Hi !
The mainreason is to improve thermal stability, without these
resistors the thermal runaway can get big. And if paralleling
outputdevices, these are a must, its the only way to ensure
proper currentsharing. Otherwise the transistor with the
lowest Vbe takes all the work, get's hotter than the others, and
the Vbe get even lower... Bang
And these resistors can add some stability to capacitive loads.
Michael
The mainreason is to improve thermal stability, without these
resistors the thermal runaway can get big. And if paralleling
outputdevices, these are a must, its the only way to ensure
proper currentsharing. Otherwise the transistor with the
lowest Vbe takes all the work, get's hotter than the others, and
the Vbe get even lower... Bang
And these resistors can add some stability to capacitive loads.
Michael
Mordor said:
This depends on the power of the amp. You can use this formula:
W = I*I*R/2
This means, if the max current is 5A, R is 0.1ohm, you need at
least 2.5Watt. This formula is for sinus. To be save, skip the /2...
If you double the outputdevices, you only need 1/4 as the current
halfes.
Michael
Lets try to help.
When the current increase, lets think about one amplifier without signal... to be easy to understand.
Lets imagine that the bias control became crazy... and sometimes big current, and sometimes normal, some bad solder....bad contact.
When bias (polarization current, stand by current, iddle current, corrente de repouso, polarizzazione) is low..... small current passes througth this resistor.... if 1 ohm resistor and 100 mA current passing.... using ohms law, you will discover that you will have a diference of potencial (voltage), that you can measure over the extremes resistor points.... with a voltmeter measuring those two resistor leads....you will find 0.1 volts... 100 milivolts...OK!.... this will disturb about nothing, because amplifier use a enormous voltage related this small one, loosen over this resistor.
But lets think now the crazy regulator start to make confusions.... and he made now a 5 ampere current passing over this resistor, because transistor is completely "on"... and full supply energy is passing from colector to emitter and them flowing througth this resistor...... using ohm law again.... 5A passing inside this poor resistor....now 5 volts will be on the resistor.... and the power on it will be 5 watts (hot!), if this resistor burns, good, will protect transistor, sometimes create more problems..better not to burn.
This way, 5 volts will be loosen over resistor.... this voltage will reduce the entire voltage that was over its transistor... and will reduce the power over the transistor.... dividing the problem.
Lets go on..... now!.... damn crazy bias multiplier potentiometer fails again.... and now 15 ampéres passing!!!....MY GOD!... My transistor can hold maximum 20 amperes.... and maximum 250 Watts dissipation unit!...... when 15 amperes is passing inside the transistor, and in consequence througth his emitter resistor, will create a big potencial difference (voltage...because some electrons will be alike too many people trying to enter a small door...one side of the cinema door full of people (electrons) and outside the cinema doors only few people that goes here, there and everywhere.... not too electrons the outside cinema door... difference of people here, is alike difference of electrons... we call this voltage) ..... a big diference of potential over the resistor extremes again..... 15 amperes over 1 ohm, will be 15 volts..... this way, over resistor, the power will be 15 volts and 15 watts power overheating it, it will became red, burn or explode, at least will smell a lot... those, are advises to the poor amplifier's owner.....the board became black, sometimes a hole, sometimes fire.....but normally a fuse blows and all thing stop!.....if not..... lets imagine your amplifier are using plus 30 volts and minus 30 volts..... we gonna study one side.... the top side..... the plus side....only 30 volts.
If this voltage is keept in 30 volts when 15 amperes are passing into transistor (what a supply!).....15 volts and 15 amperes will be over the transistor and 15 volts and the same current over the resistor...HOHOHOHOHOHÔ.... You get it "fratello".....this way, alike good friends, they divided the problem..... voltage reduced... 225 watts power over transistor, and 225 watts power over resistor..... your 250 Watts maximum transistor will be protected by the resistor.....if no resistor.... 450 watts will melt the transistor!
I made this study static... when dinamic, working, this will work too... resistor is wonderfull, and love is wonderfull too.
I piacere fratello, siamo tutti amicci, My pleasure brother, we are all friends.
regards,
Carlos
When the current increase, lets think about one amplifier without signal... to be easy to understand.
Lets imagine that the bias control became crazy... and sometimes big current, and sometimes normal, some bad solder....bad contact.
When bias (polarization current, stand by current, iddle current, corrente de repouso, polarizzazione) is low..... small current passes througth this resistor.... if 1 ohm resistor and 100 mA current passing.... using ohms law, you will discover that you will have a diference of potencial (voltage), that you can measure over the extremes resistor points.... with a voltmeter measuring those two resistor leads....you will find 0.1 volts... 100 milivolts...OK!.... this will disturb about nothing, because amplifier use a enormous voltage related this small one, loosen over this resistor.
