crossover distortion - what is it?
- Thread starter oyster
- Start date
Help Support The Boogie Board:
It's essentially a "kink" in the waveform where the wave crosses between positive and negative polarity. A cold biased amp will usually have crossover distortion. A properly biased amp will minimize this.
In the waveform below, the top waveform is correct. The lower one shows crossover distortion. While the image makes reference to transistors, it is the same with tube amplifiers.
-Steve
In the waveform below, the top waveform is correct. The lower one shows crossover distortion. While the image makes reference to transistors, it is the same with tube amplifiers.
-Steve
i'm not 100% sure cause i'm not a techy, i just play and know what i like to hear, but i think it has something to do with the cold bias of the power tubes. i guess people say rectos are like they. love 'em or hate em, but everyone tries to copy them. my Roadster 2x12 sounds better than any amp i have ever played under 3 Grand.
Hi-Fi guys like to bias their amps so this "notch" just dissappears.Way too cold for guitar amps.In fact,I have tried,while watching the wave on a scope, to see this "notch" and I couldnt get my MKII cold enough for the notch to appear.But I can tell you the thin,fizzy distortion comes long before you even approach that "notch".My point is that some sources tell you the proper bias point is when this "notch" disappears.If a tech tells you he uses this method to bias,dont do it,it is way to cold for guitar amps.
The odd thing on this for me is that I prefer to have the bias set a little on the cold side as it gives the gain a better articulation. Peavey sets the 5150 that way and biasing it hotter gives it a better clean sound but takes some of the brutality out of the Lead channel.
I have a bias pot on a Peavey XXX I own and I've experimented back and forth with different tube types, EL34s and 6L6s, and find that I like the range a little lower than what most would calculate this out to be. By that I mean that most techs would set it in the 50-70% plate disipation so around 32-44ma. I think around 26-30ma sounds better. There is a little difference in the warmth of the clean channel but I usually run reverb or delay through the loop so it works out just fine and in a live setting it is really not that noticeable.
My Point, you ask?
My Dual Rec set to SS/Bold take the current to 26ma for 6L6s. Right where I think it should be. Agressive gain, good cleans - musical.
I have KT77 in it right now and at performance volumes I'm getting that additional harmonics with the lead notes that I LOVE so I am going to abandon my plans for getting this amp modified with the adjustable bias.
I am curious as to what this setting puts the KT77s at and will most likely plug my BiasRite on it to check it out soon.
I think most folks spend too much time worrying about this and should just experiment with finding the preamp and power amp tubes that gets you close to the sound tha you want with the amp you have. If it cannot be done, maybe you are trying to do it with the wrong amp.
Just my 2 cents.
I have a bias pot on a Peavey XXX I own and I've experimented back and forth with different tube types, EL34s and 6L6s, and find that I like the range a little lower than what most would calculate this out to be. By that I mean that most techs would set it in the 50-70% plate disipation so around 32-44ma. I think around 26-30ma sounds better. There is a little difference in the warmth of the clean channel but I usually run reverb or delay through the loop so it works out just fine and in a live setting it is really not that noticeable.
My Point, you ask?
My Dual Rec set to SS/Bold take the current to 26ma for 6L6s. Right where I think it should be. Agressive gain, good cleans - musical.
I have KT77 in it right now and at performance volumes I'm getting that additional harmonics with the lead notes that I LOVE so I am going to abandon my plans for getting this amp modified with the adjustable bias.
I am curious as to what this setting puts the KT77s at and will most likely plug my BiasRite on it to check it out soon.
I think most folks spend too much time worrying about this and should just experiment with finding the preamp and power amp tubes that gets you close to the sound tha you want with the amp you have. If it cannot be done, maybe you are trying to do it with the wrong amp.
Just my 2 cents.
Kramer,you are right.what sounds good to you is the proper bias point.The plate voltage has alot to do with what is the "safe" range.If your plate volts are high enough the 26-30ma's you select could very well be 50-70% disipation.But anyway sassuming the 32-44ma were 50-70% going to 26-30 is not likely going to put you into crossover distortion range.To me there are only two requirements to proper bias,first is that your tubes are not redplating and second,that they sound good to you.Some techs use the crossover method and to me that is just too cold for guitar amps,sure using a scope may seem "scientific" and precise,but it takes the most important piece of test equipment out of the formula,your ears.As to your comment about worrying too much about this and finding the right preamp and power tubes.If you have an adjustable bias you can use power tubes that otherwise wouldnt sound good with the non-adjustable bias resistor,and you can then concentrate on the preamp tubes.
