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.
Not an equivalent tube.
Help wanted re tube
Recently the sound level on one of my speakers dropped below that of the other. By trial and error I tracked the fault to my tube based CD/SACD player. I decided to try changing the tubes. One tube was an 6H3ne for which I had a spare but the other was a GE 9 pin tube marked LN188-5 and 6829, a tube I am not familiar with. Can any of you tube experts out there provide any info on the tube and on which tubes might be compatible?