The tube datasheets specify typical characteristics for the tubes. I have understood that these values represent some average new tube and in real life there are variation from these values.
When testing unused NOS tubes, what would be normal tolerances from these values? Datasheets doesn't specify any tolerances. Used tubes of course have more variation but I'm talking about unused tubes.
When testing unused NOS tubes, what would be normal tolerances from these values? Datasheets doesn't specify any tolerances. Used tubes of course have more variation but I'm talking about unused tubes.
Each data sheet has, for each tube, a different test points, normally two ( with Vdc, Ia , Rk etc..)
If you have to check the tube just make a set up following the specs listed; it is evident that a little difference in % is tollerate. It is also a good test for coupling tubes.
With NOS ( but also for new) you must also check the isolament between cathode and filaments, this is a parameter that most people ignore.
Walter
If you have to check the tube just make a set up following the specs listed; it is evident that a little difference in % is tollerate. It is also a good test for coupling tubes.
With NOS ( but also for new) you must also check the isolament between cathode and filaments, this is a parameter that most people ignore.
Walter
For example typical characteristics for ECC83 are:
Va=250V, Vg=-2V, Ia=1,2mA, S=1,6mA/V.
The values you would like to measure are Ia and S.
If we allow 30% tolerance, the range for Ia would be 0,84-1,56mA and for S 1,12-2,08mA/V.
For cathode-filament resistance there are no values in datasheets. With what kind of test voltage this should be measured and what kind of values should be expected? There are maximum cathode-filament voltages mentioned in the datasheets however. I assume the test voltage cannot be higher than that.
Mil Specs
If you look at data sheets for mil-spec tubes that have the full specification, tolerances are given. For example, see:http://frank.yueksel.org/sheets/138/5/5814WA.pdf. The tolerances for this tube (similar to a 12AU7) are about 10-20%. These high-grade tubes are likely built to tighter specs than consumer tubes. Russian tube data sheets often have tolerances, as well. For example, see: http://frank.yueksel.org/sheets/112/6/6N1PVI.pdf.
- John Atwood
If you look at data sheets for mil-spec tubes that have the full specification, tolerances are given. For example, see:http://frank.yueksel.org/sheets/138/5/5814WA.pdf. The tolerances for this tube (similar to a 12AU7) are about 10-20%. These high-grade tubes are likely built to tighter specs than consumer tubes. Russian tube data sheets often have tolerances, as well. For example, see: http://frank.yueksel.org/sheets/112/6/6N1PVI.pdf.
- John Atwood
Note that ruskie tubes tend to have their normal distribution tolerances specified in the datasheet whereas western consumer grade tubes do not, the exception being long life tubes. Or SQ tubes of Philips/Mullard manufacture. In general parametric testing can be quite pointless, case and point being the EL509 series of tubes, they will have measurements all
The problem with drawing conclusions about the range from a bogey device is that component manufacturers do not generally provide the statistical sample sizes used to create the bogey (was it, for example, five, ten, twenty, fifty, a hundred, a thousand?) or the distributions of devices. It is impossible to know if the tubes have a normal distribution or are clustered on one side or the other of the bogey. Is the bogey, for example, worst case and most tubes are better? Or is the bogey an ideal, and most tubes tend to be worse, but not significantly so.
Some speculation. The distribution may not be normal for NOS tubes, because the bogey is idealized, and any spot-on tubes might have been culled from the large lot and sold as higher-tolerance. Compare this to some of the published analysis of distributions for film resistors, seen somewhere on diyAudio I believe, as an example of the problem, which often do not have a normal distribution. This is why combining multiple values to converge to the ideal value may not always work depending upon how the manufacturer has culled out more accurate components to sell under a narrower tolerance range.
Anyway, here's Tomer's commentary on the subject as it may prove interesting.
NB: Tomer was a synthesizer of other's data so he is never, of course, the definitive word on the subject and I am not presenting this as such. But his summary is simple and direct, and it was easily found in my books. I added some underlining to identify the key portions. Hopefully the OCR does not have too many uncorrected errors in it.
Getting the Most Out of Vaccuum Tubes
by Robert B. Tomer
Howard W. Sams & Co., Inc.
First Printing, 1960
Pages 64 to 66
Bogey. The average, or published, value for a tube characteristic. A bogey tube would be one having all characteristics on bogey.
