Many years ago, Paul Covey told me that if you add a spacer, or ring, to the bell socket of one of his oboes, you could get more depth in your tone. He made one for me; I tried it and really liked what I heard, but it made the pitch of the low notes point down, so I stopped using it. Fast-forward to 2018. I discovered one of these rings, similar to the type that Covey made, in the bell socket of a bell on a kingwood Yamaha. Bede Hanley (a friend and customer playing principal oboe in Auckland, Australia) had been experimenting with this same idea that I had heard from Covey so many years ago. I made 3 thicknesses of rings- .5mm, 1mm and 1.5mm to experiment with and found that they work really well with Yamahas- both the 831 model and the 841 model. Many of you have heard about this from a YouTube posting by Canadian oboist Jasper Hitchcock, another friend and customer.
Here's the story with these rings: There's a small gap, or space, between the end of the tenon on the bottom joint and the bottom of the socket of the bell of the Yamaha Custom (831 and 841) Model instruments. This space is approximately 1mm. If you fill this gap, it seems to add more lows to the tone, without affecting the pitch of the low register. Some players prefer the .5mm ring; others like the 1mm ring. I happen to like the 1.5mm ring the best.
I'm selling sets of these rings, made of aluminum so they don't add any noticeable weight to the instrument, for $30 per set, including shipping. They have a center diameter of 16mm (to match the bore of the oboe at that point) and an outer diameter of 22mm so it doesn't move around in the bell . If you put a little bit of cork grease or vaseline on the bell socket side of the ring, it sits securely in the bell but is easily removed with a little wiggling of the finger.