please explain bias.

leftwinger57

Once again my curiosity has gotten the better of me and I need to find out some info. I think it only pertends to tube amps and the question is the setting a bias and what it is and what does it do. If not set properly what happens. You do see set screws to turn on certin amps like Dynoco 70s and sockets on other amps for this I always wondered what it did for the sound.Plese no engineer here so an easy an uncomplicated explanation possible. Thanks again....

mhardy6647

tubes work based on the flow of electrons. Electrons are (by our formal definition of charge) negatively charged, so they can only flow from negative to positive. They boil off the cathode and are attracted to the plate. That is why a diode (plate and cathode only) tube can rectify AC to DC. Addition of a grid (forming a triode) controls the flow of the electrons; our music waveform (which is AC - hold that thought!) is applied to the grid. Depending on the potential (voltage) on the grid at any given instant, the electrons may be repelled or attracted by it; so the number of electrons reaching the plate at any given instant will be a "picture" of the AC (music) signal on the grid.

The trick is this: AC (alternating current) flows both positive and negative, but the electrons can only go one direction. The signal at the plate cannot "go negative"... in the absence of bias, the "bottom half" of the signal would be rectified (clipped off). Bias raises the "midpoint" of the AC music signal from zero volts to some positive value; then the tube can amplify the full swing of the signal without "going negative" - the whole signal is biased or offset.

In practice, this means that the bias voltage applied to the grid is negative; its exact value will determine how much current (i.e., how many electrons) are flowing from the cathode to the plate at "zero signal", or the quiescent current, so to speak. That sets the is the operating point for the output tube. As the signal varies on the grid, more or less electrons will flow all the way to the plate relative to that (somewhat arbitrarily chosen) zero point, and the plate current will vary above or below the bias current flow of electrons in lock step with the AC (music) signal applied to the grid. But the current flowing from the plate will never fall all the way to zero - thanks to the non-zero bias.
That's the basic idea.

heiney9

Bias applies to transistors as well. Here is a really good, in-depth explanation. Hope it's not over your head. Do you know how to Google?

http://answers.yahoo.com/question/index?qid=20071120063702AAZly50

mhardy6647

I was digging around for a good concise, accessible explanation and haven't found one yet (nor am I particularly happy with mine above). I cite tubes because it's pretty intuitive in a tube but yes of course the same holds in the realm of the solid state.

EDIT: The reference H9 posted, although a bit glib, is pretty good. I do know how to use Google :-P but that one didn't turn up in a couple of searches I just did while composing my little screed above. Kind of ironic that it's from "answers.yahoo.com"...

It might be worth pointing out that there's a whole class of transmitter triode tubes that are designed for "zero bias" and folks do make hifi amplifiers out of them... but it's not a trivial task in terms of design or construction, and it's kind of beyond the scope of what most of us will see in our living rooms or HT rooms.

heiney9

Mr. Hardy, the Google comment was not meant for you :razz:

He is asking in reference to tubes so that is appropriate, I was just explaining bias is also needed in the same fashion for transistors.

H9

pietro944

.....everyhing matters,leftwinger......except.......apparently......your questions:rolleyes:

doctor r

These are good explanations. In addition it is sometimes just as important to review how tubes work (i.e. class such as A AB1 AB2 and then the bias need and scheme is more apparent.

http://www.duncanamps.com/technical/ampclasses.html

is a good source. A lot of good information on the guitar amp sites.

tonyb

pietro944 wrote: »

.....everyhing matters,leftwinger......except.......apparently......your questions:rolleyes:

Don't see how that applies here, his question has been answered and pretty thoroughly to boot.

mhardy6647

heiney9 wrote: »

Bias applies to transistors as well. Here is a really good, in-depth explanation. Hope it's not over your head. Do you know how to Google?

http://answers.yahoo.com/question/index?qid=20071120063702AAZly50

oh, I knew that :-) Even though Google's algorithms are really good, if one doesn't quite know what to ask, one won't always get the answer. That's presumably why I missed the reference you posted (even though I've been doing database searches since the days of "Dialog" and when computer terminals were repurposed teletypes running on a 20 mA current loop interface!).

The "bias" question is interesting and the physics is fundamental to the reproduction of good sound... it seems to me that this stuff is considerably more fun and interesting when one understands the "how" at least a little bit, too.

