Overlooked item concerning
tube tester calibration



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by; CE “Sonny” Clutter (the Radiola Guy) Oct. 2006

For most tube testers there is little calibration info is available. For the few that do offer calibration data, almost none address the heater (filament) voltage. A quick way to determine if this might be the issue with yours is to set up your tube tester to test a 6L6. Place the tube in one of those test sockets adaptors (the type that has test pads on them so you can measure each pins voltage) and plug it into the tube tester. Now with the adaptor and tube plugged in the socket, set the line adjustment and measure with an accurate DVM the heater voltage (pin 2 & 7 of a 6L6).

I don't offer detailed instructions for every tube tester model there is because of the many differences in them. Most of them require a resistor value change or adjustment and to do this you have to figure out what resistor or adjustment that might be and determine the appropriate value.

Here's a brief explanation for the specific neglected calibration issue:

I have found the filament (heater) voltage low on nearly all the tube testers I have worked on in the last 10 years, the voltage will usually measure anywhere from 5.2 - 5.6 volts with the line adjust set properly. You cannot make an accurate test with the heater voltage is that low!

Most tube testers do not have an adjustment for this, so you must change the value of the series dropping resistor in the line voltage meter circuit to get the correct "line adjust" reading at the proper filament voltage. There are only two way to alter the filament voltage; the filament voltage selector switch & the line adjust rheostat. To make the adjustment, make sure the tube tester meter is *level, tube in the socket, then adjust the line voltage so that the voltage reading on the heater of the 6L6 is 6.3 volts. Then adjust line adjust calibration pot (or change the resistor value) so that the meter is set to the set line voltage mark.

*Be sure your unit is level when making the adjustment or at least make the final test with you tester level. Tilting on its side will often alter the meter reading.

Bottom line, the goal is to be sure that the heater voltage on a 6L6 is 6.3 volts with the tube in the socket ready to test and the meter reading is on the "line adjust" mark. Keep in mind; you can't calibrate the filament voltage, you can only calibrate the meter's "line adjust" setting.

IMPORTANT NOTE: a few tube testers do have a compensation adjustment for this. It's a "pot" and on some of the Hickok’s and it's often located near the meter (NOT THE SMALL "BIAS" POT THAT'S MOUNTED ON THE METER), this adjustment only affects the meter reading in "line adjust" mode. So all you have to do is measure the heater voltage of the 6L6 test tube in the socket and set up to test (but test button not depressed) and set the "line adjust" so that the heater voltage measures 6.3 volts. Then adjust the "pot" so the meter is at the "line adjust" position. If you are not sure of what you are adjusting, DO NOT ADJUST!


The above does not apply on the '539B/C as it has a direct line voltage meter. The issue addressed (above) relates to the lack of info on most tube testers (including other Hickoks) to verify the tube filament voltage as it relates to the line adjust setting. Since the 539B/C does not have a "line adjust" setting, this info does not apply. However, I have found on some 539's where the line voltage meter reads incorrectly and there is no adjustment to correct this. If this is a problem, the meter must be replaced or you need to compensate by adding a resistor in series with the meter if it reads high. There is no way to compensate if it reads low. B&K "Dyna-Quik" tube testers do not have a line adjust setting as they have a different method of maintaining the right filament voltage.

The bottom line point I make in the article is that few calibration procedures address’s the tube's filament voltage. It is just assumed that when you set the voltage to the specific setting for the tube under test that it will be correct. This should not be assumed for a variety of reasons. Since these tubes testers have no form of voltage regulation

© C.E. Clutter


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