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A topic that comes up periodically is that of the F extension and the single Bb horn (and descant). What is the advantage?
First, what it does not do: it does not change a single Bb into a double horn. At a basic level it is simply an extra slide tube that adds a length of tubing that is nominally equivalent to a 1-3 fingering on a single Bb — but in tune rather than a quarter step sharp! In other words, it creates the tube length for F horn.
On the horn seen here you use the longer slide instead of the standard stopping valve slide. Some horns have been made with 5 valves that have the stop valve and also the F extension (a topic of this prior article).
As it only adds that one tube length, used in combination with the other valves it is not long enough to be in tune with any combination (imagine the difference between the length of the F and Bb horn valve slides! You only have the Bb horn valve slides…). Typically then you are only using the F extension for a few notes key to a work being performed. So for example, if you had a low A to play, you would have to pull the F extension out a ways to get the low A to be in tune. But this is better than not having a low A at all! As that would be a missing note on a conventional four valve single Bb.
[You would have that low A on a descant, but only on the high F side; fingering it with the extension would give a much beefier sound.]
Beyond extending the low range a bit, the F extension does more than you might guess. The horn in the picture is my Alexander single Bb, a horn I rarely perform on but still enjoy playing on. Part of the appeal is the sweet tone and it is a horn like Dennis Brain played (more here).
I had an F extension I could use with the horn, but was not using it as it was constructed for a different horn — and needed some repair and alteration. In my recent visit to Patterson Hornworks (more here) I got ahold of the braces it needed, seen in this second photo wired up for soldering.
So what does it do to the playing qualities? Surprisingly, the F extension significantly beefs up the sound of the single Bb horn itself. Another benefit is the connections of the notes are better, smoother. Why? Something about the weight and how it impacts the instrument overall. It is really intriguing.
A competent repairman can probably cobble one together similar to the one I have, but a much better option is to have a horn maker build one up for your horn. These are really handy to have for not only single Bb horns but also for descant horns, something to look into if these horns are among those you play frequently.
