83 and 5Y3 Solid State Replacement Facts

All Hickok testers and most all services testers operate within a well balanced set of operating parameters. A tight balance between, AC line set voltage, transformer load (secondary voltages), test signals, and bias voltage, and the effect of the AC line sag on all the above. These inter related parameters establish the accuracy of the final test result in Gm value.

Many solid state replacements for these tubes are available on the market. Most are commonly found on eBay and the internet. Just installing these units in a tester will toss your tester out of calibration by causing a large variation in the established design parameters and their respective relationships. While some tubes may seem to test close to correct, others will be way off. Both these units provide higher voltages and much lower power consumption which directly changes the effect of line sag, and thus all the other key voltages and currents are also affected. All model testers are effected to one degree or another. In some models this can be off set by available user controls, but requires a good degree of technical know-how and some test equipment to accomplish it. Others can not be compensated for due to their circuit design. Many of the Hickok units fall in this category!

When replacing these tubes with solid state devises it is necessary to install units that are correctly designed for use in a tube tester. In the case of many models additional modifications are required to keep the correct relationships or test operating parameters to ensure an accurate test result.

A simple circuit example will illustrate the effect and problems created by use of the wrong design. The chosen example covers only one operating parameter that of a DC bias voltage to a tube under test. The example is simplified. Assume that the bias control in the tester calls for a setting to supply -13.7 volts of bias to the tube and that this is 1/2 of the controls full value. Notice this is the value as shown in Figure #1 with a 5Y3 tube as designed. Now look at Figure #2 which is using a solid state replacement for the 5Y3. Note that the circuit is the same but the voltage is higher as is the current through the circuit. The same setup conditions to the tube under test would result in a -16.07 volts of bias to the tube and this would result in a much lower Gm test result due to the higher negative voltage on the grid.

It must be remembered that this is only one parameter and several others are also effected at the same time making the problem worse over all. All this is averted with a properly designed solid state replacement and the appropriate circuit modification to the tester too.