Over time, I have managed to collect about a dozen and a half 807 tubes, which are all quite old and of unknown condition. My first 807 based transmitter, gave me a lot of grief, as I couldn't get it to produce the calculated power. After a lot of hair pulling, I discovered that the problem was not in the circuit or the construction technique, but on the 807. It was a dud. In order to avoid similar unpleasant experience in the future, I decided to build a very simple 807 (and 6AG7) dedicated valve tester.

The tester consist of the following:

• A 6 VA 40 V AC transformer, rectified (single diode) and filtered in order to provide a control grid voltage, controlled by a pot from 0 V to -40 V.
• A power transformer which has a 6.3 V winding (filament) and 2x225 V HV, at 50 mA max current. This was fully rectified with 1N4007 diodes, then put to a filter capacitor 100 uF/450V, a 1.5 kohm resistor and a zener chain (5 W zeners, 2x75V and 2x51V) to the ground.
• The top of the zener chain connects to the screen and, via a 50 mA panel meter, to the plate. There are two sockets, one for the 807 and one for the 6AG7. Of course, only one valve can be tested at a time.
• A 47R resistor on the plate and a 10 nF on the screen, to reduce unwanted oscillations.
• The cathode pin on both sockets is connected to the ground.
• The suppressor grid of the 6AG7 is also connected to the ground.

The final result can be seen in the following photo. In this photo, an 807 is tested with 250 V plate and screen voltage, -20 V grid voltage and the measured plate current is about 34 mA - this is consistent with the 807 datasheet, so this is a good valve.

The operation of this tester is simple: a voltmeter is connected via the unused socket to the grid voltage. Another voltmeter is connected via the same way to the plate voltage. The valve is plugged in and the top cap connected. The device is powered up, and after the valve warms up, the grid voltage is adjusted to perform the desired test. The regulation provided by the zener diodes is not very good, and that's why the plate voltmeter is needed.

This project was completed with nearly 100% recycled components. This includes the die cast box, which already had a rectangular hole on the cover, luckily the transformer seemed to fit through the hole just fine. The only new component used is the string of zeners. I have left some space on the top for future expansion of this rig to cover other types of valves in the future...

References

Amateur Radio magazine, November 2005, has an article by Drew Diamond, VK3XU, describing a similar device. This article was used as a source of inspiration.

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