CHARACTERISTIC SPREADS
We have said in earlier paragraphs that all tube characteristics follow the known laws of variation, and distribute themselves around an average value known as the bogey value. The degree for which each characteristic is allowed to deviate from this central value is known as the spread of that characteristic. Characteristic spreads are not given in most tube specifications which include, for the most part, only the average or bogey figure. Although the exact limits are an individual company's own prerogative to choose, they may vary from tube type to tube type and from company to company. There is, however, fairly general agreement throughout the industry as to what constitutes good engineering practice, and so, commercial practice. These practices apply to tubes sold for general renewal use and do not apply to specially selected tubes sold to some equipment manufacturers, or to those premium quality tubes sold either commercially or to the military. These special groups will be dealt with in subsequent chapters.
Transconductance is usually permitted to vary about ±40 percent of the published bogey value. This means that if the registered value for a given tube type is 2,000 micromhos, you can expect to find tubes reading anywhere from 1,200 micromhos to 2,800 micromhos in any sample you measure. This does not mean that there will necessarily be a similar spread in gain when these tubes are compared in typical applications. It is probable that the gain spread will be narrowed by at least half this amount because of the foregoing reasons.
Plate current cutoff is a published characteristic of considerable significance in many applications. This characteristic is normally controlled at about twice the published rating. In other words, if the rating sheet for the tube states that the plate current will be 50 microamperes at minus 10 volts, you can expect to find some tubes that do not reduce to this level until the grid voltage is raised to -20 volts. Once again, this is not a serious departure from specifications as far as performance is concerned because the plate current of some tubes may be only 100 microamperes at -10 volts, but it takes that extra 10 volts on the grid to reduce it the additional 50 microamperes. Either current is quite inconsequential for all practical purposes.
Plate current is usually controlled within limits of ±20 percent and plate resistance, because of its intimate relation to transconductance, is likewise a ±40 percent characteristic. The correlation between either or both of these characteristics and any measure of performance is rather low, except in the case of certain unique applications. For these applications, the limits may be quite often tightened.
Screen current is one that is controlled on the high side only, being usually about twice the published rating. The reasoning here is that you can't have too little screen current, since it is only a loss current anyhow. If a tube has a better than average plate-to-screen current ratio, it is just a more efficient tube, and no one is going to find anything wrong with it. High screen current, on the other hand, will waste power; reduce the effective screen voltage and, hence, cause the tube to cut off sooner and have lower transconductance at a given grid bias. High screen current will also contribute to excessive screen dissipation with the danger of screen emission and runaway.
Grid current is controlled at about 1 microampere for most small tubes, but is allowed to get up to as high as 5 microamperes for some power tubes. This is important to recognize when attempting to measure this characteristic on commercial type testers. There isn't any one gas current test that will apply to all tubes.
This is also true of heater-to-cathode leakage. In a later chapter devoted to tube testing and tube testers, this will be dealt with more completely; however, it is normal for tubes with different powered heaters to vary from as little as 5 microamperes of leakage to as much as 100 microamperes. Again, there isn't any universal test, such as a neon bulb checker, that can evaluate all tubes for this characteristic. Such a test is either too sensitive or not sensitive enough.
Finally, heater current is a variable and usually falls between ±10 percent of the ratio value. For example, 600- milliampere heaters that read as low as 530, or as high as 670 milliamperes, would not be outside of specifications. This applies to either series-string or parallel heater types. The significance of a 10 percent heater current variation is probably not too great, although the lower limit tubes will probably fail to operate if they are used where the line voltage is 10 percent below normal as well. The reasoning that backs up these limits is the fact that far more situations exist where line voltages are high than low. Also, the statistical number of tubes falling in the lower limit is very few; probably less than 1 percent.
Some speculation. The distribution may not be normal for NOS tubes, because the bogey is idealized, and any spot-on tubes might have been culled from the large lot and sold as higher-tolerance. Compare this to some of the published analysis of distributions for film resistors, seen somewhere on diyAudio I believe, as an example of the problem, which often do not have a normal distribution. This is why combining multiple values to converge to the ideal value may not always work depending upon how the manufacturer has culled out more accurate components to sell under a narrower tolerance range.
Anyway, here's Tomer's commentary on the subject as it may prove interesting.
