The 6829 is a variant of the 12AT7. It shouldn't be hard to find one.
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Read this thread before going to a 12at7. Yes you can substitute but th.e results may vary Try tubesandmore dot com
I would not consider the 12AX7 to be a long-term solution. In addition to the gain difference, various other parameters are vastly different than the parameters of the 6829, including "plate resistance" and "transconductance." Also, the maximum power rating ("plate dissipation" capability) of the 12AX7 is significantly less than for the 6829.
Basically, as I see it, that substitution amounts to incorporating a significant design change, which is not usually a good idea without having detailed knowledge and understanding of the entire circuit.
The 12AT7 is a much closer (although less than perfect) match.
Good luck. Regards,