140318-0916 EDT
GoldDigger:
I never had a Heathkit DC VTVM, but I somewhat recall the suggestion of a battery for its calibration.
I have a Heathkit scope and audio oscillator. Both still work in some fashion. The audio oscillator better than the scope. Whereas, my HP 200CD, 400H, and 412A do not. My Tektronix scopes are not working.
About my homemade DC voltage reference from 1964. I had a Boonton AC & DC Precision Calibrator at the time, but having a small simple 0.1, 1.0, and 10.0 source was convenient. So we made one. Probably was calibrated from the Booton. Boonton's use of the word precision did not mean extremely precise. I probably used a General-Radio potentiometer, 1454-A, to set the output voltages in combination with the HP 412 A as a null sensor.
Present characteristics of my voltage reference, 50 years after it was built. The following measurements were with a Beckman 4410. A side note: electrolytic capacitors of the 30s, 40s, and somewhat into the 50s weren't too good, poor life. By the 1960s they got better.
Last night shortly after power on, with 120 V and and varied from 80 V to 140 V, the 10 V output with 120 V in read 9.987 V. The unit was left on overnight with 120 V input.
This morning:
At 120 V in: the three outputs were 9.987-9.986, 0.9987, and 0.997 V.
Measured source resistance of the 10 V output.
9.987 V with 10 megohm meter load.
9.972 V with 1 megohm load.
9.847 V with 100 k load. Load current approximately 0.000,1 A.
Calculated internal resistance is (9.987-9.847)/1/10,000 = 10,000 * 0.14 = 1400 ohms.
This voltage change was 0.140 V, and note the change with the 1 meg load was 0.015. Looks close to a linear resistance.
Output voltage variation vs line voltage. Only load is the Beckman 4410.
080 V .... 9.976 V ...... capacitor 32 V about 12 mV ripple
090 V .... 9.980 V
100 V .... 9.981 V
110 V .... 9.983 V
120 V .... 9.986 V
130 V .... 9.987 V
140 V .... 9.988 V ...... capacitor 55 V
Peak change 90 to 140 V is 0.008 V.
Output ripple 0.60 mV.
Capacitor ripple is 12 mV.
The circuit basically consists of:
A homemade power transformer from Warrick parts.
A bridge rectifier.
1000 ufd 50 V Mallory capacitor.
Resistor to a 19 V Zener.
Another resistor from the 19 V preregulator to a temperature compensated 12 V
Zener diode.
Then a resistor divider network including Trimpots for each output voltage.
I have no idea how much drift has occurred in the basic calibration over the last 50 years. There is no frame of reference. There is likely little change.
The Boonton still works. It is based on an OA2 or similar voltage regulator tube. Still using the Beckman 4410 the Boonton characteristics are:
090 V .... 9.989 V
120 V .... 9.950 V
140 V .... 9.958 V
Peak change 90 to 140 V is 0.039 V.
Output ripple 0.12 mV.
Output internal resistance using 100 k load is 4.480 k ohms.
For those unfamiliar with measuring the output resistance of a DC supply or battery the following is a method.
Measure the open circuit voltage of the source. Assume the meter is an infinite input resistance, otherwise take its value into account.
Apply a known load current and measure the change in output voltage.
Rint = Vchange/Ichange. Do at another load current and compare. The apparent internal resistance may be somewhat different at different loads within a normal operating range of the source.
It may seem that some of these comments do not apply to the original post, but having an ability to work with Ohm's law in various ways should help get a better understanding of its use.
In working with vacuum tubes one would not usually describe the tube i-v curves in terms of Ohm's law. These are basically non-linear curves, and graphical analysis would be used.
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