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Post subject: Bias Creep
Posted: Wed May 04, 2016 7:33 am
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Which way does power tube bias drift over time?
Does a tube run hotter or colder?
Is it significant enough drift to check bias....how often?


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Post subject: Re: Bias Creep
Posted: Wed May 04, 2016 8:18 am
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As the tubes wear out, the bias usually drops in current and output power. I'd checked the amp's idle bias, maybe once every 6 months. As long as there are no apparent issues, like red plating or too early onset of distortion ---> under-biased. Cold, lifeless tone ---> over-biased.


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Post subject: Re: Bias Creep
Posted: Sat May 07, 2016 6:52 am
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Before they actually wear out, the bias slowly rises in a tube. Then as it wears out, the bias and power go down.
During normal wear, the emission of electrons blasts the control grid and slowly deteriorated its surface. This causes the grid to be slightly less effective in controlling the electron flow, so the bias goes up slowly over the months.
When the tube wears out, the cathode emissions drop off and no longer produces a stream of electrons.
For the most part, you will never see a completely worn out power tube.
Most fail from.microphonics or shorts.
Some from a bias that has drifted up and out of range of the amps adjustment. While you may be able to lower it, it's tryin' to tell you its tired.
This is where I'm a little sketchy about NOS power tubes. While many still work, many have wear that makes them unable to bias properly in an amp.
I try to compare a normal set of tubes bias and then pop in the NOS to compare.
Where most new tubes bias at, say 30mA, a comparison to NOS usually measures 45 or more, showing the wear. Or its 10mA and effectively worn out. Your amp is pickyer than a tube tester.
If you can adjust for a good bias, fine. I just have a hard time selecting, recommending, or providing warranty replacements for NOS power tubes.


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Post subject: Re: Bias Creep
Posted: Sat May 07, 2016 9:04 am
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"This is where I'm a little sketchy about NOS power tubes. While many still work, many have wear that makes them unable to bias properly in an amp."

Are you saying that NOS power tubes can likely be worn out....even without use?
Are the grids, vacuum, or other components likely effected by time?
I ask because I'm still contemplating buying a pr of nos GE 6L6GC's from Ruby Tubes source........but can buy up to three sets of new production tubes for the same price.
I would'nt mind laying out the coin for the supeior tubes, but i see no verifiable data that NOS is the superior tube.
I DO suspect that a 35 year old tube HAS to degrade over time, if nothing else, than by the internal components slowly leatching out their gasses.


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Post subject: Re: Bias Creep
Posted: Sat May 07, 2016 6:22 pm
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A tube should not degrade, just sitting on a shelf. Unless, subjected to a LOT of vibration or lost its vacuum --- it should be ok.

I've have some 60+ year-old stored NOS tubes --- that actually tested higher than new made tubes.

Tester is what would be nice.


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Post subject: Re: Bias Creep
Posted: Sun May 08, 2016 9:47 am
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Beemer's right. Tubes don't wear out sitting on the shelf.
The problem I have is that there is apparently no such thing as OOS. Old Old stock.
Nobody makes a distinction about used tubes
I guess I have a hard time believing that there are still unused tubes from 50-60 years ago.
With the demand, there are bound to be misleading claims as to actual wear, vintage and certainly quality.
Just because it's old doesn't mean it's good. In fact, I suspect that its the culls that are the leftovers.
The higher gain in a tester or bias measurement is a sign of a worn tube.
A tube, left to its own, will turn on fully.
So the test of quality is how well the grids can turn it off.
I'm sure there are power tubes left over that perform well, but from a technician's perspective, I can't predict performance like a new production tube and I can't warranty them because of their unknown heritage and impossibility of warranty replacements.


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Post subject: Re: Bias Creep
Posted: Sun May 08, 2016 10:48 pm
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I will say use what you want. Not all tubes are equal. It seems you have made up your mind so do as you will.

I like vintage good tubes.


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Post subject: Re: Bias Creep
Posted: Mon May 09, 2016 2:58 am
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"I will say use what you want. Not all tubes are equal. It seems you have made up your mind so do as you will."

My mind is far from being made up.
On the one hand I want to support new production power tubes, but aside from adjectives like "creamy....airy....crunchy" , I see very few technical descriptions or testing that proves their value.
OTOH, NOS tubes never relied upon such adjectives, and who's cost has escalated for what few are left.
.
The only thing that seems certain to me is you get what you pay for.
If new production tubes are priced according to superior engineering, components, and workmanship, then I dont mind paying it.
.
I want to see less adjectives and more data......we're not children.


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Post subject: Re: Bias Creep
Posted: Mon May 09, 2016 8:41 am
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A few facts that I gleaned from an old book have helped define the qualities I look for in a tube. Its from 1948, when tubes were at their peak of engineering.
Image
Now, I'm just a technician, so I have no comment on any tube tone.
I look for quality components that affect tone in a positive way. I know this isn't always what a picker is looking for.
So this book has convinced me that the most important part of a tube isn't the metal its made from. Its the carbon coating on the plate.
As you can see from the graph, the carbon coated plate has a much lower secondary emission characteristic.
Secondary emission is a fancy term for, "the electron bounced off the plate instead of being absorbed."
This lowers the signal level and the electron leaves with a pop, raising the noise level.
So, for me, shiny black plate tubes are not my first choice.
Image
This page describes materials and their black-body radiation properties.
The section outlined in red seems to suggest that carbon coatings greatly increase conductivity.
When they cite a range of 80 to 94 percent, I'm interpreting that as a range of carbon formulas or thicknesses.
I also suspect that the additional cost of a premium tube is wrapped up in that formula..
Image


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Post subject: Re: Bias Creep
Posted: Mon May 09, 2016 11:47 am
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TimsAudio wrote:
This page describes materials and their black-body radiation properties.
The section outlined in red seems to suggest that carbon coatings greatly increase conductivity.
When they cite a range of 80 to 94 percent, I'm interpreting that as a range of carbon formulas or thicknesses.

