OK-I'm dumb. What does it mean to bias tubes and when does it need to be done?

[Custom Deluxe]

As my title states: What does this really mean?

Can someone give me a laymans version or direct me to somewhere that explains it well:

1) what is done when tubes or an amp get the tubes biased?

2) When does it need to be done?

3) Do all tube amps needs this done when replaceing the tubes?


Thanks. I'm a little slow when it comes to electricity-side of this hobby.

[Steve G]

Im glad someone asked 'cos Im dumb too!

[barkley]

Im no expert but I'll have a crack at this one. This info may only be useful for a deville amp- the one I own. Im not sure if its true across the board.

Firstly its only the power tubes that need be biased. The preamp tubes are 'self biasing'.
To bias is to determine how much current goes to the tubes, therefore determining how 'hot' they run, thereby affecting your tone. Hotter= dirtier.
The deville and deluxe have a trimpot inside that allows you to adjust the bias. Youll need a multimetre to check the levels whilst biasing. I think its generally done when you replace tubes (like I need to do since mine blew!)

Hmmm, Please double check any/all of this info, as I have never biased an amp and I know it can be a dangerous thing to get wrong.

[Cam]

http://www.youtube.com/watch?v=JB1pZa0Vj4I

No affiliation. I never rock my tubes like he does putting them in and out. Straight pull for me. I'm afraid of breaking the key lock centre post of the tube.

http://thetubestore.com/powertubeinfo.html#q9

[marshman]

You can check your bias whenever you like, but most people don't worry about it unless they've already got the amp open for some reason, like changing tubes. There's no guarantee that a bias setting on one set of tubes will match the bias setting on the next set, or that the bias setting will remain constant throughout the life any given pair of tubes, so some guys check their bias every few months.

As I uderstand it, we can only check the bias "at idle", that is, while the amp's NOT being played. We set the idle speed (lots of carbeurator metaphors here, but most of us don't really remember how a carb works anymore, so they're not so helpful), therefore, how hard it runs when really pushing.

There's no right setting, but plenty of wrong ones. Generally, bias is about a range of acceptable values. Let's say that anything between 1-30 is possible: 1-14 are 'too cold' and the tubes will not function properly...15-22 is the 'acceptable range'...23-30 is 'too hot', and your tubes will burn up too fast--too hot can literally mean minutes. Setting your bias at the 'cool' end of the spectrum will give maximum tube life, but 'less vibrant' tone. Setting it to the higher end will really get the tubes crankin', but will reduce tube life. Largely a matter of taste, with a nod towards your tube budget.

Impressions of a non-electrical fellow who's been trying to 'get it' for about 6 years.

[BigDaddyLH]

Why are some amps self-biasing? Does this mean you can't do any similar adjustment to tweak the sound in a self-biasing amp?

Does biasing have anything to do with the fact that most amps' power tubes come in pairs? And why is it a good idea to buy such power tubes in pairs instead of individually?

[still_fiddlin]

Pretty good link here:
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#bias

YouTube has some videos. Different amps have different ways of setting the bias, so you'll have to see if you can find one for your amp.

[telo]

Another dummy here, too.

I know that barkley was saying that hotter=dirtier, what are some other side effects (tone-wise) of setting the bias too hot? Also, what does a cold bias sound like?

[therecordfable]

Buy a Mazerati it's self biasing...8)

[Beachbum]

I owned one of the early 90's Blues Devilles made in Corona. They were self biasing and you are right. You can not use a bias control to change your tone on that kind of amp because it doesn't have one. You can however install different preamp tubes which if you know or investigate the tubes can be used to alter tone. More head room etc.

I am also no expert but one thing I read constantly by those who are is to be very careful screwing around with bias settings. Even unplugged an amp contains massive amounts of stored electricity that can basically cause you to assume room temperature. It is always advised that you drain the stored energy from the caps before you do anything.

Groove tubes are color rated as to how hot they are. Hot, medium, cool. According to GT once you have installed and biased a given tube as long as you stay within the same rating there is no need for resetting the bias on a bias adjustable amp when you do your next change out . Using this type of rated tube also will allow you to alter the tone on your non biasing amp since it should accept any of the rated tubes with no need to bias the amp.

[robt57]

ignorant is different than dumb.

You can't just imagine up knowledge you do not possess.

Actually, seems pretty smart to ask what it all means here, a place where you are likely to get a great informed answer. ;)

[boris bubbanov]

We've talked about dramatically reduced tube life on amps that are left biased real hot.

Do ya'll know of instances where incorrect bias caused permanent damage to the amplifier itself?


Much obliged.

+++

I agree with Robt. I may act like I know something about guitars. When it comes to amps, you can't insult me, no matter what you ask. I assume my dog Daisy knows more about amps than I do. I still cannot figure out how I was allowed to build large Heathkit solid state amps when I was a teen. I must've lost some brain cells somewhere along the way, because I don't remember one tenth of what I once evidently knew. I refuse to be proud, if I can save a life, maybe my own life.

[robt57]

Originally Posted by boris bubbanov

I still cannot figure out how I was allowed to build large Heathkit solid state amps when I was a teen.

Like one of these?

[marshman]

Yeah, it's true that there are LOTS of little things in a guitar amp that can kill you deader'n'$@it if you touch 'em, even if the amp is unplugged--if you don't know which bits those are, do not go pokin' around in there.

That said, an amp must be ON to be biased, presuming it has adjustable fixed bias (how's that for confusing?) if it's NOT adjustable, then you have to swap resistors, which requires shutting it down and draining the caps.

If you don't know what 'draining the caps' means, then take it to a pro.

[T Prior]

the basic answer is so that power tubes operate in the range that they are intended, manufactured, selected by the designer. A designer selects a tube for a REASON, then builds the circuit around it to allow the tube to perform at optimal performance, as that's why it was selected in the first place, to perform at optimal performance.

from a nice technical article

quote

In most configurations, the vacuum tubes need a negative bias on the control grid, grid 1. The reason for this is that the tube characteristics change drastically if the control grid becomes positive: A grid current will flow, and the amplification and transfer characteristics will change. So most tubes are built for an operating range with a negative grid bias of several volts. This means that our equipment really needs three supply voltages: A low voltage for filament power (possibly AC), a positive voltage (B+) in the order of 50-350V for plate power, and a negative voltage (B-) in the order of 6-20V for grid

unquote

My Fender amps bias in at around 45v..plus or minus.


What happens if the tubes are not bias set ? Mostly they run a tad hotter or colder than intended. A few months after a new set of tubes are installed they will change naturally anyway so the bias points will shift.

Real world ? Does it matter ? Not to your ears. A bad tube is gonna go bad regardless of bias settings, a loud amp is still gonna be loud. If you change tubes every few years ( like me) life just goes on.

Honestly, I played my 70 Twin for maybe 20 years non stop, I probably changed the power tubes 3 times and I don't ever remember pulling it apart to set the bias. I probably didn't have time. I know I didn't care .

I still believe the trick is to change the tubes with the same type the amp was designed to use or as close as possible.

[Telenator]

If it's a Marshall, just play it til the tone knobs stop working! Seriously. That's when I knew my amps needed tubes.

[robt57]

New tubes over the used ones getting pulled may well put the circuit closer to running at spec than the old tired ones. Even if not biased to run exactly at spec. How many folks are even close to having their tube gear running t spec I wonder, hey it is a tube amp, still sounds best!