A kit about 3 years ago. Very cheap components compared to Ceriatone BTW. Nice speakers though and it sounded great. I originally used the electronic rectifier WZ34 Copper Cap. I now have a JJ GZ34, OK but not as good. I have used a variety of power tubes, including Tung Sol 5581s (Sovtek makes a 5881, as well), Winged C 6L6, Sovtek 6L6GC WXT+, TAD 6L6WGC-STR (supposedly "vintage" tone), and JJ 6L6s. My favorites in the Bassman are the JJs, by far.
It sounded exceptional with the Weber WZ34 rectifier and JJ 6L6s. The TADs were disappointing in the Bassman but sounded killer in my'66 Super Reverb. The 56FA is the most sensitive amp to tube changes, that I have. When switching tubes you must rebias to get the bets tone and tube longevity

I smoked the Weber power transformer in the amp after about 6 months due to a bad (shorting, cheap, crappy, weber, chinese) on/off switch. I replaced with an OEM Fender PT ($75) and it worked just fine. That WZ34 only lasted about a year, unfortunately. The JJ GZ34 has been going for 2 years now. I have a NOS Mullard GZ34 in my Super Reverb that has been in there for at least 12 years.

Bob at Eurotubes is a great reference for tubes and sells retube kits for the 5F6A for a reasonable price. His tube matching is impeccable.

A real 5881=6L6GB
 
It was the default 6L6 for Tweed fender's, Leo moved to the more powerful 6L6GC in '61/'62 or so.

The Sovtek 5881 isn't a real 5881, it was a Soviet aircraft tube that was close to a 5881 on paper, same pin-out, comparable power, but completely different tube construction.  But Mike Matthews (who at the time had just shuttered Electro Harmonix) dubbed it the Sovtek 5881 and started importing it to the west.  It became a staple in almost every mass produced amp because by the late '80's, early '90's, there wasn't any other power tube available...6L6GC were completely out of production worldwide, as were 6V6GT's, and the only source of El34's was Tesla which was under embargo and latter blown up by US bombers (Balkan War; they also blew up the Yugo car factory).  there were Chinese 6L6GB's and maybe even El34's, but they were horrible tubes at the time.

The newer Tung Sol 'reissue" is the first real, quality 5881/6L6GB made since the '70's or '80's.  But most people use 6L6GC's instead because they offer more power and headroom, and there is a greater selection available for those who want to waste time "tube rolling for tone."  6L6GC's also handle higher voltages better and many 5F6a's run higher bvoltage because of higher wall voltage.

There is another confusing layer on top of this, most Russian 6L6GC's aren't real 30-watt 6L6GC's, but are 25-watt tubes, which puts them in 6L6GB output territory, but they can handle higher voltages than 5881's/6L6GB's.  Such 25-watt 6L6GC's are the SED, Sovtek, the New Sensor (fake) Svetlanas, and EH.

Another 5881/6L6GB option is the JAN Phillips 6L6WGB, these are great NOS military tubes from the '70's and probably the most durable 6L6GB available