Originally I thought that the bias could be adjusted from 0 to -100V with a simple pot hung between those two extremes. Remember what I said about Mother Nature? It started to arc, making noise just like before with the Tube Curve Tracer which also had a pot with lots of Volts across it. So... I replaced it with the circuit of Q6, et al. Now there is less than 1 V across the pot, R60, and the TIP29 takes the heat although it conducts only 2 mA. The collector of the TIP29 now feeds the OPAMP, U5, an LM741, to buffer the voltage to the output connector. The 741 can stand only about 44V max between its power pins, so to help it, I have inserted two transistors, Q2 and Q4, to reduce the 100V available to about 30V. Thus, the 741 does not blow up, which is handy. The bases of these two Qs are driven from the output of the 741 itself. The result is that the 30 V applied across the OPAMP will slide up and down within the 100V available but will never alter size. It is always 30V regardless of the voltage that the 741 is buffering. To give current capability on output, the 741 drives a power transistor, a TIP30, which is inside the feedback loop of the 741 and therefore means that, within a couple of millivolts the output, at the emitter of the TIP30, is the same as that on the Q6 collector.
This circuit was built on a couple of terminals added after I started wiring. I tried to show it in another shot.
The price to pay for this arrangement is that the 741 (or TIP30) cannot go within a few volts of either ground or 100V. I don't mind the 100V part so much as the crucial 0 to -10V range that is necessary for all of the small-signal tubes. That's why pot, R77, is there and switch, SW1B, to choose between the R77 wiper or the 741 output. This pot is the smaller one beside the bigger one.
Some measured values:
Output ripple (up to 10KHz BW): <2.5 mV P-P on high range down to -70VDC
<1 mV P-P on low range down to -10 VDC
Wideband noise (up to 1 Mhz) adds < .5 mV P-P
Output impedance of high range: about 9.6 Ohms
Output impedance of low range: <1K Ohm
Metering this voltage means that the voltmeter would need two ranges as well, so that switch, SW1, has two sections, one to select the output voltage range, SW1B, and one to change the meter scale, SW1A. This meant another small hole to be drilled for this switch and the use of a tape wall. (See pix for LV stuff.)