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Post subject: General Question on Tube Biasing
Posted: Wed Jul 28, 2010 7:09 pm
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I believe I understand the potential ill effects of biasing a tube too hot. Most people talk about tone improvement with an increase in bias up to the safe maximum, but other than perhaps causing poor tone, is there any physical damage possible by biasing too cold? I've never seen any comments on this.

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Posted: Wed Jul 28, 2010 7:39 pm
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Biasing cold has the practical effect of strangling a tube. In cases of severe under-biasing, excessive heat can be the result (which promotes the premature failure of both the tubes and other thermal-sensitive components within the chassis).

Sonically, under-biasing usually results in the production of excessive even-order harmonics due to increased regeneration which produce a brittle, somewhat metallic tone. Most new amps are biased too cold right from the factory -- done generally to prolong the life of the lowest-bidder OEM glass.

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Posted: Wed Jul 28, 2010 11:00 pm
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"is there any physical damage" ? Not that I can see. The tube is just being shutoff early in the swing. As far as I know, the components don't care if they are off. The switching between these "on & off" states are a sonic concern. Art

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Last edited by aclempoppi on Wed Jul 28, 2010 11:43 pm, edited 1 time in total.

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Posted: Wed Jul 28, 2010 11:18 pm
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Thanks for the responses!

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Posted: Wed Jul 28, 2010 11:38 pm
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Hey RVM, this subject has a lot of room for input and discussion. Hopefully, folks will bring their views and experiences into this thread !!! Art

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Posted: Thu Jul 29, 2010 4:36 am
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"" If the bias control is set too low, current through the tubes will be low. As a result, the sound will be distorted on loud notes. The output tubes will run cool. Tube life will be longer under this condition, but other parts (capacitors) in your amplifier may be stressed by the higher voltages which appear on them. ""

http://www.roccaforteamps.com/tube_bias.htm

Have look on bias limit "notch cross-over oscilloscope snapshot. The picture talk from themself.

http://www.lynx.bc.ca/~jc/bias.html

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Posted: Thu Jul 29, 2010 5:10 am
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Isn't "under-biasing" mean you lower the set negative volts on the grid, therefore are running the tubes hotter or closer to Class "A"? And "over-biasing" means more negative volts = colder (or closer to Class B)?


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Post subject: Biasing
Posted: Thu Jul 29, 2010 6:41 am
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BMW2002Ti wrote:
Isn't "under-biasing" mean you lower the set negative volts on the grid, therefore are running the tubes hotter or closer to Class "A"? And "over-biasing" means more negative volts = colder (or closer to Class B)?


That's right, depending on who uses which terms. Over, under, hot, cold, can all mean different things to different people, and there are different types of biasing, grid-biased and cathode-biased. For a grid biased tube you are exactly right. A small negative voltage on the grid will set the amount of idle power dissipation for the tube. The more (negative) voltage that you apply to the grid, the less idle wattage for the tube, tube runs at a cooler temperature. The less (negative) voltage applied to the grid, the more idle wattage the tube dissipates, the tube runs at a hotter temperature. If the tube is too hot, that means that it is idling at a high power when you aren't even playing. I don't care for that myself, I like the headroom that you get with proper biasing, and the extended tube life is a plus. ;)

I usually refer to my biasing as hot or cold, referring to the temperature of the tubes, which also covers cathode biased tubes. Cathode biasing is different in that a tube chart and a load line are used to calculate the biasing of the tubes. With cathode biasing the temperature of the tube will be about the same for different bias points, as long as the bias is at a point on the load line. If the cathode bias point is not on the load line then the tube temperature will be different, hot or cold. And hot or cold tube temperature can indicate improper biasing.

Some people like to run their tubes hot (temperature) as they say that they like the sound of the earlier breakup. However, the hotter you run your tubes, the shorter the life. Typically, the "earlier breakup" only means that the sound out of the amp will start to distort with the volume at say 6, while biasing the tube properly will mean that you have to turn the volume up to say 9 before you hear the breakup.

Hope this isn't confusing. :)

.

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Posted: Thu Jul 29, 2010 7:52 am
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For those who have advised that excessive "under or cold" biasing can lead to various component failures, what order of magnitude are we looking at? -15%, -30%, -50%, etc....you get the idea.

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Post subject: Re: Biasing
Posted: Thu Jul 29, 2010 8:49 am
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shimmilou wrote:
Hope this isn't confusing. :)

.


