Concerning the maximum ratings of tubes:
If one goes over the maximum Va (plate voltage) or Vg (grid voltage), there could be a flash over inside the tube and an arc between two wires
If the maximum current rating is exceeded, the wires inside the tube could melt due to I2R heating
If the maximum power rating is exceeded, the tube could burn out due to excessive heat inside of it
==> What would physically happen if one exceeds a tube's maximum transconductance rating?
If one goes over the maximum Va (plate voltage) or Vg (grid voltage), there could be a flash over inside the tube and an arc between two wires
If the maximum current rating is exceeded, the wires inside the tube could melt due to I2R heating
If the maximum power rating is exceeded, the tube could burn out due to excessive heat inside of it
==> What would physically happen if one exceeds a tube's maximum transconductance rating?
A tube's transconductance is determined by the tube itself, and the conditions that it is operated under. The figures printed in the manual are typical numbers that are specified under the conditions given in the manual. The transconductance varies with the current (usually more current gives more transconductance), and declines slowly as the tube ages.
I am sure that you could find a set of conditions that may provide a higher transconductance than what is shown in the tube manual, for a given tube. If you are not exceeding the other ratings, nothing bad will happen! The numbers given in the manual are usually near the maximum for the given conditions, averaged over a lot of tubes.
As for the other ratings, exceeding them by a small amount is generally not instantly fatal, as is the case with transistors. The tubes life will be shortened. Some tubes can be operated well above their maximums without a problem, others die pretty quickly.
I am sure that you could find a set of conditions that may provide a higher transconductance than what is shown in the tube manual, for a given tube. If you are not exceeding the other ratings, nothing bad will happen! The numbers given in the manual are usually near the maximum for the given conditions, averaged over a lot of tubes.
As for the other ratings, exceeding them by a small amount is generally not instantly fatal, as is the case with transistors. The tubes life will be shortened. Some tubes can be operated well above their maximums without a problem, others die pretty quickly.
This article from 1950 might be of interest:
http://www.ginko.de/user/franz.hamberger/roehren/e5881.html
This also, note that a 12AX7 should last for over 30 years, under the right operating conditions:
http://members.aol.com/sbench101/TubeMisc/tubelife.txt
Pete B.
http://www.ginko.de/user/franz.hamberger/roehren/e5881.html
This also, note that a 12AX7 should last for over 30 years, under the right operating conditions:
http://members.aol.com/sbench101/TubeMisc/tubelife.txt
Pete B.
Tube specs are not absolutes like atomic weight of carbon or the speed of light or something.
The tube makers set the limits for designs. The tubes were made for table radios and TVs and what not, with the idea they should work reliably for a relatively long time. In the area of guuitar amps, we routinely exceed specs by a substantial amount. Leo Fender in his classic models ran 6V6s at as much as 100 volts over the tube manual maximum.
But we expect to change tubes a time or two each year. Mom and pop would not have been pleased if they had to retube everything twice a year.
So if your tube manual says 500 volts, it will work reliably at that level. if you go to 501 volts, a light show wil not result. Who is to say just how far you can go?
If you go way high, yes, the tube can arc from over voltage.
If you exceed current, a grid could melt, more likely the cathode would be damaged.
When you exceed power or current, you will see the plates start to glow red hot. At these levels, the cathodes can be damaged, and the surface of the plate can cause certain problems. Certainly tube life will be shortened.
Exceeding the transconductance is a meaningless concept. Transconductance is a characteristic, not a limit. Like asking what happens to me if I exceed my age. (In my case, I'd be older.)
The tube makers set the limits for designs. The tubes were made for table radios and TVs and what not, with the idea they should work reliably for a relatively long time. In the area of guuitar amps, we routinely exceed specs by a substantial amount. Leo Fender in his classic models ran 6V6s at as much as 100 volts over the tube manual maximum.
But we expect to change tubes a time or two each year. Mom and pop would not have been pleased if they had to retube everything twice a year.
So if your tube manual says 500 volts, it will work reliably at that level. if you go to 501 volts, a light show wil not result. Who is to say just how far you can go?
If you go way high, yes, the tube can arc from over voltage.
If you exceed current, a grid could melt, more likely the cathode would be damaged.
When you exceed power or current, you will see the plates start to glow red hot. At these levels, the cathodes can be damaged, and the surface of the plate can cause certain problems. Certainly tube life will be shortened.
Exceeding the transconductance is a meaningless concept. Transconductance is a characteristic, not a limit. Like asking what happens to me if I exceed my age. (In my case, I'd be older.)
Tube types from different manufacturers can also like different ratings. An example is 6P3C is often subbed for 6L6G (not GC), but the 6L6G can take higher voltage, 6P3C higher current.
I summed it up in my signature below: *Ratings are for transistors, tubes have guidelines* 😀
I summed it up in my signature below: *Ratings are for transistors, tubes have guidelines* 😀
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