EL34 drawing current

[SoundCity85]

I got a basic question related to the EL34 tube. This is the el34world forum after all.

How does the drawing of current relate to overall condition and the life that tube has left in it?

I just received my Maxi-Matcher 2 and having great fun in measuring my 40+ Xf blackburn tubes. I measure only 1 tube at the time, so the machine only needs to supply heating of a single tube, and comparing results is the most representative. With a plate voltage of 400V and a negative bias supply of 36V, the machine measures a current someone in between 20 and 50mA. It also measures transconductance, but surely this measurement seems to be greatly depending on the current in draws. Not enough samples measured to do statistics, but so far I think the machine is probably best at matching tubes, but really doesn't tell me much about the life of the tube itself. Unless of course the drawing of current is related to the life of the tube. On the other hand I have a NOS Xf4, I remember buying it and the getter flash really looks NOS, that measures lower in current and transconductance than many of my other tubes. I have also tubes where a lot of getter flash seems gone already, or have the little rainbow colors but they measure perfect. If I divide current in mA by transconductance in m-mHos, I get a value roughly between 5 and 7.

Any insights would be greatly appreciated.

[shooter]

the life of the tube

measured as TIME, add voltage and current, now you "might" guess.
Tubes I worked with, smart engineers decided they are bad at 17k hrs.
play an amp 24/7, play an identical amp 1/7, the second one lasts alot longer

[SoundCity85]

But is there no scientific approach possible? Those are mostly used tubes, and it would be nice to somehow get an estimate on how long they have still left and at what quality. 17.000 hours, that's just a ball park figure because the hotter it is biased, the less long it will last, at least that's what I read. So if you play 1 hour a day, they should last 45 years. So why do people replace their powers tubes every few years then?

When I look at my ECC83 with the Maxi Preamp 2, I can clearly see that transconductance drops well below 1.6 mA/V and then I assume they wore out to a certain degree

[PRR]

Say my 20 year old car (2002 Honda) rated 150HP new now makes 100HP. Does that mean it has 40 years left? Or could it throw a rod the next time I start it? Or could all gasoline engines be banned before it wears out, so it never dies, just sleeps?

I don't think tube testers are much use for "life left". A tube has some wear-out mechanisms but many drop-dead possibilities.

[SoundCity85]

Thanks for your input and your example. The Honda engine that is produced all have the same HP when they leave the factory, maybe a few percent difference at most. So if you would test them for HP on a bench after 20 years, and you see it only makes 100 HP instead of 150HP, at least you know it has significantly lost its potential.

I do see the point of the drop-dead possibilities, but on the other hand I had TAD premium selected tubes crap out with less than 500 hours played.

I was just wondering in what way it would somehow be possibly to assess the wear-out, a bit like the test bench for HP of the engine in the example.

Best regards

[shooter]

In the sport of music, the ears are your best source for "wear-out"
I did some work with a musician that only used tubes "on their death bed", his quote; "there's no better sound than a twin with power tubes ready to die"

[jim]

My Hickok 6000A manual describes a "Tube Life Test" probably devised by the marketing dept....After testing a tube..."adjust the SHUNT control until the needle reads 2000 micromho in the GOOD sector on the 0-3000 scale. Wait for the needle to stop. Hold everything constant, reduce FILAMENT voltage one step. If the needle remains in the GOOD sector of the scale, the vacuum tube has a long life expectancy and will perform satisfactorily" But we all know as Shooter pointed out that in the audio world a crappy sounding tube will still work.  They degrade so slowly that you really don't notice how bad they sound until you decide that it might be time to change them out. Jim

[Joel]

Hi Mr SoundCity85

You might want to look into RAM - Reliability, Availability, Maintainability.  There is some funky math associated with it.  Did a course in it years ago... forgotten most.

Some light reading: https://www.sebokwiki.org/wiki/Reliability,_Availability,_and_Maintainability

[shooter]

most all the systems I worked, the high fail stuff was all on the "change at......." maintenance schedule
the idea is simple, if you replace it before it fails you never have down time, most of my home got paid for from downtime $'s 

[HotBluePlates]

... With a plate voltage of 400V and a negative bias supply of 36V, the machine measures a current someone in between 20 and 50mA. It also measures transconductance ... doesn't tell me much about the life of the tube itself. Unless of course the drawing of current is related to the life of the tube. ...

For a brand new tube, the condition of the cathode will alter the plate current obtained between different individual tubes.  Some cathodes will have higher current, others lower current.

For a brand new tube with the plate/screen and bias voltages you report, plate current will vary with the tube's transconductance ("Gm").  For a given bias voltage, a higher-Gm tube will pass less plate current, a lower-Gm tube will pass higher plate current (all else equal).  This is because Gm is a measure of how effective the grid voltage is at controlling plate current.

