the sound of incorrect adjusted bias
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Guitarzan
Well-known member
Here's a link with Tech Talk but if you understand Tekkie then it will answer your question.
fatboy135
Well-known member
since 4 years ago I repairs amps I only noticed and audible issue on a bad bias adjustment on marshall amps, it was like a fuzz due the cross by zero distortion, that issue generates a little square signal that becomes in the spectrum domain in the classical high order odd harmonics
Guitarzan
Well-known member
fatboy135 said:
That only occurs with over bias right?
fatboy135
Well-known member
over paz vega 
i don´t know what are you trying to mean, so a little explanation about the bias:
all electronics components needs a voltage or current to see on its inputs to starts to drive the signal.
so we need to put a dc voltage at the input to starts the signal flows. Tubes works with a current flow of electrons from cathode to anode, that electrons are generated and force to jump thru the vacumm environment by the filaments.
So for example in triode tubes we have three parts, an anode, a cathode and a grid. The grid is the input and cathode and anode are the amplified output in voltage or current depending if it is a common cathode or common anode design.
We put the dc main power supply on the anode and we need that grid and cathode be very lower voltage compared with the anode. When we create a great potential between two plates, it creates an electron flows that try to minimize the potential looking for equipotential surfaces.
For that reasons and due the tubes needs great potentials to create that little currents ( very low efficiency ) we polarizate the anode with greats positive voltage from 150 to 250 volts and we polarizate the grid with negative voltages. The point of polarization of the grid is the bad-common know as bias point ( a bias point is not only the grid voltage, it is all the tube circuitry on dc voltages, anode, cathode, grid, resistors of all that components...). when you polarizate the tube you are thinking in the totally bias point of the tube, how do you want to manage, your input signal and the kind of circuit you´re implementing.
Of course solving the ecuation of the circuitry of the tube the anode is the highest positive voltage, the grid is usually negative voltage and the cathode is very lower than grid voltage, we need that voltages to create the electrons flow.
After this little explainment we must to know that grid voltages are negative.
In conclusion, we need to polarizate the grid with an upper dc voltage than the voltage threshold ( the voltage that starts the tube to create an inside current ) this voltage is applied on the grid and helps the tube to flows electrons from cathode thru anode. If we don´t fix that voltage the input ac signal will fixed it, the problem is that with very lower input signals the tube will not flows electrons. So with high input signal voltages we will get the tube polarizate with an ac signal, but with lower signals it will not be polarizate. think in a sinusoidal signal it has maximuns on the summit if the waverforms and minimuns when the signal passes by zero. Near the zero point it will create a square waveform due the tube doesn´t has enough voltage to be polarizate, in the spectrum domain a square wave is translate into high frecuency odd harmonics.
We polarizate the tube with a bias point to fix it in a work zone and to avoid that distortion by zero issue, so if we put a lower than threshold voltage we will get the issue i was quoting.
:roll:
i don´t know what are you trying to mean, so a little explanation about the bias:
all electronics components needs a voltage or current to see on its inputs to starts to drive the signal.
so we need to put a dc voltage at the input to starts the signal flows. Tubes works with a current flow of electrons from cathode to anode, that electrons are generated and force to jump thru the vacumm environment by the filaments.
So for example in triode tubes we have three parts, an anode, a cathode and a grid. The grid is the input and cathode and anode are the amplified output in voltage or current depending if it is a common cathode or common anode design.
We put the dc main power supply on the anode and we need that grid and cathode be very lower voltage compared with the anode. When we create a great potential between two plates, it creates an electron flows that try to minimize the potential looking for equipotential surfaces.
For that reasons and due the tubes needs great potentials to create that little currents ( very low efficiency ) we polarizate the anode with greats positive voltage from 150 to 250 volts and we polarizate the grid with negative voltages. The point of polarization of the grid is the bad-common know as bias point ( a bias point is not only the grid voltage, it is all the tube circuitry on dc voltages, anode, cathode, grid, resistors of all that components...). when you polarizate the tube you are thinking in the totally bias point of the tube, how do you want to manage, your input signal and the kind of circuit you´re implementing.
Of course solving the ecuation of the circuitry of the tube the anode is the highest positive voltage, the grid is usually negative voltage and the cathode is very lower than grid voltage, we need that voltages to create the electrons flow.
