consequences of wrong bias settings?

[Fragle]

hi all

recently i was talking to a bud of mine about biasing and stuff. basically, he told me that the bias setting doesnt really matter as long as you dont have it way too hot.

as far as i know, setting the bias low puts less stress on the tubes and the transformer and stuff, so they will last longer, but also kills your tone (well, this is a matter of preference, though. some people (particulary people who like solid state amps [img]/images/graemlins/laugh.gif[/img] ) like ice cold biased amps ^^ .
if you set it hot, your tone will become better, but the life of the tubes is shortened, up to the point where you push them past what they can handle, resulting in tube failure (and possibly damage to resistors and the transformer? ). but other than that i think there are no rules concerning biasing.

i know there are lots of amp gurus around here, so maybe at least one of them might show up and tell me if there are any other things to keep in mind.

btw: how can i find out if the bias is set too hot without measuring the voltage (or is it the current? [img]/images/graemlins/laugh.gif[/img] )? i remember something like if the grey large plates in the middle of the tube start glowing red hot (only spots or in the worst case the whole plate) it is about to die, right? and a slight blue glow is normal, right? and the.....damn, i dont know how it is called, the "heating" of the tube will always glow red no matter where the bias is set, right? so the tube will always glow red with a slight blue glow in the middle, but the middle section will also glow red hot if the tube is biased too hot and is about to die, right?

maybe someone can clarify the whole issue a bit.

greets
fragle

[fett]

Re: consequences of wrong bias settings?

Your amp will either sound like chit or melt. Surf the web. I tried, "Tube Bias". Some amps allow you to adjust the bias via a knob. Others have a fixed bias resisitor. The bottom line is you want the power tubes power balanced. A tube should not glow "Red" under any circumstances. The classic warning when screwing with tube amps is- "Tube amps store a potentionally lethal charge of electricity even if they have been unplugged for hours." Learn and know what your are doing before you attempt to open up the back.

[Guest]

Re: consequences of wrong bias settings?

You have the right idea. If your bias is set to hot, it can not only cause damage to your tubes, but also to the solder joints, the foil in the board or fry out a resistor. One should have a bias probe for a meter to get accurate results and know how to discharge a capacitor with a large resistor before starting to attempt work on an amp of this nature. Provided you know what the recommended plate current is for a certain tube for your amp, you can then determine to know how your tubes are biased and what kind of life you're going to get out of them. It is good to know if the tubes are matched and where they are in their life cycle as well.

It sounds like you are trying to determine if your tubes are biased properly by appearance. I would compare this to a cars fuel/air mixture. You can tell if the mixture is to lean if the exhaust starts glowing red, and running to hot. It is the same for tubes. I would eventually go with getting the tools and learning how to measure bias for yourself if you are a do it yourself kind of guy. In the long run, it pays for itself. A blue glow may also mean a gas leak, especially if it is the whole tube. Typically the cathode will have a red glow that is not so strong that it is making the base or the tube look like it is burning hot just as a normal exhaust will not have a red glow when the cars fuel mixture is calibrated correctly.

[Cleveland Metal]

Re: consequences of wrong bias settings?

I'm no guru, I assure you. But, I know enough to be dangerous! I'll tell what I think I know...

Bias is a negative or positive (depending on design) voltage applied to a power tube's grid, to adjust the measurement of idle current the tube is drawing.

Biasing is pretty much to get both sides of the A/B push pull amp working together evenly and commensurately, and if they are, you'll basically see a nice graceful looking sinewave between the two sides when viewed on a scope.

Biasing cold isn't damaging really as far as I know. Biasing hot certainly can be.

If an amp is biased cold, it is running the tubes somewhat innefficiently and it will get a crossover distortion (seen as a on a scope) that can sound harsh to the ears when overdriven/distorted, and unmusical. Tubes will live much longer though, hence many amps coming from the factory biased quite cold. I guess you'd say to help eliminate warranty troubles maybe.

