Tube Tester Woes

3/24/2005 11:09:19 AMStephen

I've been using a Knight 600 tube tester for about 9 months and thought I was getting good results (for a basic emission/shorts tester). While repairing a basic AA5 radio however, I was dissapointed to find that a 35W4 recifier tube which tested fairly high for emissions (no shorts)actually produced only about 10% of required B+ voltage at the plate when in the radio. What gives? I thought emission was directly related to how much voltage the tube would produce for rectification or amplification. I spent hours checking every cap and resistor only to substitute another 35W4 (with same test results) and Viola, the B+ voltages were normal. I'm also finding some tubes that show shorts actually work just fine in radios. Should I spring for an expensive dynamic or transconductance tester?

3/24/2005 12:43:31 PMLou

I find your case a very interesting one. Every 35W4 I have ever tested, as long as the emission was OK, worked in the circuit it was put in.

Perhaps there was a short in the unit that did not show up in the emissions test.

I always use a transconductance tester for "important" tubes like IF amplifiers and RF oscillators. However, you can start with an emissions test and if you don't seem to get the desired result, check again on a transconductance tester. It's good to have both.

I try never to use tubes that have any shorts. They could cause eventual component failures.

3/24/2005 7:15:05 PMStephen

:I find your case a very interesting one. Every 35W4 I have ever tested, as long as the emission was OK, worked in the circuit it was put in.
:
:Perhaps there was a short in the unit that did not show up in the emissions test.
:
:I always use a transconductance tester for "important" tubes like IF amplifiers and RF oscillators. However, you can start with an emissions test and if you don't seem to get the desired result, check again on a transconductance tester. It's good to have both.
:
:I try never to use tubes that have any shorts. They could cause eventual component failures.

Lou,
Don't suppose you have any recommendations on good transconductance testers??

Stephen

3/24/2005 12:55:13 PMThomas Dermody

Depending on the tester, it may or may not load the tube as it would be loaded in a radio. A detector diode can be almost dead and have absolutely no affect on radio reception because the detector circuit in a radio does not load the tube much at all. In this case, the tube tester actually loads the tube more than the radio, so it tests bad even though it will work in the radio. If your tester is not loading the rectifier properly, your test will be inaccurate. I have had fine results with a properly repaired Eico 625 tester, which uses regular emissions testing. It loads output and rectifier tubes fairly well. RF tubes are loaded less so. As the settings go, 1 is the least, and 4 is the most. 1 and 2 are used for various RF and 1st audio tubes, whereas #3 is used for power tubes. #4 is rarely used for such things as high voltage rectifier tubes and the target screen of magic eye tubes.

When you test the resistors in your tube tester, YOU MUST DISCONNECT THEM TO GET ACCURATE READINGS. If, after you have tested the resistors, you find that none are at fault, your tester may just load the 35W4 tube improperly.

As for shorts:
Depending on how your tube is used in a set, a partial short may or may not affect performance. Also, the neon light will sometimes light up on one side even if the tube is not shorted. A short passes AC. If the neon light is only lighting up on one side, it is receiving DC. Resistors in the neon bulb circuit may have drifted. A resistor across the bulb that drifts high will allow more voltage to flow through the bulb, making it easier to light. Tubes that light the bulb on one side simply have such good emissions (usually is the case) that they pass enough DC to light the bulb on one side.

Sometimes a tube will light the bulb on both sides and still not have a short that is measurable with a multi-meter. This type of short is a high resistance--semi-capacitance short where the elements are really close together but are not really touching much if at all. It may or may not affect performance. Television circuits are greatly affected by these types of shorts. Sometimes I can blow these partial shorts and direct shorts apart by setting the two offending elements across the high voltage test supply. For instance, grid 2 and grid 1 are shorted. I throw grid 2 lever up and grid 1 lever down (as my tester is wired....you have to know how your tester is wired in order to perform this....it may not be possible with your tester). I then set my selector to the high current 4 position. I press the merit button momentarily several times to blow the short apart. Several short bursts blow the short apart. Holding the merit button down may damage the power supply, though it is more likely to damage a resistor, which isn't so bad because they are easily replacable. With my tester I don't ever have any trouble in either department, so sometimes I hold the button down and tap the tube until the elements are completely burned apart (sparks will fly within the tube each time the elements touch). Unless you know your tester, you cannot just perform radical tricks like this or you may damage it severely.

Ever so slightly gassy tubes can cause the shorts light to light up on either one side or both sides.

Other shorts that appear on testers but do not affect actual circuitry are those illusional shorts created by an element getting warm next to the hot cathode, should you leave the tube in the tester socket for a long time. If another element gets hot enough, it will also emit electrons to some degree. Current will be able to flow in two ways in the tube, which will light both sides of the neon bulb.

Sometimes when this happens in actual circuits, it can affect performance. In audio circuits it can introduce distortion. Sometimes it has no affect.

