tube biasing
- Thread starter crow55
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ildar
Guest
Teach them racial stereotypes.
Tubes control the current flow of electrons in the circuit.
Tubes, in the most simple way, consist of a cathode, control grid, and a plate. With no control grid, nothing would stop (let alone control) the flow of electrons from cathode to plate.
The cathode is the negative element in the circuit being controlled, and the plate is the more positive element. By adding the control grid between the two and varying the voltage on the control grid, the current flow between cathode and plate can be "modulated".
When the grid goes a bit more positive, more current flows. When the grid goes more negative, less current flows. In this way, a small voltage change, can control a large current change. This is amplification.
If we apply an alternating voltage (like our guitar signal) to the grid, the current flow in the tube will vary at the same rate as the signals voltage - and we've amplified the guitars signal.
At some point, for any given tube design, there are certain limits and thresholds in its operation.
In other words, at some point, the grid voltage might be high enough, that the current flow becomes so great, that the tube self destructs. We don't want to go there.
At another point, if the grid voltage is very low, sometimes (usually in fact) even more "negative" than the cathode, the tube will just about "shut down" and no current will flow.
The idea in the design of the circuit, is to have the signal voltage vary up and down (remember its audio and alternating) around some set voltage point that provides suitable current flow. That voltage point, is an offset from the "zero volt" or "chassis ground" voltage point in the circuit. That is because almost universally the grid must be more negative than the cathode to properly control the current flow.
The word that circuit designers use for this voltage offset is "bias". In other words the signal is "biased" a bit negative in relation to ground.
There are a LOT of things left out here.... but generally, if its possible to set the bias on the amp, then it should be set when the output tubes are changed. You cant set it and forget it. Reason being because tubes vary all over the place in manufacture, even two tubes off the same production line will "bias" differently.
A word on tubes. Because they vary they should be matched, both for bias level, and transconductance. Transconductance is a big word that means "how the tube reacts to changes in signal at the grid". You can have two tubes that give the same current flow at a certain bias voltage, yet at a different bias voltage, are wildly out of line with each other. Obviously this is not what we want in our amplifier.
Setting the bias usually involves observing the current flow "at idle" (no signal) and turning a little control hidden in the circuit of the amplifier, to adjust the grid voltage, and therefore the tubes "idle current".
The idle current method of "biasing" is the most popular method. Its easy. Its cheap. It needs little technical smarts. It also is totally inaccurate. Some "gurus" have suggested that its the only way to "measure" the bias, as opposed to observing the operation on an oscilloscope. Those gurus don't know how to use a scope, are afraid of it, and feel at home with a multimeter.
A really good tech will have made his own dual trace current monitoring setup for his scope, and will set the bias according to how those tubes perform, not how many milliamps they use at idle. The tech will determine the ceiling of operation for those actual tubes, and adjust accordingly.
If you're just some guy that wants to set up his amp - use the idle current method, and try a bit more and a bit less idle current and see how it sounds. Observe your amps tubes when playing to make sure they don't go cherry red. Be happy. Have ok tone too.
If you're Clapton and want a certain amount of compression to your tone, every time, I can guarantee you that the tech is not setting up the amp by the idle current method.
Discussion of compression and clarity and other nice stuff.... is for another thread.
:icon_biggrin:
Tubes, in the most simple way, consist of a cathode, control grid, and a plate. With no control grid, nothing would stop (let alone control) the flow of electrons from cathode to plate.
The cathode is the negative element in the circuit being controlled, and the plate is the more positive element. By adding the control grid between the two and varying the voltage on the control grid, the current flow between cathode and plate can be "modulated".
When the grid goes a bit more positive, more current flows. When the grid goes more negative, less current flows. In this way, a small voltage change, can control a large current change. This is amplification.
If we apply an alternating voltage (like our guitar signal) to the grid, the current flow in the tube will vary at the same rate as the signals voltage - and we've amplified the guitars signal.
At some point, for any given tube design, there are certain limits and thresholds in its operation.
In other words, at some point, the grid voltage might be high enough, that the current flow becomes so great, that the tube self destructs. We don't want to go there.
At another point, if the grid voltage is very low, sometimes (usually in fact) even more "negative" than the cathode, the tube will just about "shut down" and no current will flow.
The idea in the design of the circuit, is to have the signal voltage vary up and down (remember its audio and alternating) around some set voltage point that provides suitable current flow. That voltage point, is an offset from the "zero volt" or "chassis ground" voltage point in the circuit. That is because almost universally the grid must be more negative than the cathode to properly control the current flow.
The word that circuit designers use for this voltage offset is "bias". In other words the signal is "biased" a bit negative in relation to ground.
There are a LOT of things left out here.... but generally, if its possible to set the bias on the amp, then it should be set when the output tubes are changed. You cant set it and forget it. Reason being because tubes vary all over the place in manufacture, even two tubes off the same production line will "bias" differently.
A word on tubes. Because they vary they should be matched, both for bias level, and transconductance. Transconductance is a big word that means "how the tube reacts to changes in signal at the grid". You can have two tubes that give the same current flow at a certain bias voltage, yet at a different bias voltage, are wildly out of line with each other. Obviously this is not what we want in our amplifier.
Setting the bias usually involves observing the current flow "at idle" (no signal) and turning a little control hidden in the circuit of the amplifier, to adjust the grid voltage, and therefore the tubes "idle current".
The idle current method of "biasing" is the most popular method. Its easy. Its cheap. It needs little technical smarts. It also is totally inaccurate. Some "gurus" have suggested that its the only way to "measure" the bias, as opposed to observing the operation on an oscilloscope. Those gurus don't know how to use a scope, are afraid of it, and feel at home with a multimeter.
A really good tech will have made his own dual trace current monitoring setup for his scope, and will set the bias according to how those tubes perform, not how many milliamps they use at idle. The tech will determine the ceiling of operation for those actual tubes, and adjust accordingly.
If you're just some guy that wants to set up his amp - use the idle current method, and try a bit more and a bit less idle current and see how it sounds. Observe your amps tubes when playing to make sure they don't go cherry red. Be happy. Have ok tone too.
If you're Clapton and want a certain amount of compression to your tone, every time, I can guarantee you that the tech is not setting up the amp by the idle current method.
Discussion of compression and clarity and other nice stuff.... is for another thread.
:icon_biggrin:
Funky Phil
Senior Member
- Messages
- 324
I would strongly advise you to find a really good amp tech (one that plays and owns a few nice Marshalls, Fenders etc)
Avoid techs that don't play guitar and/or tell you that valves and transistors sound the same.
The insides of valve amps are very unfriendly places (even when unplugged)...thet will bite you :evil4:
Avoid techs that don't play guitar and/or tell you that valves and transistors sound the same.
The insides of valve amps are very unfriendly places (even when unplugged)...thet will bite you :evil4:
Volitions Advocate
Hero Member
- Messages
- 1,239
In my experience.. you want to find an old world guy that repairs organs or something. They're the best because they never take shortcuts, because they learned how to do it back when there were no shortcuts.
Its really not a big deal on most modern amps that actually have adjustments.
You can make yourself a bias probe with an octal base and an octal socket... some buss wire and test lead wire.. some banana ends, and a bit of potting.
HA!~~ I sold em by the score for $40 each once. Those were the days.
You can make yourself a bias probe with an octal base and an octal socket... some buss wire and test lead wire.. some banana ends, and a bit of potting.
HA!~~ I sold em by the score for $40 each once. Those were the days.