     It is worth knowing that Gm varies with plate current; higher current results in a higher Gm.

So now we have 2 different effects (cathode condition, Gm) intrinsic to the individual tube that causes plate current to vary sample-by-sample.  Even for brand-new tubes, the measured values of these two factors vary above & below the value of "an average tube" as indicated on the data sheet.
__________________________________

My Hickok 6000A manual describes a "Tube Life Test" ... Hold everything constant, reduce FILAMENT voltage one step. If the needle remains in the GOOD sector of the scale, the vacuum tube has a long life expectancy ...

Earlier I talked about "all else equal."  So what happens when all else is not equal?

A cathode in good condition has strong emission even when it is not heated to full operating temperature (which happens when Heater Power is lowered by lowering Heater Voltage).  The theory behind Hickok's Life Test is to lower heater voltage of a 6.3v tube by 21.9% to 5v, and re-check Gm (which is indirectly measured by measuring plate current in their testers).  If cathode-emission/Gm stays above some limit, the cathode is judged to be strong, and the tube judged to have plenty of remaining life.  A tube whose Gm/emission drops below the limit is judged to have a weak cathode & little remaining life.

... I just received my Maxi-Matcher 2 ... so far I think the machine ... really doesn't tell me much about the life of the tube itself. ...

... is there no scientific approach possible? ...

Hickok's Life Test is the closest thing to a logical Gold Standard to test for remaining tube life, and even then it shows a definitely-dead tube but cannot tell how many hours of remaining life a "good tube" has.  It's an exercise in reading tea-leaves.

After getting neck-deep in tube-testing, I have found nothing beyond Hickok's Life Test that shows the condition of the cathode to provide an indication of the remaining tube life.  And if the tube doesn't suffer a catastrophic failure (broken glass/air leak, arcing/leakage/short-circuit, open element) or a subjective failure (excessive hum, microphonics, noise), then cathode condition is the sole remaining factor on how long a tube will continue to function.

If you'd like to read more, Tomer explained all this in 1960 (and even showed how a tube could be "bad" in one circuit and "good" in another).

[pdf64]

...For a given bias voltage, a higher-Gm tube will pass less plate current, a lower-Gm tube will pass higher plate current (all else equal).  This is because Gm is a measure of how effective the grid voltage is at controlling plate current...

Excellent post, but I'm confused by the above; to my thinking it's the other way around 

[PRR]

> to my thinking it's the other way around

I think there are two effects:
 * raw emission, diode test (zero grid)
 * how well grid controls current

[HotBluePlates]

...  For a given bias voltage, a higher-Gm tube will pass less plate current, a lower-Gm tube will pass higher plate current (all else equal).  ...

Excellent post, but I'm confused by the above; to my thinking it's the other way around 

Consider EL34 vs 6L6 in the same socket of an amplifier.

Bias the EL34 for a given current-draw, then swap in the 6L6.  The latter tube will pass much more current, and needs a larger bias voltage to bring down the idle current.  That's because the EL34's Gm is very much higher (around 11mA/volt rather than ~5-6mA/volt), and much less bias-voltage is needed at G1 to tame plate current.

The same effect happens on a smaller scale with various 6L6s or various EL34s:  The ones with higher Gm don't require as-large a bias voltage to tame their plate current.

You're left with Chicken & the Egg:  a tester like a MaxiMatcher (or an initial setting of an L3-3) applies fixed plate, screen & bias voltages.  Keeping those fixed parameters, a higher-Gm sample might initially show a lower plate current reading.  The readings are also more-different among individual samples in a fixed-bias test than if the L3-3 is configured for cathode bias, where tubes will settle in to more-similar plate current & show less Gm-variation (though the absolute value of bias is different among the tubes tested, according to their individual differences).

[SoundCity85]

... With a plate voltage of 400V and a negative bias supply of 36V, the machine measures a current someone in between 20 and 50mA. It also measures transconductance ... doesn't tell me much about the life of the tube itself. Unless of course the drawing of current is related to the life of the tube. ...

For a brand new tube, the condition of the cathode will alter the plate current obtained between different individual tubes.  Some cathodes will have higher current, others lower current.

For a brand new tube with the plate/screen and bias voltages you report, plate current will vary with the tube's transconductance ("Gm").  For a given bias voltage, a higher-Gm tube will pass less plate current, a lower-Gm tube will pass higher plate current (all else equal).  This is because Gm is a measure of how effective the grid voltage is at controlling plate current.

     It is worth knowing that Gm varies with plate current; higher current results in a higher Gm.

So now we have 2 different effects (cathode condition, Gm) intrinsic to the individual tube that causes plate current to vary sample-by-sample.  Even for brand-new tubes, the measured values of these two factors vary above & below the value of "an average tube" as indicated on the data sheet.
__________________________________

My Hickok 6000A manual describes a "Tube Life Test" ... Hold everything constant, reduce FILAMENT voltage one step. If the needle remains in the GOOD sector of the scale, the vacuum tube has a long life expectancy ...

