Yngwiestein
New member
So a phase inverter basically takes the input and splits into two signals, one an upside-down mirror image of the other so that it can be used across a bank of power tubes in push/pull? Am I understanding this right?
About Phase Inverters
- Thread starter Yngwiestein
- Start date
glassman
Electron Herder
Re: About Phase Inverters
That is the basic function of the phase inverter...but the signals are not a mirror image of each other. Typically, on "leg" of a phase inverter will have slightly more gain than the other; this has the effect of altering the duty cycle from what would be 50% - 50% in a high end audio amp to somewhere around 45%-55% in most guitar amps (black face Fenders and Marshalls for instance). Some of the early Boogies were in the 40%-60% range for duty cycle. This "imbalance" helps to sweeten the clean sound. An example of a balanced phase inverter amp would be most of the early 70's silver face Fenders.
That is the basic function of the phase inverter...but the signals are not a mirror image of each other. Typically, on "leg" of a phase inverter will have slightly more gain than the other; this has the effect of altering the duty cycle from what would be 50% - 50% in a high end audio amp to somewhere around 45%-55% in most guitar amps (black face Fenders and Marshalls for instance). Some of the early Boogies were in the 40%-60% range for duty cycle. This "imbalance" helps to sweeten the clean sound. An example of a balanced phase inverter amp would be most of the early 70's silver face Fenders.
Yngwiestein
New member
Re: About Phase Inverters
Does it mean that it would work one power tube more than another since it's pumping a higher % to another or does it go back and forth so that each tube gets either 40% or 60% [hypothetically speaking of course
]?
Does it mean that it would work one power tube more than another since it's pumping a higher % to another or does it go back and forth so that each tube gets either 40% or 60% [hypothetically speaking of course
]?
Last edited:
glassman
Electron Herder
Re: About Phase Inverters
Your output tubes see different signals; both in amplitude and duration. One "side" will begin to break up before the other. If you are using any amount of distortion at all, you already have an altered signal feeding the phase inverter. The phase inverter imbalance really only affects the clean sound where you have an otherwise "unaffected" sinusiodal waveform from your preamp.
Bottom line...your output tubes never see the same signal and one side gets a "hotter" signal than the other.
Your output tubes see different signals; both in amplitude and duration. One "side" will begin to break up before the other. If you are using any amount of distortion at all, you already have an altered signal feeding the phase inverter. The phase inverter imbalance really only affects the clean sound where you have an otherwise "unaffected" sinusiodal waveform from your preamp.
Bottom line...your output tubes never see the same signal and one side gets a "hotter" signal than the other.
Yngwiestein
New member
Re: About Phase Inverters
Now I need to know how tube types affect the phase inverter. Can upgrading the preamp tube for the phase inverter make a difference? Like if you use a high gain JJ vs. a JJ or if you go with a 12at7 instead of a 12ax7. Does that do anything to the sound?
Now I need to know how tube types affect the phase inverter. Can upgrading the preamp tube for the phase inverter make a difference? Like if you use a high gain JJ vs. a JJ or if you go with a 12at7 instead of a 12ax7. Does that do anything to the sound?
glassman
Electron Herder
Re: About Phase Inverters
There's not a lot of opportunity to tone shape at the phase inverter. If the curcuit is designed for a 12ax7, a 12at7 will lower the signal level to the output tubes and not much else. If you go up in gain, like substituting a 12ax7 for a 12at7, it will push your output tubes a little harder...but...it may also reduce your bandwidth. Usually the bass frequencies are the area of loss.
The most effective changes to a phase inverter is a curcuit change and even that is a subtle change.
There's not a lot of opportunity to tone shape at the phase inverter. If the curcuit is designed for a 12ax7, a 12at7 will lower the signal level to the output tubes and not much else. If you go up in gain, like substituting a 12ax7 for a 12at7, it will push your output tubes a little harder...but...it may also reduce your bandwidth. Usually the bass frequencies are the area of loss.
The most effective changes to a phase inverter is a curcuit change and even that is a subtle change.
glassman
Electron Herder
Re: About Phase Inverters
Class A amp curcuits are identical to Class AB through the phase inverter. Class A amps use a resistor between the cathode and ground to control the current through the output tubes while a Class AB amp uses a negative DC voltage applied to the grid to control current.
The phase inverters are the same in form and function. A Plexi Marshall and a Vox AC30 use the same phase inverter.
Class A amp curcuits are identical to Class AB through the phase inverter. Class A amps use a resistor between the cathode and ground to control the current through the output tubes while a Class AB amp uses a negative DC voltage applied to the grid to control current.
The phase inverters are the same in form and function. A Plexi Marshall and a Vox AC30 use the same phase inverter.
Rich_S
HomeGrownToneBrewologist
Re: About Phase Inverters
You went one question too far there, glassman. Good stuff up until the Class A part. The resistor-to-ground thing you described is a cathode-biased output stage, rather than fixed-bias, which has the separate bias supply. The operating class A or AB has little to do with whether the output stage is cathode biased or not. The marketing yahoos often get them confused, and tend to call all cathode-biased amps (Vox AC30s are the classic example) "Class A" when they really mean "Cathode Biased".
