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I've been looking for an F tuba, and have looked at many different models, yet never got a chance to try any of the models I've looked at yet. I am looking for an F tuba with a 6 valve 4+2 setup, I want your opinion on which models are the absolute best, the MOST COST EFFECTIVE and the ones to avoid. What would you say?

I have not played enough 4+2 F tubas to give you any really useful info, but I am lucky enough to have 2 good horns.
I have a Rudy 6 valve, that is a great sounding horn. I also have a 6 valve Alex, that plays in tune!!!

I haven't played many F tubas in general, but I own a B&S 3100 that I am very pleased with. The 6th valve adds enough flexibility in available fingerings to be very accurate in all registers and to compensate for user headspace error. The fourth valve C comfortably full and in tune when I pay attention. I don't know if it characteristic of the model, but I was pleasantly surprised by the response of the false tone register.

Play as many Fs as you can get your hands on. I'm not entirely convinced that the 6th valve is necessary on mine, so don't discount 4+1s until you've played a bunch of bass tubas in general.

My experience is probably not representative. Have fun!