NB: Tomer was a synthesizer of other's data so he is never, of course, the definitive word on the subject and I am not presenting this as such. But his summary is simple and direct, and it was easily found in my books. I added some underlining to identify the key portions. Hopefully the OCR does not have too many uncorrected errors in it.
Getting the Most Out of Vaccuum Tubes
by Robert B. Tomer
Howard W. Sams & Co., Inc.
First Printing, 1960
Pages 64 to 66
Bogey. The average, or published, value for a tube characteristic. A bogey tube would be one having all characteristics on bogey.
CHARACTERISTIC SPREADS
We have said in earlier paragraphs that all tube characteristics follow the known laws of variation, and distribute themselves around an average value known as the bogey value. The degree for which each characteristic is allowed to deviate from this central value is known as the spread of that characteristic. Characteristic spreads are not given in most tube specifications which include, for the most part, only the average or bogey figure. Although the exact limits are an individual company's own prerogative to choose, they may vary from tube type to tube type and from company to company. There is, however, fairly general agreement throughout the industry as to what constitutes good engineering practice, and so, commercial practice. These practices apply to tubes sold for general renewal use and do not apply to specially selected tubes sold to some equipment manufacturers, or to those premium quality tubes sold either commercially or to the military. These special groups will be dealt with in subsequent chapters.
Transconductance is usually permitted to vary about ±40 percent of the published bogey value. This means that if the registered value for a given tube type is 2,000 micromhos, you can expect to find tubes reading anywhere from 1,200 micromhos to 2,800 micromhos in any sample you measure. This does not mean that there will necessarily be a similar spread in gain when these tubes are compared in typical applications. It is probable that the gain spread will be narrowed by at least half this amount because of the foregoing reasons.
Plate current cutoff is a published characteristic of considerable significance in many applications. This characteristic is normally controlled at about twice the published rating. In other words, if the rating sheet for the tube states that the plate current will be 50 microamperes at minus 10 volts, you can expect to find some tubes that do not reduce to this level until the grid voltage is raised to -20 volts. Once again, this is not a serious departure from specifications as far as performance is concerned because the plate current of some tubes may be only 100 microamperes at -10 volts, but it takes that extra 10 volts on the grid to reduce it the additional 50 microamperes. Either current is quite inconsequential for all practical purposes.
Plate current is usually controlled within limits of ±20 percent and plate resistance, because of its intimate relation to transconductance, is likewise a ±40 percent characteristic. The correlation between either or both of these characteristics and any measure of performance is rather low, except in the case of certain unique applications. For these applications, the limits may be quite often tightened.
Screen current is one that is controlled on the high side only, being usually about twice the published rating. The reasoning here is that you can't have too little screen current, since it is only a loss current anyhow. If a tube has a better than average plate-to-screen current ratio, it is just a more efficient tube, and no one is going to find anything wrong with it. High screen current, on the other hand, will waste power; reduce the effective screen voltage and, hence, cause the tube to cut off sooner and have lower transconductance at a given grid bias. High screen current will also contribute to excessive screen dissipation with the danger of screen emission and runaway.
Grid current is controlled at about 1 microampere for most small tubes, but is allowed to get up to as high as 5 microamperes for some power tubes. This is important to recognize when attempting to measure this characteristic on commercial type testers. There isn't any one gas current test that will apply to all tubes.
This is also true of heater-to-cathode leakage. In a later chapter devoted to tube testing and tube testers, this will be dealt with more completely; however, it is normal for tubes with different powered heaters to vary from as little as 5 microamperes of leakage to as much as 100 microamperes. Again, there isn't any universal test, such as a neon bulb checker, that can evaluate all tubes for this characteristic. Such a test is either too sensitive or not sensitive enough.
Finally, heater current is a variable and usually falls between ±10 percent of the ratio value. For example, 600- milliampere heaters that read as low as 530, or as high as 670 milliamperes, would not be outside of specifications. This applies to either series-string or parallel heater types. The significance of a 10 percent heater current variation is probably not too great, although the lower limit tubes will probably fail to operate if they are used where the line voltage is 10 percent below normal as well. The reasoning that backs up these limits is the fact that far more situations exist where line voltages are high than low. Also, the statistical number of tubes falling in the lower limit is very few; probably less than 1 percent.
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