The 5881 valve (5881 tube) is a power output tube used in some makes of guitar amplifier.
The 5881 is towards the lower end of the power spectrum for output valves. It can handle a maximum of 23W and so a pair of 5881 valves would give you about 46W tops.
5881 tubes (5881 valves) are actually very similar to the 6L6 valve but lower power (23W -v- 30W) and in a more squat tube. They were introduced in 1954.
Any amplifier which uses a 6L6 can have a 5881 valve instead but you will need to rebias. Also, if the amp was not designed for 5881 valves then you should get a tech to check that the plate voltage is not too high for these valves (they can handle less plate voltage than a 6L6GC.)
Technically, the 5881 valve is an audio frequency beam pentode (pentode just means it has 5 electrodes. In this case it has a cathode, and anode and three control grids. Their primary use is in audio amplifiers. You’ll find 5881 valves in pairs more often than not. One ‘pushing’ the loudspeaker, the other ‘pulling’ it (you’ve probably heard of ‘push-pull’ amplifiers).
A pair of 5881 valves would give you 46W maximum and a quad of 5881 valves would give you double that – 92W.
The 5881 was manufactured in the USA by GE, RCA, Sylvania and Westinghouse. It is currently manufactured by Harma, Tung Sol, JJ valves and others.
If you want a sound which is somewhere between the 6L6 and the 6V6 then the 5881 tube is the one for you. Many say it gives an awesome vintage tone but these things are so much in the ear of the listener. I often wonder how many if these differences would turn out to be imaginary if a proper A-B test were to be carried out!
The 1959 Fender bassman featured these valves.