When you run across tubes that show a partial short on the tester, you can try blowing the elements apart. Tap the tube many times during and after this. Take several tests while tapping the tube. Depending on how much you love your radios, you may or may not want to use tubes in this condition. Often when you blow elements apart, they stay apart, but sometimes they can short again. Honestly, any tube can short out, so I guess you take a risk even if you choose to use a different tube than the one that showed a short.

Thomas

:I've been using a Knight 600 tube tester for about 9 months and thought I was getting good results (for a basic emission/shorts tester). While repairing a basic AA5 radio however, I was dissapointed to find that a 35W4 recifier tube which tested fairly high for emissions (no shorts)actually produced only about 10% of required B+ voltage at the plate when in the radio. What gives? I thought emission was directly related to how much voltage the tube would produce for rectification or amplification. I spent hours checking every cap and resistor only to substitute another 35W4 (with same test results) and Viola, the B+ voltages were normal. I'm also finding some tubes that show shorts actually work just fine in radios. Should I spring for an expensive dynamic or transconductance tester?

3/24/2005 7:13:14 PMStephen

:Depending on the tester, it may or may not load the tube as it would be loaded in a radio. A detector diode can be almost dead and have absolutely no affect on radio reception because the detector circuit in a radio does not load the tube much at all. In this case, the tube tester actually loads the tube more than the radio, so it tests bad even though it will work in the radio. If your tester is not loading the rectifier properly, your test will be inaccurate. I have had fine results with a properly repaired Eico 625 tester, which uses regular emissions testing. It loads output and rectifier tubes fairly well. RF tubes are loaded less so. As the settings go, 1 is the least, and 4 is the most. 1 and 2 are used for various RF and 1st audio tubes, whereas #3 is used for power tubes. #4 is rarely used for such things as high voltage rectifier tubes and the target screen of magic eye tubes.
:
:When you test the resistors in your tube tester, YOU MUST DISCONNECT THEM TO GET ACCURATE READINGS. If, after you have tested the resistors, you find that none are at fault, your tester may just load the 35W4 tube improperly.
:
:As for shorts:
:Depending on how your tube is used in a set, a partial short may or may not affect performance. Also, the neon light will sometimes light up on one side even if the tube is not shorted. A short passes AC. If the neon light is only lighting up on one side, it is receiving DC. Resistors in the neon bulb circuit may have drifted. A resistor across the bulb that drifts high will allow more voltage to flow through the bulb, making it easier to light. Tubes that light the bulb on one side simply have such good emissions (usually is the case) that they pass enough DC to light the bulb on one side.
:
:Sometimes a tube will light the bulb on both sides and still not have a short that is measurable with a multi-meter. This type of short is a high resistance--semi-capacitance short where the elements are really close together but are not really touching much if at all. It may or may not affect performance. Television circuits are greatly affected by these types of shorts. Sometimes I can blow these partial shorts and direct shorts apart by setting the two offending elements across the high voltage test supply. For instance, grid 2 and grid 1 are shorted. I throw grid 2 lever up and grid 1 lever down (as my tester is wired....you have to know how your tester is wired in order to perform this....it may not be possible with your tester). I then set my selector to the high current 4 position. I press the merit button momentarily several times to blow the short apart. Several short bursts blow the short apart. Holding the merit button down may damage the power supply, though it is more likely to damage a resistor, which isn't so bad because they are easily replacable. With my tester I don't ever have any trouble in either department, so sometimes I hold the button down and tap the tube until the elements are completely burned apart (sparks will fly within the tube each time the elements touch). Unless you know your tester, you cannot just perform radical tricks like this or you may damage it severely.
:
:Ever so slightly gassy tubes can cause the shorts light to light up on either one side or both sides.
:
:Other shorts that appear on testers but do not affect actual circuitry are those illusional shorts created by an element getting warm next to the hot cathode, should you leave the tube in the tester socket for a long time. If another element gets hot enough, it will also emit electrons to some degree. Current will be able to flow in two ways in the tube, which will light both sides of the neon bulb.
:
:Sometimes when this happens in actual circuits, it can affect performance. In audio circuits it can introduce distortion. Sometimes it has no affect.
:
:When you run across tubes that show a partial short on the tester, you can try blowing the elements apart. Tap the tube many times during and after this. Take several tests while tapping the tube. Depending on how much you love your radios, you may or may not want to use tubes in this condition. Often when you blow elements apart, they stay apart, but sometimes they can short again. Honestly, any tube can short out, so I guess you take a risk even if you choose to use a different tube than the one that showed a short.
:
:
:
:Thomas
:
::I've been using a Knight 600 tube tester for about 9 months and thought I was getting good results (for a basic emission/shorts tester). While repairing a basic AA5 radio however, I was dissapointed to find that a 35W4 recifier tube which tested fairly high for emissions (no shorts)actually produced only about 10% of required B+ voltage at the plate when in the radio. What gives? I thought emission was directly related to how much voltage the tube would produce for rectification or amplification. I spent hours checking every cap and resistor only to substitute another 35W4 (with same test results) and Viola, the B+ voltages were normal. I'm also finding some tubes that show shorts actually work just fine in radios. Should I spring for an expensive dynamic or transconductance tester?