After this little explainment we must to know that grid voltages are negative.
In conclusion, we need to polarizate the grid with an upper dc voltage than the voltage threshold ( the voltage that starts the tube to create an inside current ) this voltage is applied on the grid and helps the tube to flows electrons from cathode thru anode. If we don´t fix that voltage the input ac signal will fixed it, the problem is that with very lower input signals the tube will not flows electrons. So with high input signal voltages we will get the tube polarizate with an ac signal, but with lower signals it will not be polarizate. think in a sinusoidal signal it has maximuns on the summit if the waverforms and minimuns when the signal passes by zero. Near the zero point it will create a square waveform due the tube doesn´t has enough voltage to be polarizate, in the spectrum domain a square wave is translate into high frecuency odd harmonics.
We polarizate the tube with a bias point to fix it in a work zone and to avoid that distortion by zero issue, so if we put a lower than threshold voltage we will get the issue i was quoting.
:roll:
Guitarzan
Well-known member
What I meant was (over bias = too much voltage) = clipping on signal.
fatboy135
Well-known member
ok, setting the bias so high could also generates a similar fx, but i have seen it only on solid state circuits with transistors and op amps.
if you fixed the bias so high you are limiting the posible input signal, think that in amplifier circuits we only could achieve theoretical the maximum power supply voltage, so if we attack our circuit with a very high input signal or if we fixed a high bias point we will surely achieve the maximum power supply feeds, and we will distort the signal.
When a signal reaches the maximum and the trafo couldn´t feed it no longer, then all the rest of the signal will be amplified, excepts the parts that reaches the headroom, the consequence is a square waveform, and you could imagine the next....
Well that matter are for solid state due its construction, but remember that tubes are not the same than transistors, the features of the tubes do they to be more nice for music because it answers like the human ear.
When you are getting so high levels on tubes they don´t cut the higher crests. they compress them, very similar to the human ear, for that reason until you reach a high distortion on a tube you will hear a good compress, for that reason trash metal players likes transistor amps and transistor stomp boxes like metal zone, because you could distort a signal with a transistor but you could only saturate or overdriven it with a tube.
if you fixed the bias so high you are limiting the posible input signal, think that in amplifier circuits we only could achieve theoretical the maximum power supply voltage, so if we attack our circuit with a very high input signal or if we fixed a high bias point we will surely achieve the maximum power supply feeds, and we will distort the signal.
When a signal reaches the maximum and the trafo couldn´t feed it no longer, then all the rest of the signal will be amplified, excepts the parts that reaches the headroom, the consequence is a square waveform, and you could imagine the next....
Well that matter are for solid state due its construction, but remember that tubes are not the same than transistors, the features of the tubes do they to be more nice for music because it answers like the human ear.
When you are getting so high levels on tubes they don´t cut the higher crests. they compress them, very similar to the human ear, for that reason until you reach a high distortion on a tube you will hear a good compress, for that reason trash metal players likes transistor amps and transistor stomp boxes like metal zone, because you could distort a signal with a transistor but you could only saturate or overdriven it with a tube.
slhguitar
Well-known member
I think he means the Total Harmonic Distortion of the circuit can get boosted by an overly cold bias. I do not claim to have any knowledge of these things, I just think thats what fatboy is tring to say.
fatboy135
Well-known member
if i improve my english maybe i could be the king of electronics :lol:
so first lesson: my name is fatboy what is your name :lol:
i think that electronics are easier than english i fails a lot of english lessons due i was paint some boogie amps in the classroom :lol:
so first lesson: my name is fatboy what is your name :lol:
i think that electronics are easier than english
i fails a lot of english lessons due i was paint some boogie amps in the classroom :lol:fatboy135
Well-known member
hey, look my emoticons, they describe a perfect diagonal :roll:
Guitarzan
Well-known member
Yo comprendo, Gordo.
fatboy135
Well-known member
nice try. better say " te entiendo gordo " it sounds more spanish :wink:
i will announce on the board classifieds and exchange of spanish lessons for english lessons, i´m very newbee in that language
i will announce on the board classifieds and exchange of spanish lessons for english lessons, i´m very newbee in that language
Guitarzan
Well-known member
:lol: LOL
