My dumb question for the day is...what is the difference between a 6AU8 and a 6UA8? I'm asking because I have an old O scope that calls for a 2-6AU8's and I have a bunch of 6UA8's. Thanks for the help.

Have not heard of 6UA8...maybe you were thinking 6U8?
The 6AU8 and 6U8 are both triode-pentodes however the pin-out is different so they will not substitute.

ditto on what chad just said there is no 6ua8 try 6U8A

You are both right. I screwed up when I typed it. Thank you both for the information.

Mike,

As Chad says, the 6U8A will NOT sub for a 6AU8. However there are the following acceptable swaps for a 6AU8: 6AW8, 6BA8, 6BH8, and if a parallel filament circuit (your case of a scope most likely) also 6CX8, 6EH8, and 6X8/6X8A. Using one of your 6U8As may cause damage to the scope and/or the tube.

Charles

*source of info: Sams Tube Substitution Book