One of the best dissertations on the subject yet......should qualify as a "sticky".

Outstanding job, shimmilou!

Arjay

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Post subject: Tube biasing
Posted: Thu Jul 29, 2010 8:55 am
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Hi RVM lead,

Typically +/- 10 % of the proper bias set-point is acceptable without adverse affect to tubes or components. But remember that tubes and components have tolerances too. Typical capacitors have a plus or minus 20% tolerance, and resistors will typically be +/- 10% tolerance. That means that for a typical capacitor rated at 0.047 uF, the actual value could be as high as 0.056 uF, or as low as 0.037 uF and still be considered OK. If your capacitor and resistor values are on the high side of their value and the bias is set low, it will be a cumulative affect. So there are acceptable limits to components and for tube biasing, but I think that setting the bias properly becomes even more important if you consider the tolerances of the other components.

I hope that is what you meant. :) If you mean what percentage of failure is caused by improper biasing, that is dependent on how far out of spec it is and how much time it is operated this way. And that can vary wildly depending on the ambient temperature that it operates in and the quality of the components. :)

.

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Post subject: Re: Tube biasing
Posted: Thu Jul 29, 2010 12:10 pm
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shimmilou wrote:
Hi RVM lead,

Typically +/- 10 % of the proper bias set-point is acceptable without adverse affect to tubes or components. But remember that tubes and components have tolerances too. Typical capacitors have a plus or minus 20% tolerance, and resistors will typically be +/- 10% tolerance. That means that for a typical capacitor rated at 0.047 uF, the actual value could be as high as 0.056 uF, or as low as 0.037 uF and still be considered OK. If your capacitor and resistor values are on the high side of their value and the bias is set low, it will be a cumulative affect. So there are acceptable limits to components and for tube biasing, but I think that setting the bias properly becomes even more important if you consider the tolerances of the other components.

I hope that is what you meant. :) If you mean what percentage of failure is caused by improper biasing, that is dependent on how far out of spec it is and how much time it is operated this way. And that can vary wildly depending on the ambient temperature that it operates in and the quality of the components. :)
.


Yes, that's exactly what I was after. So once you're outside of the +/- 10% range of recommended bias setting, you begin to run the risk of component damage. Further out-higher risk.

It's been mentioned that many amp mfg. purposely bias the tubes cold. Would we find them staying within that -10%? That's really not a large tolerance.

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Posted: Thu Jul 29, 2010 1:27 pm
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Hey shimmilou,great explaination.
Have you had a chance to write some things on the subject of capacitors and other components,and their functions in the amp like we had talked about a few weeks ago?
no hurry....


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Posted: Thu Jul 29, 2010 3:06 pm
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Rebelsoul wrote:
Hey shimmilou,great explaination.
Have you had a chance to write some things on the subject of capacitors and other components,and their functions in the amp like we had talked about a few weeks ago?
no hurry....


Yes I have! It is pretty much finished and I'm going to post it on my website. I just have to redo my site a little and then I'll post the capacitor paper and the biasing paper that I posted before, so they will be in the same place for referencing. I should have it up by Friday, maybe sooner. Look under my post for the www button to my site, and check back often. Right now I have some Strat reviews and pictures on there, but will delete the pictures and "about me" page to make room for the papers. I may need a new site in the future, the one that I have now won't let me post in the word.doc or text.txt format, only text. When I do put it up, feel free to copy and paste, as I may move the stuff in the future. Thanks for your interest! :D

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Post subject: Re: Tube biasing
Posted: Thu Jul 29, 2010 3:20 pm
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RVM lead wrote:
It's been mentioned that many amp mfg. purposely bias the tubes cold. Would we find them staying within that -10%? That's really not a large tolerance.


From what I've seen, typically if the manufacturers bias to hot or too cold from the factory, it is well outside of a +/- 10%. Otherwise, it wouldn't really make much difference. Mostly though, I see biasing as too hot from the factory (meaning temperature of the output tubes). Lots of people changing tubes without making sure why the tubes need changing, or what caused the tubes to go bad if they are bad. My BJr was running so hot (temperature) that the white labels turned brown and small black spots of discoloration are showing inside the tubes. I have ordered caps and a bias trim-pot to correct the bias before I use it any more. And I wouldn't want to put new tubes into that hostile environment. :)

.

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