Hello!
I have a question. We discuss very often the pros and cons of fixed vs self-bias for output stages. But I do not see often hybrid bias, where self-bias and fixed bias are combined, into something like this:
What are the drawbacks of this approach? I can understand that it is a bit more complex, even when the current source is replaced by a resistor. But I can see a big advantage, which is reducing the power dissipation of the bias component, while still retaining some of the self-bias advantages. Using a CCS instead of the bias resistor makes it even better.
I'm sure it is used somewhere, but not very often. Any ideas?
Regards,
Jose
I have a question. We discuss very often the pros and cons of fixed vs self-bias for output stages. But I do not see often hybrid bias, where self-bias and fixed bias are combined, into something like this:
What are the drawbacks of this approach? I can understand that it is a bit more complex, even when the current source is replaced by a resistor. But I can see a big advantage, which is reducing the power dissipation of the bias component, while still retaining some of the self-bias advantages. Using a CCS instead of the bias resistor makes it even better.
I'm sure it is used somewhere, but not very often. Any ideas?
Regards,
Jose
I'm not aware of any special drawbacks, it does exactly what you describe; slightly more efficienct than pure cathode bias, slightly less efficient than pure fixed bias.
I think you missed the fixed bias to the grid, allowing to reduce the cathode voltage, hence the dissipation.
The design using both fixed negative bias and CCS at the cathode has its advantage.
Fixed bias would reduce the B+ voltage and adjustment of Plate-Cathode voltage.
The CCS controls the amount of current flowing through the power tube regardless of the voltage change.
Lastly high quality bypass capacitor need to be used at the cathode because 100% of the AC will go through it.
Johnny
Fixed bias would reduce the B+ voltage and adjustment of Plate-Cathode voltage.
The CCS controls the amount of current flowing through the power tube regardless of the voltage change.
Lastly high quality bypass capacitor need to be used at the cathode because 100% of the AC will go through it.
Johnny
I experimented with this in a Fender guitar amp, years ago.
Of course, given a fixed B+ voltage, introducing any cathode bias will reduce Va-k (plate-cathode). This will reduce power output, since the available output voltage swing will be reduced, along with the Va-k. The more cathode bias is introduced, the more power output is reduced.
However, in my experiment with push-pull 6L6GCs, with a sine wave input, the clipping behavior with cathode bias was 'rounder' (for lack of a better descriptor). The output waveform at the OPT secondary clipped softly (rounded edges on the output waveform) compared to the same output stage with fixed bias.
The fixed bias version clipped with much more squared-off (harder) edges on a clipped sine wave. The difference was very evident.
Adding some cathode bias to the fixed bias stage (while adjusting for Ia in the output tubes) made the clipping softer than from a pure fixed bias output, but not as soft/rounded as from a pure cathode bias output.
The clipping behavior of the 'hybrid' cathode/fixed bias output stage was intermediate between a purely fixed bias output stage and a purely cathode bias output stage. Very much like you would expect! The amount of rounding was adjustable by varying the proportion of cathode bias to fixed bias. It was quite predictable.
For guitar amps, I actually prefer the sound of a warmer biased output stage with cathode bias, because it makes a softer, more 'round' sounding overdrive. That suited my guitar playing style more than the harder, 'more-bark-to-it' sounding overdrive of a fixed bias Fender amp with the typically cold biased class AB output stage. It probably works the same way in a hi-fi tube amp.
Of course, given a fixed B+ voltage, introducing any cathode bias will reduce Va-k (plate-cathode). This will reduce power output, since the available output voltage swing will be reduced, along with the Va-k. The more cathode bias is introduced, the more power output is reduced.
However, in my experiment with push-pull 6L6GCs, with a sine wave input, the clipping behavior with cathode bias was 'rounder' (for lack of a better descriptor). The output waveform at the OPT secondary clipped softly (rounded edges on the output waveform) compared to the same output stage with fixed bias.
The fixed bias version clipped with much more squared-off (harder) edges on a clipped sine wave. The difference was very evident.
Adding some cathode bias to the fixed bias stage (while adjusting for Ia in the output tubes) made the clipping softer than from a pure fixed bias output, but not as soft/rounded as from a pure cathode bias output.
The clipping behavior of the 'hybrid' cathode/fixed bias output stage was intermediate between a purely fixed bias output stage and a purely cathode bias output stage. Very much like you would expect! The amount of rounding was adjustable by varying the proportion of cathode bias to fixed bias. It was quite predictable.
