2/19/14 It has come to my attention that someone has scammed at least one person out of money using the content of this blog post. If you have reached this post under the impression that this tube tester is for sale, you are being scammed as well. To reiterate, the Mercury 1100C tube tester that I own and discuss in this post is NOT and has NEVER been for sale. If you’re reading this message it is still not for sale despite what anyone may have told you.
When you’re working on your old radio how do you know the tubes are good? They made a device for testing tubes called, non-shockingly, a tube tester. There are different types of testers out and I have a midlevel one that falls into the category of emission tester. What does this mean? First I have to give a very simplistic description of how a tube works. Tubes contain a vacuum and work by passing an electric current through a filament which causes it to heat up. This heat causes the cathode to emit electrons which, depending on the design of the tube, may or may not be the same thing as the filament. When a positively charged anode (also called a plate) is placed inside the tube the electrons are attracted to it which creates an output current. The release of electrons is known as thermionic emission and attracting them with an anode is known as the Edison Effect. You can read more at Wikipedia if you want. If the plate is negatively charged, no current will be generated. So, you have a device that will pass current in one direction but not the other. This is called a diode. If an additional element, called a control grid, is inserted between the cathode and anode you can control the current out of the tube by varying the current on the control grid. This is called a triode. By varying the current on the grid, the current out of the tube can be controlled. If the output voltage is larger than the grid voltage this results in signal amplification. Through the same basic principles more complex tubes can be created. Wikipedia has lots on them.
So, an emission style tester works by measuring the plate current for a constant grid and plate current. Now you see why it is called an emission tester. These style of testers do have downsides. Wikipedia lists that they don’t measure the ratio of current out for voltage in (transconductance), don’t perform tests at real load, voltage and currents, as well as a few other cons. As a result of this, these style of testers can say that a good tube is bad and a bad tube is good. Still, it shows if the tube functions and this type of tester is much more affordable than the fancier ones.
Now to the actual tester. This is my Mercury Model 1100C Tube Tester than my father was nice enough to pick up for me off of Ebay. It comes in a simple hinged box which contains the tester and the extremely important manual that has the settings for the tester. The list of settings, called the tube data chart, lists the positions to set all of the knobs and switches in to test the tube. Without the chart, the tester is worthless. The manual describes the 1100C as a “compact, ultra-modern tube tester” that is “one of the few truly obsolescence-proof tube testers.” I’m glad to know it!
Knob A selects which switch to connect to knob B and shorts the others to ground. Knob B sets the filament voltage and Knob C sets the meter sensitivity. The switches in the middle, one per tube pin, control whether the tube pins are connected to voltage, ground, or open. The meter has a 0-100 scale to allow comparison readings between the same type of tubes according to the manual. The little neon bulb on the lower left hand corner tells you if there is a short or leakage between the element selected by the switch in the K position and the other elements.
To test the tube, you first identify the type of tube by locating the label on the tube. For this post I’ve picked a 6S8-GT Triple-Diode tube which is why it has 4 entries on the chart. From left to right, the columns list Tube, knob A position, knob B position, knob C position, which switch to set to K, and which switch(s) to set to open. The tube must be tested with the tester in each configuration to test each part of it. The dot beside the tube type means you should read from the diode portion of the meter.
With the tester configured correctly, the tube is inserted into the appropriate spot and allowed to warm up. Next, the switch to test the tube is pushed and the meter read. For this test the tube is “OK”.
That’s pretty much it with this tester. You set it up and then the tester tells you if something happens in the tube. I’ve read that back in the day, normal stores used to have similar, but larger, testers for their customers to test their tubes in. If it failed, they’d have one to sell ya. Imagine that, electronics you could work on yourself!
For fun here’s a picture of the tube glowing and some vintage tube boxes.