ACCURACY OF A FUNKE W19 TESTER
As you can understand I wanted to know how good the accuracy of the W19 is, because I had fully restored it. Actually the one I have is now amazingly precise. So when you read 10.0, it is 10.0 and not 10.1 with this tester. This could only be achieved by re-calibrating the analog meter. Originally it was not so good, and had errors of a few percent depending on what scale, and what sensitivity. The main error was unlinearity which comes from mounting it not horizontal. It means at Funke they sacrificed some accuracy for a nice housing "design" with the meter mounted at some angle. Note, this is always a problem for an analog meter. They knew that, I am sure.
It does have an error of the filament voltage, with tubes that draw very little current.
You see here a TRUE RMS Voltmeter, and it measures 6.7 Volts heater voltage instead of 6.3 Volt, when testing an ECC88 tube. The heater voltage of this tester can not be adjusted, so it is one winding for 60mA heaters, and 3Ampere heaters as well. When it gives 6.3 Volts at 3Ampere, then "only" an increase to 6.7 Volts at very small current is not even bad. This is a good transformer. The problem with this however, is that tubes tend to give "better" values at higher filament voltage. The plate current improves from that specially with older tubes. And the effect is even more with DHT tubes. So an older DHT tube may easily read 10% better than it is. As long as you know this, you know what you're doing. We do this test here on a worst case, so we take not a DHT tube, and we take not a used tube. Still, the result is larger as you might expect.
In the next picture, we repeat the Funke W19S measurement with the same ECC88, on a more versatile tester, the Russian L3-3. With the L3-3, you can perform any test you like, so imitating a W19 is very easy. We just set it the same as the Funke W19 card Nr 1088, which is for the ECC88, including the TOO HIGH filament voltage of 6.7V the W19S uses. The result is really as expected. You can see both testers show the same number: 10.0 mA very precise. At least this gives a very good feeling about the good function of each, and the good calibration.
Let's go back now to the fact that we tested teh tube at too high filament voltage of 6,7V. Look at the next picture, and we will re-test the same tube, but this time at the correct filament voltage of 6.3 Volts. You see this makes a certain difference.
This tube measured 10mA on the Funke W19. When this test was repeated on the L3-3, with the correct filament voltage for this tube, it measured 9.5 mA instead of 10mA.
How important is 5% ? Here is one way to find that out. Go to a supermarket, and pay 95 Euro for 100 Euro of goods. Tell the cashier, 5% is not unimportant, grab the goods and walk out. What happens now will demonstrate how important 5% can be ;)
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