Bias hotter and the tubs don't live as long. But, they are working in a more efficient range and sound better or more musical with better dynamic feel and harmonically richer. Over a certain point (%85 or so), you start blowing fuses, getting hum, melting tubes and damaging transformers.

You commonly bias by signal generator/scope (which will give you an idea of other troubles in the amp as well) and meter to check your work as far I know. The metering is important to check as some amps are a bit vague in their scope pattern I'm to understand. Some may have little or no wave and be biased properly. And of course you can bias by meter alone if you just wish to check the mA.

As some people I've heard do, going by ear is not a good way, hehe... and I Can't say I know how you could visually check overbias on an amp, as different tubes react differently as far as red glow on certain parts inside, or blueishness etc.

Not sure I've been much help....

[Guest]

Re: consequences of wrong bias settings?

More technical, but that is well explained Cleveland.

[fett]

Re: consequences of wrong bias settings?

Find a tech and have him do it if you think the amp doesn't sound right. "Danger, Danger, Will Robinson". The "Tubes" had a song called: "White Punks on Life Support". [img]/images/graemlins/laugh.gif[/img] The wrong bias may elect the wrong guy. [img]/images/graemlins/laugh.gif[/img]

[taxistationary]

Re: consequences of wrong bias settings?

You've got the fundamentals down. Yes, it's normal to have a slight blue glow. About the red glowing: that is the filament glowing.
The filament heats the cathode to send off a neagtive electrical charge (electrons).
Then from there, the positively charged plate accepts the negitive voltage from the cathode.
Between the cathode and plate, there are 3 grids (in pentodes): The control grid (bias), then screen, and then supressor grid. The grids all do different things. The control grid has a slight negative charge to lower the voltage between the cathode and plate (biasing).
The screen grid has a postive charge. It's main reason for being there is to draw electrons through the control grid to find there way to the plate.
Then, the suppressor grid is there to pretty much slow the electrons down so they don't slam the plate. The supressor grid has more uses, I just don't want to pull my books out.
I hope that helps, because it took a while to type out!! What a mouth full! [img]/images/graemlins/laugh.gif[/img]

P.S: Im not a guru either. Pete's the guy that know his **** around here, I just read books!

[CZSER]

Re: consequences of wrong bias settings?

Cleve: after trying out 70%, 60% and 82% in 50 watter SLO clone (now loaded with Tung Sol 6550 [img]/images/graemlins/laugh.gif[/img]) I can tell U it's useless 82% (way too compressed and tubes lives 1/2), it's fine 60% (out of crosover distortion) and sweetest 70% let the amp run relaxed, well defined, yet smooth and punchy.
Frag: to establish the bias point U need to know/measure your plate voltage and the power tube Plate Dissipation (by tube type)
See this: http://www.thinlizzy.de/html/tubes.html
Easy reading.
Me no guru [img]/images/graemlins/nono.gif[/img], But also read a lot.
My two cents.

[Fragle]

Re: consequences of wrong bias settings?

yes, basically i want to check my bias by appearance. i can tell that it isnt biased too cold because of its sound, but i'm not sure if it is a bit hot or not. on the other hand, i've been running the amp with the new tubes for about 3 months now, without a single problem [img]/images/graemlins/smile.gif[/img]

can anyone take a pic of their tubes (switched on, obviously [img]/images/graemlins/laugh.gif[/img] ) so i can check if mine are running normal or not?

i'm particulary asking because the tube glows quite red/orange in my opinion, and i'm not sure if thats normal.
but as i said above, the big grey plate in the middle is not glowing, only the filament.

greets

[CZSER]

Re: consequences of wrong bias settings?

