Help wanted re tube

I suggest you order a new 6829. Per the provided link, the gain is 47 compared to that of 100 with a 12AX7.

Not an equivalent tube.

None of the tube sellers I have tried, including the links provided have 6829 tubes. Until I can find one or two the 12ax7 seems to be working Ok despite the difference in gain. 

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,
-- Al

There is a sleeve of 5 6829 GE NOS tubes on ebay for $18.

Here's the link:

https://www.ebay.com/itm/JAN-6829-VACUUM-TUBE-SLEEVE-OF-5/222797486643?hash=item33dfc3da33:g:n1AAAOS...