If I write a Euphonium part that implies more melodic stuff that is more high in the register, can I use a 3rd or 4th line C clef for this Euphonium instead of using the bass clef or should I always use the Bass clef?
This melodic lines may use a lot of extra lines above the staff so, shouldn't be more correct and easier to read if I use a 4th line C clef? (which I read somewhere It is called the tenor clef I think).
In my opinion, an experienced euphonium player should be able to read tenor clef. It's expected of trombone players, and plenty of euphonium plays also play trombone music, so they should have spent the time learning tenor clef.
And if you're writing something that's this high so consistently, I'm assuming it's written for a more advanced player. Ergo, tenor clef should be a fine notational choice.
Alternatively, you can always go the 8va route and just write it one octave lower in the bass clef.
UK tradition brass band euphonium players expect treble-clef parts that treat the instrument as a Bb transposing instrument. So a written treble clef middle C is performed as an open note (no valves) and sounds a concert Bb a ninth below middle C.
The advantage as far as writing higher-sounding parts is that instead of writing a bass-clef Bb, which is second line up on the bass clef stave, you'd write treble clef middle C, which is first ledger line below the stave.
Treble clef parts like this mean you can exploit the euphonium's considerable range without needing to use other clefs or many ledger lines.
In my experience (UK-based) I've yet to meet a euphonium player who didn't read treble clef transposing parts. Bass-clef reading euph players do exist, but they're rarer. If you find a trombonist who doubles on euph you can give them concert pitch or transposed parts using any clef you like and they'll be able to do the mental gymnastics to play the correct pitch.