Filaments allways glow orange, the warnin is the plate's red glow.
You wont be able to establish your bias by appearance. You might be at 80%, 70%, 40% and look all the same.
If U want to bias youself, get your amp to a tech & tell him to measure the plate voltage with all knobs to "0" (idle). Write it down in the back of th am with permanent marker: "B+= xxx Volts". See the link I posted. Calculate Bias point. Get a bias probe(to use with multimeter) or a "Bias King". DO IT YOURSELF.

[charvelguy]

Re: consequences of wrong bias settings?

I think it would help if we knew the type of amp and what type of tubes are being discussed here as well, then someone here may be able to help you in specifics further. Otherwise, I'd look on the net for pics of tube amps and audio equipment running similar types of type tube compliments. I'm sure there are several if you spend a few hours to look about. The best and surest way is not by sight, but by meter and ear. I used to drive my tech nuts with a rebias almost every week on a Marshall. Some nights it would sound so sweet and warm, other nights, the mojo just wasn't there even tho it would be the same room.
I wasn't exactly sure if it was me, the amp, the tubes or the bias.
I finally just chalked it up that was just the nature of the amp and had the bias set where I thought it sounded good and left it alone.

[Fragle]

Re: consequences of wrong bias settings?

its a laney gh 50 l with jj el34 tubes, but i'm gonna sell this amp anyway (as you can see in my other thread).

actually, i dont want to bias it myself (i'm quite scared of high voltages etc [img]/images/graemlins/smile.gif[/img] ), but i just want to check if its running normal - judging by ear i'd say yes.
i also wanted to know what can go wrong if the bias is set wrong. to me it seems like the bias setting is 100% preference as long as the plates dont glow red hot. is that true, or does the bias setting also affect other parts than the tubes - and obviously the sound [img]/images/graemlins/smile.gif[/img]

greets
fragle

[charvelguy]

Re: consequences of wrong bias settings?

You'll have ideal parameters or an operating range of plate voltages you want to work within, and within that you have a matter of preference or 'sweet spots'. Not sure if they manufacture amps with outside bias pots that can exceed what the amp can handle, but it sure wouldn't suprise me if they didn't make it idiot proof (no offense implied)
Some amps like Sundown came with a bias adjust on the front panel.. I'm sure there are other boutique amps having this feature as well but they slip my mind at the moment.

Sounds like your amp is running fine and has a normal appearance but the best way to check is to take it to a tech and spend an hour of bench time tweaking it if you really want to get particular.


As it has been mentioned, if you bias to hot, it could potentially cause harm to the amp itself; blow fuses, fry resisters, melt solder and copper, harm transformers etc.
To cold and your amp sounds more harsh. I'm not even going to go into a variac here..but typically that would also be another way of varying plate voltage and that will have its obvious dangers as well.

[Twisteramps]

Re: consequences of wrong bias settings?

If you bias too hot, you're going to kill the tubes. When the tubes die, they could take out other components depending on how much bad luck you have. [img]/images/graemlins/smile.gif[/img]

Plates are those usually grey things in the middle of the tube. The danger sign is when it glows cherry red or you see red spots.

Some guys just set their bias by ear - which is fine. I prefer to check mine with a voltmeter just so I KNOW it's ok and can also put the amp back to where it sounded good to me every time.

There is a huge range in which your powertubes will work. The big thing is to not bias them too hot. Depending on the amp, you may want to run 70% dissipation, or lower... it just depends. There is no magic number, only a certain point after which tubes will either age much too fast or die quickly. Keep it under 70% and you'll be ok. To simplify it - if you don't see cherry red glowing spots on your plate, and the amp doesn't sound bad - then you're ok.

Pete

[slo100]

Re: consequences of wrong bias settings?

Here's a great link:

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

Remember, when you open the hood of a tube amp and don't know exactly what you are doing, especially when plugged in, don't take any risks!!! Electricity is a killer and even an unplugged tube amp with charged capacitors can do some damage if you touch the wrong thing.

There are tons of links, information, and resources on the Internet so spend some time reading! Tinkering around inside your amp without doing the reading and research is NOT knowing enough to be dangerous!