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Old 9th April 2006, 07:35 PM   #1
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Default The Trials, and Tribulations of Bias

Hello everyone, thanks for looking

My question (like every question I ask) seems like it will be easy to answer but always ends up taking a very long time. What does Bias do in a tube amplifier? I see countless schematics that read "Bias supply (insert odd voltage here)"

How much current is used in a bias?

Is bias simply a direct current?

Please help me understand! Bias will probably be in my second project, as my first is self-biased
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Old 10th April 2006, 12:41 AM   #2
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Hi Alexmoose,

Ooph! This is a very basic question to which my reply must necessarily be incomplete. I think the most satisfactory way for you will be to try find a source (here or book) fully explaining tube basics. I say this not with disrespect for your knowledge, but it is a very wide subject.

To start, tubes are normally (at least in audio) used with the control grid (G1) negative to the cathode to control the plate current. This means that input impedance is very high as this grid just creates a field and does not draw any current. Applying an input signal to the grid varies its potential either side of the set bias and thus controls the flow of plate current etc. Should it ever be driven to a voltage positive to the cathode, grid current will be drawn and the input impedance greatly diminished (that is used in transmitters but not in audio).

It is this fixed negative voltage on the grid that is called grid bias or just bias. It can be supplied by a voltage source (as said no current flows), and will vary from 0,5V to a few volts for voltage amplifiers, to several tens of volt for power stages.

You said your first project was self-biased. If I understand that correctly you mean that there was a resistor in series with the cathode. The plate current flowing through it would have placed the cathode a few volts positive to ground (common), which is the same as saying that G1 was that few volts negative to the cathode, thus "biased". Alternatively one could ground the cathode, but then you will need a separate voltage supply (just like a battery) from which to tap the necessary negative bias to G1.

In low power (voltage amplifying) stages cathode bias is used simply because it is convenient - no separate supply required. Similarly in power stages, but here there are advantages in using a separate (fixed) negative voltage to supply say -30V or whatever the tubes demand to the grids.

From here on we will have to proceed to the relative characteristics of each method, which will require pages of narrative. This will be better covered, as said, if you can get a book on the subject of tube amplification or some web-site. I hope that this very basic description will at least get your thoughts in the right direction.

Regards.
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Old 10th April 2006, 01:03 AM   #3
arnoldc is offline arnoldc  Philippines
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Same predicament with alex here. Never made a fixed bias SET yet.

I suppose with fixed bias, I can get away with that big cathode capacitor, it's bypass cap, less heat? Does that translate to better sound?
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Old 10th April 2006, 01:29 AM   #4
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I answered Alex's question very generally, not being sure whether he was referring to any specific type of use.

If we are talking SET, I think you mean whether you can DO away with the big capacitor and less heat from a cathode resistor.

I am not a SET fan myself, but the general principle would be valid for any power stage. When you start driving the output tube it draws more plate current, thus increasing the bias. This may land you at an operating point with more distortion than for low signals. It will probably also limit the maximum available output power because of the higher bias, unless you use such a large bypass capacitor that it momentarily keeps the bias the same. This can be the case for speech and music signals containing not too much energy. Using fixed bias obviates this "back-biasing", but at the expense of the extra complexity of a separate supply.

One could also use a zener diode(s) in place of the cathode resistor which will give fixed bias, if you can get the right value and wattage.

Regards.
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Old 10th April 2006, 01:34 AM   #5
arnoldc is offline arnoldc  Philippines
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Hi Johan, I wasted some bandwidth reading the fixed vs cathode bias posts for SET at the Audio Asylum and based on what I've read (their preferences) might well stick to cathode bias.

Cheers!
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Old 10th April 2006, 02:10 AM   #6
kevinkr is offline kevinkr  United States
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I have generally preferred fixed bias to cathode bias in SET amplifiers for several reasons;

1. Eliminates the need for expensive, high quality cathode bypass capacitors - u know what they say, there is no better capacitor than no capacitor.

2. Eliminates the dissipation in a cathode power resistor, and allows the use of a lower voltage power transformer. Better efficiency is the result as energy is not wasted in needlessly heating a cathode resistor.

3. You can fine tune the operating point of your output tubes based on personal sonic preferences and based on variations in the tubes themselves.

Exceptions:

Cathode bias does provide a dc feedback mechanism not present in fixed bias circuits which makes variations in tube transconductance much less of an issue, and is probably the best approach for high transconductance tubes like the 6AS7.

Small signal stuff where cathode bias generally results in more consistent operating points without the need for tweaking bias voltages.
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Old 10th April 2006, 02:15 AM   #7
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Quote:
Originally posted by Johan Potgieter
Hi Alexmoose,

Ooph! This is a very basic question to which my reply must necessarily be incomplete. I think the most satisfactory way for you will be to try find a source (here or book) fully explaining tube basics. I say this not with disrespect for your knowledge, but it is a very wide subject.

To start, tubes are normally (at least in audio) used with the control grid (G1) negative to the cathode to control the plate current. This means that input impedance is very high as this grid just creates a field and does not draw any current. Applying an input signal to the grid varies its potential either side of the set bias and thus controls the flow of plate current etc. Should it ever be driven to a voltage positive to the cathode, grid current will be drawn and the input impedance greatly diminished (that is used in transmitters but not in audio).

It is this fixed negative voltage on the grid that is called grid bias or just bias. It can be supplied by a voltage source (as said no current flows), and will vary from 0,5V to a few volts for voltage amplifiers, to several tens of volt for power stages.

You said your first project was self-biased. If I understand that correctly you mean that there was a resistor in series with the cathode. The plate current flowing through it would have placed the cathode a few volts positive to ground (common), which is the same as saying that G1 was that few volts negative to the cathode, thus "biased". Alternatively one could ground the cathode, but then you will need a separate voltage supply (just like a battery) from which to tap the necessary negative bias to G1.

In low power (voltage amplifying) stages cathode bias is used simply because it is convenient - no separate supply required. Similarly in power stages, but here there are advantages in using a separate (fixed) negative voltage to supply say -30V or whatever the tubes demand to the grids.

From here on we will have to proceed to the relative characteristics of each method, which will require pages of narrative. This will be better covered, as said, if you can get a book on the subject of tube amplification or some web-site. I hope that this very basic description will at least get your thoughts in the right direction.

Regards.

To be very short, Bias Voltage is used to establish the correct pottential difference between the Cathode, and Grid 1? that makes perfect sense to me. (I am in AP physics, we study electric fields, however the college board finds vaccum tubes to be "out dated"...jerks!)

However my follow-up question is, how can a power soruce safely supply a negative voltage? in other words, where does one put the other wire of their bias voltage source?
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Old 10th April 2006, 03:11 AM   #8
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The negative bias supply is connected between ground and the grid circuit: positive goes to ground and negative goes to the bottom end of the grid resistor, usually via a potentiometer arrangement that allows the bias voltage to be adjusted, to select the required operating point for the tube. Note that the grid resistor needs to be of a much lower value than with self-bias. Typically, for beam tetrodes such as the 6L6 family, it should be less than 100k; for pentodes such as the EL34, it can be about 250k.

Kevin's points are valid but he failed to mention one other disadvantage of self-bias, namely, the need for higher B+ so that the plate-cathode voltage is not reduced.
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