Hello! I am a newby to tube sound and I have met some thing I cannot find information about. In item descriptions of tubes at Ebay some sellers write that the tubes are measured at Amplitrex and give a list of parameters. And there is some percentage which I cannot understand. What does it mean? Is it a measurement?
The sellers uses the figures to convince You that the tubes are in good shape.
As tubes are used they degrade, presenting some numbers from some kind
of measuring device is a way to "proof the state" of the tubes.
One must take in consideration that new tubes , even the tubes made in the -60,
has quite large differences, +-20% is not uncommon. Only specially selected
tubes was more accurate.
Designers was aware of this and made their designs insensitive of tube
parameters by various methods, negative feedback is one such technique.
One common parameter quotes by ebay sellers are % of current drawn at
some specified or implied working point, where very used tube tend to
decline. Tube documentation has one or a few typical working point
that the tester (might) reference.
See page 3 on https://frank.pocnet.net/sheets/030/e/ECC83.pdf as an example,
Plate at 250V is the most common reference for this tube , the tube
also is known as 12ax7 in us.
Some testers don't test at 250V thus the figures are less accurate.
I thought I understood the issue with percents, but I have just found out that it may be written that a tube has 120% lifetime. How can it be? And what does it mean?
Usually that is just an emission test. It indicates that the valve gives 120% of the nominal datasheet emission at a certain operating point.
As a valve (tube) ages it's cathode emission will drop. This does give some sort of indication of a valves health but it is not a reliable test of life left.
Valves die from many varying factors. Emission from the cathode slowly declines, gas (air) gets in slowly or various gasses are liberated from the electrode structures over time or through misuse (over dissipation of the anode or even the grid/screen grid) extremely rarely on indirectly heated types the heater may go open. I have only seen this twice in over 20 years messing with valves. Directly heated types do get open filaments far more often. Even just from vibration during transport.
Cheers
Matt
As a valve (tube) ages it's cathode emission will drop. This does give some sort of indication of a valves health but it is not a reliable test of life left.
Valves die from many varying factors. Emission from the cathode slowly declines, gas (air) gets in slowly or various gasses are liberated from the electrode structures over time or through misuse (over dissipation of the anode or even the grid/screen grid) extremely rarely on indirectly heated types the heater may go open. I have only seen this twice in over 20 years messing with valves. Directly heated types do get open filaments far more often. Even just from vibration during transport.
Cheers
Matt
tubes have large variance in parameters when manufactured.
120% is compared to a "standard tube" , meaning that this tube conducted
with more current ( not necessarily a good thing) then the "standard tube".
I have understood, thank you.
But then if I (or anybody else) make measurements to a tube and get, e.g., 12 mA while the datasheet for this tube has 10mA in it, does it mean that the tube has 120% lifetime or 80%?
It does mean that this tube started life with 12mA or more current drawn
at the working point and has faded ( or not) to 12mA.
It has nothing to do with estimated lifetime, in fact no measurements
will predict remaining lifetime of a tube.
Lifetime is limited not only on emission, it's limited by a number of factors
that will "kill a tube" such as :
-microphonics
- broken pins/sockets
- gas ( from internal emissions of gas)
- broken glass
- contaminated g1 resulting in inability to keep bias
- internal shorts
etc
The only way to get a good tube that is expected to work well is to but a new tube that has not been exposed to use. There is a fair supply of popular
tubes from a small number of factories.
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