The #83 is a mercury vapor rectifier tube which is used in Hickok and other  Mutual Conductance tube testers. Some dealers recommend and offer a #5Z3 or a solid state device as a replacement the #83. I disagree on both accounts. The 5Z3 is a better choice for replacement over a solid state device, but there is a reason that the #83 was chosen by Hickok, B&K and other manufacturers.  More info on this subject can be found on my web site article.

I am sold out of my good used #83's and offering a limited quantity of NOS tubes from various manufacturers. All are vintage us made brands.