155-0059-00
The 155-0059-00 is a Tek-made HF Amplifier chip. The Tek custom chip catalog calls it a "Gain Trim Amplifier". It comes in a 12 lead TO-8 package and is used in the vertical amplifiers of the 400 MHz and 500 MHz scopes from the 7000 series.
The final vertical gain adjustment is after this amplifier and it typically leads into either a 155-0064-00 or 155-0065-00 output amplifier which drives the vertical deflection plates of the CRT.
Repair issues
Note that this IC can fail by becoming non-linear. It will continue to amplify but will show non-linear gain and clipping when using a step calibrator.
According to Modification 050-0650-00, Vertical HF Amplifier Replacement, the emitter resistors need to be selected according to a code printed on the device.