SFPat

I have problems playing in the low register (below A below the staff) with the Ambassador. Upon inspection, I see that a small amount of the plating is missing near the bottom of valve #3, which after searching this forum, I've learned is probably the cause of my problem. What are my options short of buying a better trumpet (which I will do sometime in the future). Can I replace the valve, assuming I can find a suitable one? It otherwise plays well and has a good tone. Valves 1 and 2 look good with no visible signs of wear and there is good compression from all three valve slides (they all go "pop" when removed).

ComeBackKid

Not so fast. Unless the area of worn plating extends up and into the lowest 3rd valve port (the one that aligns with the outlet pipe going to the 2nd valve), it is not involved in controlling the sealing of the valve. Here is a way to test for sure.
(1) Remove the third valve slide and press down the 3rd valve and then put your finger over the outlet pipe (upper one) and blow into the mouthpiece and feel for loss of pressure (leakage). Then replace the slide. (this tests the sealing of the 3rd valve in the down position).
(2) Remove the 2nd valve slide and press down on the 2nd valve. Then place your finger over the outlet pipe (lower) and again blow. Compare the rate of leakage to what you experience before (this tests the 3rd valve in the open/up position assuming that you are correct about the other valves being fine). Then replace the slide.

If you do not notice a significant amount of leakage in one or the other positions of the 3rd valve, then the valve is OK.

My experience has been that with a leaking valve(s), there is an airy/hissy sound to the tone and it affects all registers. From what you are describing, it sounds like either a mouthpiece issue (I have trouble with low notes with some of my MP's) or a valve alignment issue (have you checked the corks and felts?). Give it a try.

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Come-Back Kid
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SFPat

Thanks for the info. I'll check for the leak as recommended.

I doubt if it is a mouthpiece issue though as I can play the low register fine on my son's Bundy using the same mouthpiece (Bach 7c).