But lets think now the crazy regulator start to make confusions.... and he made now a 5 ampere current passing over this resistor, because transistor is completely "on"... and full supply energy is passing from colector to emitter and them flowing througth this resistor...... using ohm law again.... 5A passing inside this poor resistor....now 5 volts will be on the resistor.... and the power on it will be 5 watts (hot!), if this resistor burns, good, will protect transistor, sometimes create more problems..better not to burn.
This way, 5 volts will be loosen over resistor.... this voltage will reduce the entire voltage that was over its transistor... and will reduce the power over the transistor.... dividing the problem.
Lets go on..... now!.... damn crazy bias multiplier potentiometer fails again.... and now 15 ampéres passing!!!....MY GOD!... My transistor can hold maximum 20 amperes.... and maximum 250 Watts dissipation unit!...... when 15 amperes is passing inside the transistor, and in consequence througth his emitter resistor, will create a big potencial difference (voltage...because some electrons will be alike too many people trying to enter a small door...one side of the cinema door full of people (electrons) and outside the cinema doors only few people that goes here, there and everywhere.... not too electrons the outside cinema door... difference of people here, is alike difference of electrons... we call this voltage) ..... a big diference of potential over the resistor extremes again..... 15 amperes over 1 ohm, will be 15 volts..... this way, over resistor, the power will be 15 volts and 15 watts power overheating it, it will became red, burn or explode, at least will smell a lot... those, are advises to the poor amplifier's owner.....the board became black, sometimes a hole, sometimes fire.....but normally a fuse blows and all thing stop!.....if not..... lets imagine your amplifier are using plus 30 volts and minus 30 volts..... we gonna study one side.... the top side..... the plus side....only 30 volts.
If this voltage is keept in 30 volts when 15 amperes are passing into transistor (what a supply!).....15 volts and 15 amperes will be over the transistor and 15 volts and the same current over the resistor...HOHOHOHOHOHÔ.... You get it "fratello".....this way, alike good friends, they divided the problem..... voltage reduced... 225 watts power over transistor, and 225 watts power over resistor..... your 250 Watts maximum transistor will be protected by the resistor.....if no resistor.... 450 watts will melt the transistor!
I made this study static... when dinamic, working, this will work too... resistor is wonderfull, and love is wonderfull too.
I piacere fratello, siamo tutti amicci, My pleasure brother, we are all friends.
regards,
Carlos
resistors
I have found through testing that most resistors R33 and lower have a strong positive tempco. R10 went up to R50! These were ceramic "plate" non-inductive resistors comonly used for emitter resistors. "Bathtub" types suffer from this as well. This will throw your bias readings off and cause you to set bias to low when setting hot.
I used an HP 34401A in 4 wire mode (kelvin connection), nulled. My heatsource was a soldering iron under the body. High tech ;-). I didn't care about the numbers, just the trends and types of resistors. No list as it's easy to test yourself.
I'm thinking of paralleling some flameproofs. 1R0 seem pretty stable.
-Chris
I have found through testing that most resistors R33 and lower have a strong positive tempco. R10 went up to R50! These were ceramic "plate" non-inductive resistors comonly used for emitter resistors. "Bathtub" types suffer from this as well. This will throw your bias readings off and cause you to set bias to low when setting hot.
I used an HP 34401A in 4 wire mode (kelvin connection), nulled. My heatsource was a soldering iron under the body. High tech ;-). I didn't care about the numbers, just the trends and types of resistors. No list as it's easy to test yourself.
I'm thinking of paralleling some flameproofs. 1R0 seem pretty stable.
-Chris
Very good friends, thanks cooperation.
I remember that those emitter resistor have one more use, and this one is very good too.
When you have a lot of transistors in parallell in the output.... each one with their own characteristic gain.
The one with enormous gain, will drive bigger current... and this will make a bigger voltage over emitter resistor.
The other one, with minimun gain, will drive good current, and this will make a small voltage over emitter resistor.
This way, you protect the higher gain units....because they will drive more current, because have more gain....but, the voltage will be less then the others transistors....Colector to Emitter voltage will be reduced because some voltage will stay over the emitter resistor... this way.... something alike equalize current will happens.... and this way, no trouble to put different gains transistor in Parallell.
If no resistor, the one with more gain will work hot... others not!... the one with more gain can burn, because others do not conduct enough current.... the work is not equally divided.
regards,
Carlos
I remember that those emitter resistor have one more use, and this one is very good too.
When you have a lot of transistors in parallell in the output.... each one with their own characteristic gain.
The one with enormous gain, will drive bigger current... and this will make a bigger voltage over emitter resistor.
The other one, with minimun gain, will drive good current, and this will make a small voltage over emitter resistor.
This way, you protect the higher gain units....because they will drive more current, because have more gain....but, the voltage will be less then the others transistors....Colector to Emitter voltage will be reduced because some voltage will stay over the emitter resistor... this way.... something alike equalize current will happens.... and this way, no trouble to put different gains transistor in Parallell.
