Sag and Bias
- Thread starter vashondan
- Start date
I'm no electrical engineer but the best description I've heard is that BIAS is like the idle on your car. Current flow. It is an optimal setting for best performance with your amp and the power tubes installed. High and low BIAS settings will give different results in tone. I just replaced the power tubes on my Blues Junior. It has a fixed BIAS (no adjustment). I bought a power tube pair set for a middle range. I believe the Blues Junior tends to run hot. (high bias). Some amp manufacturers have adjustable BIAS and some have fixed BIAS. Mesa Boogie is one of the best examples of the fixed BIAS amp. Randall Smith(MESA Boogie fame) wrote a good article explaining BIAS and why he chose a fixed BIAS for his amps. Well done. Those guys are all about tone.
Mike
Mike
Rancid Rumpboogie
Blues Mangler
Aw, now you've gone and put out all kinds of stuff to confuse the poor guy. Put simply, Bias is a good reason to not buy a tube amp so you don't have to deal with it every time a tube burns out. Vash doesn't have a tube amp, just a Mustang, so Vash, simply twiddle the two knobs until you find the setting that sounds best to you ... you don't have any tubes to fry, so there's nothing to worry about.
Yeah, as usual, I'm just stirring up s**t, but couldn't resist. ;D
Yeah, as usual, I'm just stirring up s**t, but couldn't resist. ;D
You bet. Many like cathode-biased tube amps that allow fast and easy tube swapping to determine what your ear likes best. Amps that have sag, the 5E3 circuit for example, can have that characteristic altered just by swapping in different rectifier types or power tubes.
Lots of fun, but man it can get time-consuming for those suffering from OCD.
Rancid Rumpboogie
Blues Mangler
I knew that, Falconer. I was just making a taco salad and wanted a bunch of tube-meisters to throw a bunch of free 'maters at me. ;DNot all tube amps require re-biasing when changing tubes.
If you don't want to mess with it, and it really is an easy thing to do, then buy a cathode-biased amp. You can swap or replace tubes to your heart's content.Â
I have a Lee Jackson XLS1000 combo that can take almost any tube you care to stuff in it with its "Smart Tube" auto-biasing.
http://www.leejackson.com/XLSC.htm
If you want a butt-kicking powerful do-it-all tube combo amp with a built in attenuator, get your hands on one of these. They will also give you hemorhoids trying to get one of the heavy buggers into a back seat (Mine wieghs close to 100lbs).
Crossroads
Thump the Bottom
What is it with you and your SSS? That's the acronym for Solid State Snobery. Were you beaten up by a tube amp as a young child? ;D
Rancid Rumpboogie
Blues Mangler
No. I was beaten up by tube amps for years on the road lugging the heavy buggers around, carrying spare tubes and other "emergency kit" stuff. I have three great tube amps and LOVE them. I used to be a "tube snob" myself. But I also have a Mustang III and LOVE it every bit as much. The MIII changed my tune. And I've heard the Quilter amps, and they are incredible. Both are way more versatile than any one-trick-pony tube amp. Buy one good modeling amp and you are done. Buy a tube amp, and unless you REALLY KNOW EXACTLY WHAT YOU'RE LOOKING FOR BEFORE YOU BOUGHT IT, it's a crap-shoot ... you might love it and you might not. So when I see inexperienced players myopically focused only on a tube amp and going "what will this or that one sound like", it just makes me gnash my teeth.
CapnDenny1
Student Of The Blues
The tube bias adjusts the operating point of the tube. What you would really like is a tube that is linear, which means the output signal increases exactly the same as the input signal. If you input signal goes up by lets say 1 volt, and that causes your output voltage to go up by 10 volts. (Just example numbers) Then if you increase the input signal to 2 volts the output should be 20 volts. That would be a linear gain. Unfortunately for hi-fi types and fortunately for guitar types, it doesn't work that way. When you increase the input to 2V the output may only go to 17 volts. That's a non-linear response, otherwise known as distortion, or clipping, or whatever.
When you adjust the bias you will move the voltage that comes out the output, how much gain there is, and also where and how much the non-linear distortion starts to happen.
Lets take the example above. If we increase the bias on that tube amp, the the 1V might give 15 volts output, but a 2V input might only give you 17. It starts distorting much earlier in terms of the output.
The same thing happens at the other end with low signals. If the bias is too low, then you get distortion at the other end, at say 1/10V input voltage.
They try to balance the tube so it gives as clean a signal as possible over the full range of the amp.
By running the bias really high you can make the amp have tons of distortion even at moderate output levels and even more when you crank it up. Unfortunately, the tubes will get very hot and it will shorten their life. I read somewhere that EVH had his tubes all biased so hot that they were only good for about 1 show. I guess if you are EVH you can afford that.
The sag is a flaw in the power supply rectifier tubes from back in the day. They were not really very good. A lot of the power was lost in the rectifier tube itself. When you played a really loud chord or something, then the amp needs more power, but the rectifier can't supply it, so the amp is starved for power. This is sometimes called compression. It compresses the dynamics of the guitar signal, kind of like a pedal will do. It lowers the amplitude of the peak signal, simply because the amp can't get the power from the rectifier. You can eliminate this by increasing the size of the power transformer and changing the rectifier tube to a solid state rectifier (diodes). Probably a larger filter cap as well. But that is one of the effects that people like about tube amps.
That's the engineering perspective, at least as I understand it.
I don't really like a lot of sag, or distortion. But I do like tube amps, so go figure.
When you adjust the bias you will move the voltage that comes out the output, how much gain there is, and also where and how much the non-linear distortion starts to happen.
Lets take the example above. If we increase the bias on that tube amp, the the 1V might give 15 volts output, but a 2V input might only give you 17. It starts distorting much earlier in terms of the output.
The same thing happens at the other end with low signals. If the bias is too low, then you get distortion at the other end, at say 1/10V input voltage.
They try to balance the tube so it gives as clean a signal as possible over the full range of the amp.
By running the bias really high you can make the amp have tons of distortion even at moderate output levels and even more when you crank it up. Unfortunately, the tubes will get very hot and it will shorten their life. I read somewhere that EVH had his tubes all biased so hot that they were only good for about 1 show. I guess if you are EVH you can afford that.
The sag is a flaw in the power supply rectifier tubes from back in the day. They were not really very good. A lot of the power was lost in the rectifier tube itself. When you played a really loud chord or something, then the amp needs more power, but the rectifier can't supply it, so the amp is starved for power. This is sometimes called compression. It compresses the dynamics of the guitar signal, kind of like a pedal will do. It lowers the amplitude of the peak signal, simply because the amp can't get the power from the rectifier. You can eliminate this by increasing the size of the power transformer and changing the rectifier tube to a solid state rectifier (diodes). Probably a larger filter cap as well. But that is one of the effects that people like about tube amps.
That's the engineering perspective, at least as I understand it.
I don't really like a lot of sag, or distortion. But I do like tube amps, so go figure.
Rancid Rumpboogie
Blues Mangler
I'm just happy if my tube amps sound good and the tubes aren't getting fried. ;D