The approximatuve answer is 1h < 5000h
Tubes ( like other components ) have a probability of failure consisting av many components,
time is just one of them, external factors, random internal fails and problems with the circuit
it works in ( a non optimised circuit might stop working for a certain degradation of tubes
properties while a better circuit would continue for a long time)
Thus my answer is the exact answer !
You can of course measure the voltages at each terminal and see if they are correct. If not, this could mean the tube is in bad health. But there is no way of predicting when it will actually stop working.
You could check the tube current and dissipation, because the closer (or over) it is to it's maximum, the shorter the life. But it's all relative.
Jan
Jan
Maybe there is a methodto do this if you measure plate resistance or transconductance repeatedly, after a known number of operating hours. The characteristics of a bogey tube are published, and anything up to abut 60% of those values is acceptable normally, so it ought to be possible to plot a graph and interpolate from the change in characteristics how the tube is trending to being end-of-life.
Would be an interesting plot, if anyone has one?
Would be an interesting plot, if anyone has one?
Supposing tubes degrade linearly (which in lack of better suggestions is a reasonable assumption), what old hector suggests has merit.
Emission and transconductance worsen with age, so comparing actual fresh values to datasheet specs can at least give you a reasonable hint.
I never search for excuses NOT to measure something.
Emission and transconductance worsen with age, so comparing actual fresh values to datasheet specs can at least give you a reasonable hint.
I never search for excuses NOT to measure something.
Check how the tube is being used.
For example, a pentode / beam power output tube, is it used: In pentode / beam power mode, or used in Ultra Linear mode, or Triode wired mode?
Measure the voltages of all the elements (including the filament voltages), the current of the cathode, plate (and screen, if possible). Measure or determine the g1 grid resistor value; note the type of bias that is used.
Calculate the screen dissipation, plate dissipation, and the total screen and plate dissipation.
Check all the tube data specifications, and compare to the actual numbers of how the tube is being used.
Also, check the voltages and currents in your amplifier, and compare to the family of plate curves for that tube.
If the numbers badly match the tube curves, your tube most likely is beyond its useful life.
An amplifier that has lots of negative feedback can "hide" a bad tube to the extent that you may not notice it, unless you make comprehensive tests of output power, harmonic and intermodulation distortion, frequency response, and damping factor, and then compare those measurements to the amplifier published specifications.
A tube that is used near or beyond its specifications, will have short tube life.
A tube that is used more conservatively will have long tube life.
A quality tube will generally last longer.
A poor quality tube will generally not last long.
Account for the un-usual infantile failures that may occur.
Use a timer module so you know the actual total hours that the amplifier is on.
It is both as simple and as complicated as that.
Now that you have determined how hard the tube is run, or how conservatively the tube is operated, you have a decision to make:
Operated near to, at, or beyond maximum ratings, either re-design/modify the amplifier, or purchase spare tubes.
Operated more conservatively versus the maximum tube ratings, sit back and enjoy the music.
Or, stop worrying and just listen to the music.
Your Mileage May Vary
For example, a pentode / beam power output tube, is it used: In pentode / beam power mode, or used in Ultra Linear mode, or Triode wired mode?
Measure the voltages of all the elements (including the filament voltages), the current of the cathode, plate (and screen, if possible). Measure or determine the g1 grid resistor value; note the type of bias that is used.
Calculate the screen dissipation, plate dissipation, and the total screen and plate dissipation.
Check all the tube data specifications, and compare to the actual numbers of how the tube is being used.
Also, check the voltages and currents in your amplifier, and compare to the family of plate curves for that tube.
If the numbers badly match the tube curves, your tube most likely is beyond its useful life.
An amplifier that has lots of negative feedback can "hide" a bad tube to the extent that you may not notice it, unless you make comprehensive tests of output power, harmonic and intermodulation distortion, frequency response, and damping factor, and then compare those measurements to the amplifier published specifications.
A tube that is used near or beyond its specifications, will have short tube life.
A tube that is used more conservatively will have long tube life.
A quality tube will generally last longer.
A poor quality tube will generally not last long.
Account for the un-usual infantile failures that may occur.
Use a timer module so you know the actual total hours that the amplifier is on.
It is both as simple and as complicated as that.
Now that you have determined how hard the tube is run, or how conservatively the tube is operated, you have a decision to make:
Operated near to, at, or beyond maximum ratings, either re-design/modify the amplifier, or purchase spare tubes.
Operated more conservatively versus the maximum tube ratings, sit back and enjoy the music.
Or, stop worrying and just listen to the music.
Your Mileage May Vary
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I've seen small-signal tubes run 100,000 hours (11 years) and working great.
Home-size (not radio station) power tubes working AT their rating may be weakening at 5,000 hours. Or burn-up at 1 hour dues to manufacturing defect. OTOH working de-rated 10,000 hours can happen.
This is little different from any technology more 'advanced' than dirt. I got a holiday tree with LEDs all over. The -timer- crapped-out the second week. I can see the cord will not last forever. But if I splice and hot-wire I think most of the LEDs will be working for Yule 2030. Hey, the gutter-lights from our last house are just starting to die, and that's over a decade.
Home-size (not radio station) power tubes working AT their rating may be weakening at 5,000 hours. Or burn-up at 1 hour dues to manufacturing defect. OTOH working de-rated 10,000 hours can happen.
This is little different from any technology more 'advanced' than dirt. I got a holiday tree with LEDs all over. The -timer- crapped-out the second week. I can see the cord will not last forever. But if I splice and hot-wire I think most of the LEDs will be working for Yule 2030. Hey, the gutter-lights from our last house are just starting to die, and that's over a decade.
Tubes that are run at their maximum ratings need to be spaced from each other, and need open air, not closed under a small closed cover.
The military have done tests and repeatedly removing a tube for testing shortens its life.
When new take note of all the AC and DC voltages and the power output. When output starts to drop at the same input level then you know something is wrong and you compare all voltages. You'll quickly discover which tube is weak.
When new take note of all the AC and DC voltages and the power output. When output starts to drop at the same input level then you know something is wrong and you compare all voltages. You'll quickly discover which tube is weak.