I needed a circuit that would be relatively simple but would provide a high plate and screen grid voltages as well as a stepping control grid voltage with steps of ½ V, 1V each, etc. For the plate drive I used a half sine wave straight off a high voltage transformer winding since I realized that the plate current would follow the same characteristic path going up the wave as coming down. The wave form need not be precise, calibrated or any particular shape as long as it rose and fell in a non-abrupt fashion. It did not even have to be consistently the same shape each time it rose or fell. The shape of the resulting curve is determined solely by the characteristics of the tube under test. This eliminated any need for a precision high-voltage ramp generator but I still needed to acquire the transformer for this...
I wanted to have several tube sockets for the various existing base types but eventually settled on four: 7 and 9 pin miniature plus octal sockets. I also included a 4 pin socket to allow testing old rectifier tubes.
The stepped bias generator is a cheesy 4-bit R-2R ladder type digital-to-analog converter driven by a counter advanced by the 60 Hz wave from another winding on the transformer.
The filament voltage came from a transformer ripped out of an old ReadRite tube checker from the 1940's which provided many filament voltages from 1.1 V to 110 V AND a switch to select them.
Finding a switching method to accommodate all of the various and sundry tube base pin-outs proved to be futile at best so I avoided the whole issue and used patch cords with each numbered pin and each drive signal brought out to 5-way banana connectors. This gave me ultimate connection flexibility and prevented me from going mental trying to figure out a good switching method.
Finally, the biggest concern was measuring the plate current. I didn't measure the cathode current since it is the sum of ALL element currents including the screen grid. The place where the plate current is measured (at the plate) was elevated to about 400V at the top of the wave. So after dividing the plate voltage down to 0-6V with a resistor divider so OP-AMP ICs could work with it, a large gain, very-well-balanced differential amplifier was needed. The LMC6082 dual precision OP-AMP did this very well and to boot its signal range includes ground so it could be wired up as single-supply.
Both plate current and plate voltage readings were then output on BNC connectors to an oscilloscope operating in A-B mode so the final chart of these two quantities could be plotted against each other.
Some people have written asking for a clear copy of the schematic since the one that shows up was pretty fuzzy. I have removed it and replaced it with a PDF version. The green line encloses all of the circuit on the small hand-wired circuit board. A couple of parts of the circuit are expanded upon in step 7.
There were a couple of surprises in the build and I will talk about those later.