I just bought a 34401 from Craigslist and the seller threw in a free 3468. They both agree with my pair of 3456's that my DIY LM399 standard is outputting around 9.998. Measured 9.9991 on a trio of good but long out of cal meters a year ago. So the time has come for me to build a proper standard using what I've learned on this thread.
Attached is my design for everyone to shoot holes in. Component selection somewhat driven by contents of my junkbox. 3K 0.01% divider resistors likely from same production batch but I should be able to verify TC tracking by putting 4 at a time as a whetstone bridge and watching null as I vary temp.
I'm considering placing the LM399's upside down on the vectorboard with RTV and wiring all 4 leads with 30 ga wire to avoid mechanical stress, to slot or not slot, etc. Would that likely help or just add more thermocouples?
Main use would be to calibrate my meters, and to drive a KVD, but no reason not to make it as noise free as practical.
Bought a 3455 years ago for $20 if I promised not to return it if it didn't work. Well it didn't, most front panel switches bad as well as unknown problems after that. Saw a thread on the 3455 reference module, so removing that and making it into a calibrator also tempting.
John