Earlier I talked about "all else equal."  So what happens when all else is not equal?

A cathode in good condition has strong emission even when it is not heated to full operating temperature (which happens when Heater Power is lowered by lowering Heater Voltage).  The theory behind Hickok's Life Test is to lower heater voltage of a 6.3v tube by 21.9% to 5v, and re-check Gm (which is indirectly measured by measuring plate current in their testers).  If cathode-emission/Gm stays above some limit, the cathode is judged to be strong, and the tube judged to have plenty of remaining life.  A tube whose Gm/emission drops below the limit is judged to have a weak cathode & little remaining life.

... I just received my Maxi-Matcher 2 ... so far I think the machine ... really doesn't tell me much about the life of the tube itself. ...

... is there no scientific approach possible? ...

Hickok's Life Test is the closest thing to a logical Gold Standard to test for remaining tube life, and even then it shows a definitely-dead tube but cannot tell how many hours of remaining life a "good tube" has.  It's an exercise in reading tea-leaves.

After getting neck-deep in tube-testing, I have found nothing beyond Hickok's Life Test that shows the condition of the cathode to provide an indication of the remaining tube life.  And if the tube doesn't suffer a catastrophic failure (broken glass/air leak, arcing/leakage/short-circuit, open element) or a subjective failure (excessive hum, microphonics, noise), then cathode condition is the sole remaining factor on how long a tube will continue to function.

If you'd like to read more, Tomer explained all this in 1960 (and even showed how a tube could be "bad" in one circuit and "good" in another).

Thanks for taking the time to write up this very nice piece of info!

[pdf64]

Consider EL34 vs 6L6 in the same socket of an amplifier...

That's the example I was thinking of when I was trying to work through it myself, but I still managed to get in a muddle 
I've got it now, many thanks!

I think there are two effects:
 * raw emission, diode test (zero grid)
 * how well grid controls current

So for a valve in fixed bias, the 2 effects may act to balance out its idle plate current as it ages?

[shooter]

In my 'ol life, you could plot tube decay pretty easy, the primary plot;  grid bias over time.  you needed to bump bias say 4-6 times a year to hold plate in spec.

[pdf64]

...If you'd like to read more, Tomer explained all this in 1960 (and even showed how a tube could be "bad" in one circuit and "good" in another).

On p115 of Tomer, is 'triodes' a typo that should say 'diodes'? 2nd paragraph, sentence reads 'Tubes are operated as triodes in this tester, and raw AC voltage is applied to all elements, including the grid'.
It sounds a rather savage test, about as subtle as dropping it onto a concrete floor. Yup, that's another bad 'un 

[2deaf]

I got a basic question related to the EL34 tube. This is the el34world forum after all.

How does the drawing of current relate to overall condition and the life that tube has left in it?

It doesn't.  Until you get to extremes.

So you have your EL34's (or my EL34's) idling with 400V on the plates, 400V on the screens, and -36V on the control grids.  There is a general correlation between transconductance (g_m) and plate current under these conditions and the larger the difference between transconductances, the higher the correlation.  The g_m I'm talking about is at 400V/400V/-36V and this g_m is way smaller than the often quoted g_m of 11S.  EL34's that have a significantly larger g_m have a higher plate current at 400V/400V/-36V.

Brand new EL34's have wild variations in g_m and, therefore, wild variations in plate current at 400V/400V/-36.  Transconductance does decrease with age, but we don't know if lower current is due to aging or due to the wild variations between newborn tubes.  Now if you happen to know the history of the plate current on a particular EL34, then the plate current drawn today might tell you something. 
         

[2deaf]

...For a given bias voltage, a higher-Gm tube will pass less plate current, a lower-Gm tube will pass higher plate current (all else equal).  This is because Gm is a measure of how effective the grid voltage is at controlling plate current...

Excellent post, but I'm confused by the above; to my thinking it's the other way around 

I was just thinking about the "all else equal" thing and what PRR said.  Suppose that one of the else equals is that the OV grid curves are the same for both tubes.  Now all of the grid curves below 0V would move up with decreased g_m and the given bias voltage grid curve would pass higher plate current at the else equal plate voltage. 

But if all else is going to be equal, then the rate of change of the spacing between grid curves is going to have to be the same.  Under these conditions, the grid curves would never reach zero amperes for the lower g_m tube.  I'm pretty sure this doesn't happen in the wild.

[HotBluePlates]

On p115 of Tomer, is 'triodes' a typo that should say 'diodes'? 2nd paragraph, sentence reads 'Tubes are operated as triodes in this tester, and raw AC voltage is applied to all elements, including the grid'.

Yep, that page says "triodes" twice where it should say "diodes."