A client gave me a beautiful original Gates Sta-Level to bring back to life. It had all original components but had not been turned on in over ten years. After testing all the tubes, I brought it up on my Variac with just a solid state rectifier installed. The power supply filter caps were good, but not great so they got replaced first. With the new PSU caps and the tubes installed the unit was working ok, but had a very strange frequency response. Above 1kHz it was fairly even, though not quite up to spec. Below 1kHz it increased steadily to about +12dBm at 30Hz! It turned out the feedback caps from the 6V6 output stage to the 12AU7 were leaky. I replaced those and the coupling caps and things are now back to factory spec.