I love the information found in old tech manuals as it relates to dated (don’t hate me) technology, the folks who wrote them really understood the tech and often presented it in a straightforward manner. Tim, you obviously have a strong grasp of the subject, and physics is only a hobby for me (as it relates to cosmology and astronomy), however as it reads to me the percentage discussed are the variance (in %) of the subject material, (both coated and uncoated nickel in this case) from that of a “Black Body”, a black body, being a theoretical perfect absorber and therefore a perfect emitter. To me this suggests the carbon coating (of undetermined thickness) brings nickel closer to the radiation value of a black body and increases its efficiency. I realize this contributes nothing to the OP’s question, lol, but is only my opportunity to join in a conversion regarding electronic issues that I am woefully uneducated about.


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Post subject: Re: Bias Creep
Posted: Mon May 09, 2016 12:27 pm
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This long excerpt from a book detailing the end of American Vacuum manufacturing I find to be very interesting....."The End Game";

https://books.google.com/books?id=JscDj ... &q&f=false

it makes you wonder just how good tubes WERE in the mid 70's to 80's, when tube factories were either being purchased from each other.....or being shut down....or even tube equipment dumped off an overseas coast.
But alas....American tubes are still alive and well in Russia...


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Post subject: Re: Bias Creep
Posted: Tue May 10, 2016 11:15 am
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TimsAudio and ckmckool, that is some good information there.

Things change over time as with anything when it comes to business and bottom line. Methods and materials are made cheaper.

I understand that the coatings within vintage tubes are different than today. I have also read where users claim a good vintage tube to far outlive some of the modern counterparts. It seems though through description of others that some modern manufacture have upped their game producing better product lately, especially China.
But failure rates still seem to be fairly common.

Check your tubes and bias whenever possible or at least periodically. Make notes of bias current and voltages at the tubes for record and later comparison.

I guess there is more tone description relating to musical amplifier tubes for modern manufacture than vintage. Most musicians do not know, do not use or do not want to pay for good vintage tubes. The comparisons are fewer and farther between than a shootout of say two or three modern type tubes.

When it comes to vacuum tubes I believe there will be less new information and information as there once was in their golden era. Tubes back then had to compete using better practices and materials, especially do to military and government contracts. Things feel off for a while but seems the industry may be stepping up for the demand at hand. Still notice that some of the new superior products also carry a heavier price tag similar to vintage product.

I believe TAD was mentioned in your other thread. A lot of people seem to like TAD quality in general.


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Post subject: Re: Bias Creep
Posted: Wed May 11, 2016 8:15 pm
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Well, while I am finding good material, I still feel like I'm in tube kindergarten.
I don't have a grasp of the math, only the concepts. And even that I'm not sure of.
There is another ambiguity that I'm confronted with in the percentages of black-body radiation, as well as the one you described. Are they only talking about thermal characteristics? Or does that relate in some way to the electrical emissivity and plate heating? My tube mysteries continue...

Don't get me wrong. Vintage tubes can sound great. I recommend a few different premium vintage preamp tubes. Telefunken and Bugle Boy outperform any modern production tube.
Many pickers can only afford one in the V1 slot, but there is nothing you can do to the circuitry to acheive the same quality results as an excellent tube in V1.


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Post subject: Re: Bias Creep
Posted: Wed May 11, 2016 8:50 pm
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"There is another ambiguity that I'm confronted with in the percentages of black-body radiation, as well as the one you described. Are they only talking about thermal characteristics? "

From what I understand, black colored plates were employed mostly in the earlier days of tube engineering because of the cooling effect of black body radiation. At the time, this was the best science insofar as plates being able to shed heat.
But later it was determined that convection cooling (cooling the glass envelope itself) is a far more effective means of cooling a tube, thus gray plates.
I am guessing it was more expensive to manufacture black plates for minor cooling benefits.
As to materials, I'd guess that cathode coating, alignment, and vacuum would be also important.
.
I'm so ready for tube manufacturers to stop with the juvenile adjectives (crunchy...creamy....airy) and start advertising what quantified characteristics make their tubes superior for the money.



http://www.tungsol.com/tungsol/img/edit ... sol-ad.jpg


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Post subject: Re: Bias Creep
Posted: Thu May 12, 2016 1:22 am
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TimsAudio wrote:
There is another ambiguity that I'm confronted with in the percentages of black-body radiation, as well as the one you described. Are they only talking about thermal characteristics? Or does that relate in some way to the electrical emissivity and plate heating? My tube mysteries continue...

My Understanding is that a perfect black body (an object the absorbs and emits all wavelengths of EMR) is a theoretical model used as a constant in assessing how a material deals with EMR . Objects that come close to the constant are considered good transmitters. Its been a while since I studied all this planks law stuff, and the math is waaay over my head. I think as far as the tube thing goes , the closer the plates get to a black body the more efficient the tube will be.


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