When I got to this point in the calibration of my 3A8, I did not have this tool, but it quickly became clear that the Tektronix calibration method was just a matter of watching the Nuvistor grid current come out of the input BNC and charge up a capacitor. This could take a long time, because we are supposed to watch the capacitor charge for several seconds, then adjust and watch again. Rinse and repeat. This could take ages!
I quickly realized that it would be much quicker and easier to hook up my HP 419A, watch the current, and adjust the pot for a null. The HP 429A is an analog DC current meter with a zero center scale that will measure down into the pA range. After letting everything warm up for an hour, it was easy to adjust the grid current nulling pot while turning the HP 419A range switch to more and more sensitive scales. In less than a minute, the current was down to a fraction of the Tek spec.
This was done on both channels.
When I was done, I let everything "burn in" overnight, and retweaked the next day: no significant changes. These Nuvistors were probably the originals, so I suspect they were already well seasoned. I'll check them again someday, but if you use new Nuvistors, you'll probably want to redo this after a few weeks of use.
- User:Jadney 16 August 2021