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One of my favorite artifacts at the Pavek is this Hickok classroom demonstrator, donated in 1990 by Dr. Frederick Koelsch, a retired chemistry professor at the U of M. It’s the only one I’ve ever seen. The bottom half is a conventional 532 tube tester, but mounted in the top lid is a Bakelite panel with four milliammeters and engraved circuitry illustrating Hickok’s patented design.

The top meter on Dr. Koelsch’s 532 demonstrator displays the total plate current through the tube under test. The meter on the left shows the plate current when grid voltage is most negative, while the meter at the right shows plate current with the grid signal voltage most positive. The meter at the bottom of the panel shows the difference between the two states and is calibrated in micromhos.

 


Hickok 532 demonstrator
 
Much has been written about the circuit, designed by Job R. Barnhart in 1934. It allowed Hickok to produce the best-selling dynamic mutual conductance tube tester on the market until the patent expired in 1965.
The terms “Mutual Conductance” and “Micromho” were coined by Professor Louis Alan Hazeltine in 1919. Dynamic Mutual Conductance (Gm) is the measure of change in plate current produced by a change in grid voltage.

Most “emission type” tube testers measure the current created by a fixed voltage between the plate and cathode, usually with all the grids tied to the plate.

Barnhart’s circuit applies operating voltages separately, to all the elements of the tube being tested. It then puts a known AC signal on the control grid and measures the difference between the plate currents produced by the changing grid voltages. The true dynamic mutual conductance is then displayed on the direct reading meter.

The keys to the performance of the Hickok circuit are accurately calibrated potentiometers, a sensitive and accurate Gm meter, and the absolute predictability of the phase relationships of the voltages produced by the secondary windings of the power transformer.

The image on the left illustrates the Hickok design in its simplest form. The cathode of the tube under test is tied to the negative lead of the 150 volt plate supply and to the wiper of the bias control. The control grid is tied the the negative side of the 130 volt screen supply.
The wiper of the bias pot is shown closest to the negative end of the voltage divider across the screen supply. At this point the cathode and the grid are at the same potential, in other words, zero grid bias.

 

Here the wiper of the bias control is shown closest to the positive end of the voltage divider. As the wiper on the bias pot is adjusted toward R118 the cathode becomes positive in relation to the grid. Grid voltage is always measured in relation to the cathode. If the cathode is 40 volts positive in relation to the grid, that's the same as saying the grid is 40 volts negative.

 

If a five-volt transformer winding is added between the cathode and the bias pot, then the grid will swing five volts above and five volts below whatever voltage is dialed in on the bias pot.

 

Although the high-voltage winding for the 130 volt screen supply (5Y3) has a conventional center-tapped winding, the secondaries (#2 & #3) for the plate supply (83) are separated by the Gm meter and English pots.

When P2 of the 83 is positive at the same time that the grid end of Secondary #5 is high, the Gm meter will want to swing toward the high end of the scale.

 

When P1 of the 83 is positive at the same time that the cathode end of Secondary #5 is high, the Gm meter will want to swing toward the low end of the scale. Since the Gm meter can't swing back and forth 60 times per second, it will settle in at the average of the two readings, thus providing a true mutual conductance reading for the tube under test.