In designing a vacuum tube tester there are many factors to consider: the maximum test voltage, the maximum plate current, the range of the negative supply for the grid and the voltage/current rating for the heater to name a few. There are always trade-offs between cost and performance. Here we focus on the high voltage aspect. For an operating voltage as high as 800V DC in etracer the components need to be carefully selected to guarantee a maintenance -free operation for a very long period of time. And like an old saying “Quality doesn’t come cheap” components that are designed for higher voltage applications tend to be bulkier and pricier compared to their lower-voltage counterparts. It is found that several tube testers products and kits in the market have poor components selections. They are attractive at first glance because of the cheaper price tag but they might not last long and they might cause severe damage to the device or incur safety issues if something went wrong.
Gas test can be often found in traditional tube testers like a TV-7. The assumption for gas test is for a gassy tube there exists a non-zero grid current due a leakage path between electrodes (plate, cathode and grid) under negative grid bias. The more the amount of gas inside the DUT the higher the grid current. However, according to  the grid current is a sum of at least four types of currents and leakage due to gas is only one source of it. Figure 1 is excerpted from  showing the four sources of grid current. By observing figure 1 we can see even the direction of the grid current is not necessary fixed.
Figure 1. Sources of grid currents
Nonetheless it might be desired to add the “gas test” function to etracer as it might reveal some interesting behaviors of the DUT against different resistances in the grid bias circuit.
The maximum current measurement capability of etracer is designed to be 300mA. This capability is more than adequate for testing and tracing most of common audio tubes such as 300B, KT88 and 6550. However, it might be desired to have a higher current measurement capability for testing tubes like 6C33C or KT150. This application note describes the modification method needed to increase the current measurement capability to 600mA. Below figure shows a traced curves set with a Telefunken EL156 with a screen voltage of 500V!! under pentode connection. Please note the maximum plate dissipation is set to 100W to allow traces in the high current region.
Introducing the Sbox12 automatic wiring box for etracer. The Sbox12 is designed to sit underneath the etracer Model-01 chassis and performs automatic wiring for the users.
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