I have a NOS Tung Sol 5998 tube for sale that I bought a few months ago from Skylab.
I used it for about an hour in my Crack, it was dead silent in terms of harmonics/buzzing and sounded great but I'm in the process of letting go of some of my gear since I went from an office to a cubicle at work.
Please let me know if you have any questions and PM me if you are interested.
Paypal only please
Shipping to CONUS only
I'll pay for shipping