If no resistor, the one with more gain will work hot... others not!... the one with more gain can burn, because others do not conduct enough current.... the work is not equally divided.
regards,
Carlos
Hi millwood,
I guess no harm as long as this is designed in. Most people just assume a resistance is constant within the tolerance range, but this is not always true. I like my bias circuit to look after that alone. A slight negative temperature coeffcient makes me happy.
This can affect other circuits, like current sources and resistors that may set gain. What works in the lab and fails in the field may just be component selection. (works in spice but not on bench too)
Carlos, I normally match at least output transistors and diff pairs, the principle is the same. There just isn't as big a difference in work load for the outputs. My experience is mostly in repair, so my viewpoint is from that direction. It always pays to know what your parts are doing.
-Chris
I guess no harm as long as this is designed in. Most people just assume a resistance is constant within the tolerance range, but this is not always true. I like my bias circuit to look after that alone. A slight negative temperature coeffcient makes me happy.
This can affect other circuits, like current sources and resistors that may set gain. What works in the lab and fails in the field may just be component selection. (works in spice but not on bench too)
Carlos, I normally match at least output transistors and diff pairs, the principle is the same. There just isn't as big a difference in work load for the outputs. My experience is mostly in repair, so my viewpoint is from that direction. It always pays to know what your parts are doing.
-Chris
I wouldn't advice to use resistors as fuses, thinking they would blow (and become an open circuit) if they heat too much.
Some of my experiments with resistors:
-when testing my gainclone with a friend, we only had 2 small 1/4W metal film resistors, 10R. Together they made a nice 5R 1/2W dummy load.
We first tried to send a small signal to the amp, and looked at the output on the scope.
Ouah, nice!
The sine is clean, how can such a mess of wire, an IC and 2 1000uF caps give this?
So we tested further, turning the volume pot further and further
The sine was still as nice, so we continued turning the pot.
Then, we saw some smoke, and turned everything off.
The smoke came from the resistors. They were black
we were sending them more than 40W
And they still measure 10.0 Ohm each
-another experience:
sending also 40W in a resistor, but a 10W ceramic one (the green resisors)
at first it smells like burnt, but it's only the dirt on its surface that burns (by manipulating it), and after a minute or two, it became incandescent!
It's value didn't change either
Some of my experiments with resistors:
-when testing my gainclone with a friend, we only had 2 small 1/4W metal film resistors, 10R. Together they made a nice 5R 1/2W dummy load.
We first tried to send a small signal to the amp, and looked at the output on the scope.
Ouah, nice!
The sine is clean, how can such a mess of wire, an IC and 2 1000uF caps give this? So we tested further, turning the volume pot further and further
The sine was still as nice, so we continued turning the pot.
Then, we saw some smoke, and turned everything off.
The smoke came from the resistors. They were black
we were sending them more than 40WAnd they still measure 10.0 Ohm each
-another experience:
sending also 40W in a resistor, but a 10W ceramic one (the green resisors)
at first it smells like burnt, but it's only the dirt on its surface that burns (by manipulating it), and after a minute or two, it became incandescent!
It's value didn't change either
Hi Bricolo,
I normally see a resistor drop it's resistance, then rise as it fails. You generally can't trust an overheated resistor to remain in spec.
Try hooking up some sub 1R0 resistors and heat them up. Some types may remain close in value, others will drift substantially as their temperature rises. Note that this is a non-destructive test.
Once the parts cool to their original temperature, the value should also return to the original value. You will need a four wire measuring setup to accurately measure small changes in resistance, more so with low values of resistance.
-Chris
I normally see a resistor drop it's resistance, then rise as it fails. You generally can't trust an overheated resistor to remain in spec.
Try hooking up some sub 1R0 resistors and heat them up. Some types may remain close in value, others will drift substantially as their temperature rises. Note that this is a non-destructive test.
Once the parts cool to their original temperature, the value should also return to the original value. You will need a four wire measuring setup to accurately measure small changes in resistance, more so with low values of resistance.
-Chris
throw them out
Hi Bricolo
I guess the only answer to that is ... it depends. Depends on the construction of the part and how long the overload lasted. Different value ranges behave differently too. Overheated resistors may increase in value, generally as they are opening up. On a short overload, they may not change in value, or decrease in value as the failure begins.
These are just observations based on repairing equipment over a 30 yr period. There will always be exceptions to this. Generally speaking, I feel it's safe to consider any overheated part to be bad. If you don't discard them, they will keep showing up. Besides, they are not expensive parts (standard types).
-Chris
Hi Bricolo
I guess the only answer to that is ... it depends. Depends on the construction of the part and how long the overload lasted. Different value ranges behave differently too. Overheated resistors may increase in value, generally as they are opening up. On a short overload, they may not change in value, or decrease in value as the failure begins.
These are just observations based on repairing equipment over a 30 yr period. There will always be exceptions to this. Generally speaking, I feel it's safe to consider any overheated part to be bad. If you don't discard them, they will keep showing up. Besides, they are not expensive parts (standard types).
-Chris
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