Thomas,
Thanks for the detailed info. Guess I'll check all the resistors in the tube tester. I'm not sure I understand what you mean by "both sides of the bulb." Unfortunately, I do not know what the 10 3-position switches really do on a tester. Some are set in the up position, some in the down position and most are in the center position. For my tester, you simply move those switches that are in the center position to the up position momentarily while you watch the neon bulb for shorts. I don't suppose those switches correlate to the tube pins do they?
Stephen

3/24/2005 7:16:22 PMThomas Dermody

Yes, the switches are for the corresponding tube pins. Obtain a circuit diagram for your tester to fully understand it (possibly on this site under references). A neon bulb has two electrodes within it, which a current is passed between that lights the gas. If DC is applied to the bulb, only one of the two electrodes will glow. If AC is applied to the bulb, both electrodes will glow. Observe closely.

3/24/2005 11:51:54 PMNorm Leal

May not be a problem with the tube tester just limitation? A 34W4 can handle 100 ma of current. Most likely your tube tester can't supply this much current.

An emission tester is just as good as transconductance tester for rectifiers. Diodes don't have gain. A tester like Eico 666 does a reasonable job testing rectifiers. It can supply more current than most.

Norm

:Yes, the switches are for the corresponding tube pins. Obtain a circuit diagram for your tester to fully understand it (possibly on this site under references). A neon bulb has two electrodes within it, which a current is passed between that lights the gas. If DC is applied to the bulb, only one of the two electrodes will glow. If AC is applied to the bulb, both electrodes will glow. Observe closely.
:
:T

3/26/2005 10:47:59 PMStephen

:Hi
:
: May not be a problem with the tube tester just limitation? A 34W4 can handle 100 ma of current. Most likely your tube tester can't supply this much current.
:
: An emission tester is just as good as transconductance tester for rectifiers. Diodes don't have gain. A tester like Eico 666 does a reasonable job testing rectifiers. It can supply more current than most.
:
:Norm
:
::Yes, the switches are for the corresponding tube pins. Obtain a circuit diagram for your tester to fully understand it (possibly on this site under references). A neon bulb has two electrodes within it, which a current is passed between that lights the gas. If DC is applied to the bulb, only one of the two electrodes will glow. If AC is applied to the bulb, both electrodes will glow. Observe closely.
::
::T

Thanks Norm and Thomas, you guys are a great source of information for us neophytes! One last question, though...should I have a dynamic transconductance tester for amplification tubes?

Stephen

3/30/2005 1:33:23 PMMarv Nuce

Norm/Thomas,
Nuther tube tester question. Cleaned up a 50's vintage Superior Instruments TV-11, and it seems to perform well. No parts were replaced. Roll chart is still a problem. Can't find schematics online, and don't know if its a transconductance or emissions tester. Purchased NOS 27's for a rebuild, and each one measured barely into the green (good) area on the meter. The one original 27 in the set tested in the weak area on the meter, yet performs much better (more volume across the dial) in the first RF amp location. What am I missing?
marv

3/30/2005 3:36:52 PMThomas Dermody

Check your tester for drifted resistors (remove one lead of each resistor while testing). This causes low readings. Also be sure that you are adjusting the line test properly. I'll let someone else explain what a transconductance tester has as far as tests. There are more and they are slightly more complicated. If your tester simply has a shorts-gas test and then a merit test, then it is likely a simple emissions tester.

3/30/2005 11:47:14 PMMarv Nuce

Thomas,
By your description, it must be an emissions tester, (cathode/plate current??) but without a schematic, and rusty, dusty, moldy components, I'm hard pressed to define their required values. Line voltage adjusted to its marker for each test. The 2 tubes described in earlier email were tested under identical conditions, but the weaker of the 2 performed better in the TRF set. Old wives (service technician) tale "If it ain't broke, don't fix it"
marv

:Check your tester for drifted resistors (remove one lead of each resistor while testing). This causes low readings. Also be sure that you are adjusting the line test properly. I'll let someone else explain what a transconductance tester has as far as tests. There are more and they are slightly more complicated. If your tester simply has a shorts-gas test and then a merit test, then it is likely a simple emissions tester.

3/31/2005 2:52:06 AMeasyrider

:Thomas,
:By your description, it must be an emissions tester, (cathode/plate current??) but without a schematic, and rusty, dusty, moldy components, I'm hard pressed to define their required values. Line voltage adjusted to its marker for each test. The 2 tubes described in earlier email were tested under identical conditions, but the weaker of the 2 performed better in the TRF set. Old wives (service technician) tale "If it ain't broke, don't fix it"
:marv
:

Marv

Send me an email if you want a schematic for this tester

3/31/2005 11:22:59 AMLou

The tube tester I have had the best experience with is a B and K 900. This is a rugged tester that was primarily used for the color tv and entertainment electronics industry in the 1960's. It is easy to use and gives good, repeatable results.