For guitar amps, I actually prefer the sound of a warmer biased output stage with cathode bias, because it makes a softer, more 'round' sounding overdrive. That suited my guitar playing style more than the harder, 'more-bark-to-it' sounding overdrive of a fixed bias Fender amp with the typically cold biased class AB output stage. It probably works the same way in a hi-fi tube amp.
Understood @jan.didden, you keep B+ the same for both cases. One of the benefits of fixed bias (and hybrid fixed bias) is that you need less B+ voltage to reach the same result, you do not "lose" 60V in the cathode resistor. Here is a simulation of both situations, 430V B+ for self-bias (left), 378V for hybrid bias (right):
One advantage to a hybrid fixed/self bias output stage would be that the output tubes would still have some protection from severe 'runaway' (underbias meltdown) from the current limiting action of the self-bias resistor(s). I always thought that would be a desirable feature in a guitar amp that's being overdriven a lot of the time.
The safest guess would be the fixed bias value, or 50k ohms.
The max value in real life will vary between tubes, of course, depending on their predilection for self-immolation.
My hunch is that testing would reveal the max value of grid leak resistor for a hybrid fixed/self-biased output tube to be intermediate between fixed and self bias. Probably have to test to know for sure.
The max value in real life will vary between tubes, of course, depending on their predilection for self-immolation.
My hunch is that testing would reveal the max value of grid leak resistor for a hybrid fixed/self-biased output tube to be intermediate between fixed and self bias. Probably have to test to know for sure.
What we are calling hybrid bias can be set up as a continuum between any percentage of cathode bias and fixed bias.
The resulting performance characteristics of the output stage will then be dependent on the percentages of each bias type that make up the total required bias voltage. I have used this method on several amplifiers to provide some protection when severely over driven and improved temperature stability over full fixed bias. Can be helpful with very high S tubes in stabilizing the bias point without as much loss as full cathode bias.
A nice feature if you make the fixed bias voltage adjustable is the ability to easy "tune" the idle current without having to change the cathode resistor.
Watch out when fixed bias is paired with a CCS. As the % amount of fixed bias voltage is increased the "headroom" voltage in the CCS decreases and you can run out of operation voltage for the CCS and at that point your CCS circuit no longer functions correctly. In your example you show 7.9V across the CCS so if the required output tube bias voltage decreases by that voltage - the voltage to operate the CSS will also decrease and at some voltage the CCS will fail to operate. Spice will lie to you about this as the Spice CCS will work with any voltage or no voltage at all unlike "real" CCS circuits that need several volts for opperation.
The resulting performance characteristics of the output stage will then be dependent on the percentages of each bias type that make up the total required bias voltage. I have used this method on several amplifiers to provide some protection when severely over driven and improved temperature stability over full fixed bias. Can be helpful with very high S tubes in stabilizing the bias point without as much loss as full cathode bias.
A nice feature if you make the fixed bias voltage adjustable is the ability to easy "tune" the idle current without having to change the cathode resistor.
Watch out when fixed bias is paired with a CCS. As the % amount of fixed bias voltage is increased the "headroom" voltage in the CCS decreases and you can run out of operation voltage for the CCS and at that point your CCS circuit no longer functions correctly. In your example you show 7.9V across the CCS so if the required output tube bias voltage decreases by that voltage - the voltage to operate the CSS will also decrease and at some voltage the CCS will fail to operate. Spice will lie to you about this as the Spice CCS will work with any voltage or no voltage at all unlike "real" CCS circuits that need several volts for opperation.
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I would not say that LTSpice lies. This is just a circuit to show my point, simple, using an ideal current source. A real CCS has, of course, a minimum and maximum voltage range, and many other parameters. When designing a real practical circuit one needs to take care of everything, working voltage, power up sequence, failure mode, etc etc.
jcalvarez,
You said:
"What are the drawbacks of this approach?"
Drawbacks:
More parts
Longer, more complex design process
. . . And one thing more . . . Performance:
When the amplifier is subjected to Overload, Clipping, etc.:
An RC self bias network Recovers Faster to its quiescent conditions; Versus:
A CCS and parallel C self bias circuit, Recovers Slower to its quiescent conditions.
Advantages:
There are many of them, already listed in this thread.
You said:
"What are the drawbacks of this approach?"
Drawbacks:
More parts
Longer, more complex design process
. . . And one thing more . . . Performance:
When the amplifier is subjected to Overload, Clipping, etc.:
An RC self bias network Recovers Faster to its quiescent conditions; Versus:
A CCS and parallel C self bias circuit, Recovers Slower to its quiescent conditions.
Advantages:
There are many of them, already listed in this thread.
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Thank you @6A3sUMMER, very informative, as always, and something I wanted to hear, about drawbacks.