Besides, I still don't understand them dang newfangled transistor things... dang "holes" moving around in lattices... what's that all about? ;-)

heiney9

Nelson Pass has a great article on the importance of bias, but I didn't post the link because I fear even though it breaks it down to the layman it covers a lot and one needs atleast a basic understanding of how an audio circuit flows.

H9

P.s. Ah, wth, I've posted it before but here it is "The Sweet Spot" and "Leaving Class A" are great, to the point explanations about proper biasing and it's positive and negative effects (pun intended :razz:)

https://passlabs.com/technical-articles/

doctor r

Thanks H9.

Missed those before :sad:

polkfarmboy

Tubes bias is all over the forum :cheesygrin:

leftwinger57

YES IS THE EASY ANSWER TO THE GOOGLE QUESTION.why use that when I can bother some one like you to explain. All kidding aside I did read the article and one thing did strike me that the guy said not to leave your tubes on which is totally opposite of what most of you tube owners here have said. I know he was talking more about guitar amps then audio 2 chl amps so well does it matter if you leave them on or off.We've been through this many times and my Adcom gfa555 is off until I use it. As far as being over my head well yea, I got the whole negative charge in the tube and it strives to hit the positive plate beyond that I really would be the wrong guyto ask.

cstmar01

Bias is also important for the life of the tubes as well. if you run them higher then a tube life can go down quickly.

I for one do not leave my power amps on all the time, only when I intend to listen for a long period of time. There is no way I could afford replacing my tubes (16XKT88 or 6550)'s as often as I would need to.

I know there can be an argument as well as to the circut (I think) regarding how the tube is being biased which also leads to how long the tube will last etc...I can't fully remember how it went.

heiney9

leftwinger57 wrote: »

YES IS THE EASY ANSWER TO THE GOOGLE QUESTION.why use that when I can bother some one like you to explain. All kidding aside I did read the article and one thing did strike me that the guy said not to leave your tubes on which is totally opposite of what most of you tube owners here have said. I know he was talking more about guitar amps then audio 2 chl amps so well does it matter if you leave them on or off.We've been through this many times and my Adcom gfa555 is off until I use it. As far as being over my head well yea, I got the whole negative charge in the tube and it strives to hit the positive plate beyond that I really would be the wrong guyto ask.

Tubes have a finite life, so no you shouldn't leave them on for extended periods of non-listening for that reason and because they can be a fire hazard. I don't turn my tubes on if I know I am going to leave shortly after powering them up nor will I leave them unattended for long periods of time, but that doesn't keep me from powering them up on Sunday morning in anticipation of an extended listening session or listening off and on all day while I'm home.

Will I turn them on and leave them while I go work in the yard for 3-4 hours.....no, I won't leave them unattended for that long. But the more you run tubes, especially power tubes, the more life you use up.

H9

mhardy6647

That finite life is pretty long. Western Electric 300B were built for, if memory serves, 30,000 hours MTBF.

Preamp tubes will often last for decades... although in all cases the lifespan of tubes depends on the operating points chosen for them in the circuit (and, of course the health of the other components in the device).

McIntosh's vintage amplifiers, for example, famously tended to run even their output tubes quite conservatively.

heiney9

mhardy6647 wrote: »

That finite life is pretty long. 300B were built for, if memory serves, 30,000 hours MTBF.

It can be, espcially for the really good old stock tubes. Mullard EL34's seem to run a lifetime as well as the 5AR4 rectifiers. The newer production tubes, not so much in many cases. You should to easily get 10,000+ hours out of OS Telefunken signal tubes, etc., etc.

mhardy6647

yeppers.
And then you had companies like GTA (Golden Tube Audio) that sometimes built amplifiers that were designed to immolate the tubes in 'em. D'oh.

heiney9

This scenario would be much harder on the tubes than if they were just left on

Power up the tubes---listen for an hour---power down tubes---work outside in the yard for 3 hours---come back in and power up the tubes---listen for 2 hours---power down tubes---go grocery shopping for a couple hours---come home power up the tubes---listen for an hour---power down go out for the evening---come home power up listen for an hour---power down go to bed.

Rather than leaving them powered up for say 8 hours and listening periodically.

H9