My Marshall 18 Watter clone is an example of a cathode biased Class AB amp.
To answer bognerfan's question, single ended amps do not use a phase inverter - they use one output tube to amplify the entire wave. ALL single-ended amps are Class A - the other operating classes don't apply to single-ended. A Fender Champ is a good example of a single-ended amp.
The PI is required when you go to a push-pull output stage - the output tubes are wired in pairs, and one pushes the positive direction while the other pulls the negative. The negative side tube needs an inverted signal - thus the need for a phase inverter (also less-descriptively called a "driver" stage). So, now we have a push-pull amp with a phase inverter - it can be Class A or AB, and it can be cathode-biased or fixed-bias, but it's the push/pull part that determines the need for a PI.
There are different type of PI circuits, but the most common is the long-tailed-pair (LTP), used in Marshalls and most Fenders. It has the highest gain of the common types. In modern (Blackface and beyond) Fenders, the LTP is used in the Deluxe Reverb and the larger 6L6 amps. The smaller Princeton has a different PI, with different distortion characteristics, but it's still push/pull.
Swapping PI tubes will have less effect on tone than earlier preamp positions, although it does determine how hard you push the output tubes. I don't think you'll hear much difference in big amps running 6L6's or EL34's. It makes a bigger difference in lower-powered amps like an 18 Watter. The 12AX7 long-tailed-pair in that amp drives the living snot out of the little EL84 output tubes. When scaling a plexi circuit down to lower-powered EL84 tubes, many guys use a 12AT7 in the PI, because the drive level of the 12AX7, while acceptable for the original EL34s, is just too much for the EL84s.
You went one question too far there, glassman. Good stuff up until the Class A part. The resistor-to-ground thing you described is a cathode-biased output stage, rather than fixed-bias, which has the separate bias supply. The operating class A or AB has little to do with whether the output stage is cathode biased or not. The marketing yahoos often get them confused, and tend to call all cathode-biased amps (Vox AC30s are the classic example) "Class A" when they really mean "Cathode Biased".
My Marshall 18 Watter clone is an example of a cathode biased Class AB amp.
To answer bognerfan's question, single ended amps do not use a phase inverter - they use one output tube to amplify the entire wave. ALL single-ended amps are Class A - the other operating classes don't apply to single-ended. A Fender Champ is a good example of a single-ended amp.
The PI is required when you go to a push-pull output stage - the output tubes are wired in pairs, and one pushes the positive direction while the other pulls the negative. The negative side tube needs an inverted signal - thus the need for a phase inverter (also less-descriptively called a "driver" stage). So, now we have a push-pull amp with a phase inverter - it can be Class A or AB, and it can be cathode-biased or fixed-bias, but it's the push/pull part that determines the need for a PI.
There are different type of PI circuits, but the most common is the long-tailed-pair (LTP), used in Marshalls and most Fenders. It has the highest gain of the common types. In modern (Blackface and beyond) Fenders, the LTP is used in the Deluxe Reverb and the larger 6L6 amps. The smaller Princeton has a different PI, with different distortion characteristics, but it's still push/pull.
Swapping PI tubes will have less effect on tone than earlier preamp positions, although it does determine how hard you push the output tubes. I don't think you'll hear much difference in big amps running 6L6's or EL34's. It makes a bigger difference in lower-powered amps like an 18 Watter. The 12AX7 long-tailed-pair in that amp drives the living snot out of the little EL84 output tubes. When scaling a plexi circuit down to lower-powered EL84 tubes, many guys use a 12AT7 in the PI, because the drive level of the 12AX7, while acceptable for the original EL34s, is just too much for the EL84s.
Last edited:
PVFan
Fudgeitallologist
Re: About Phase Inverters
I thought part of the definition of class A was that it was NOT push-pull, as in your "single-ended" amp description that has no phase splitter. But I was confused even before this thread by amps that switch between A and AB, and which I thought use a PI for both, since I didn't think the driver stage (the PI) and associated circuit was actually removed when you flipped the A/AB switch. That seemed too weird.
Can you clear me up on that? What is the difference between class A and AB in an amp that has a PI ?
Nobody better call me Walters. I'll sue.
I thought part of the definition of class A was that it was NOT push-pull, as in your "single-ended" amp description that has no phase splitter. But I was confused even before this thread by amps that switch between A and AB, and which I thought use a PI for both, since I didn't think the driver stage (the PI) and associated circuit was actually removed when you flipped the A/AB switch. That seemed too weird.
Can you clear me up on that? What is the difference between class A and AB in an amp that has a PI ?
Nobody better call me Walters. I'll sue.
krankguitarist
Krankitupologist
PVFan
Fudgeitallologist
Re: About Phase Inverters
I never found anything that covered all the bases so well. It will take me a little while to understand the answers to my questions though.
http://www.aikenamps.com/ClassA.htm
First of all, what is a class A amplifier?
For audio amplification, a class A amplifier can be either single-ended or push-pull. Now, you might be thinking, how can a push-pull amplifier be class A? Doesn't one side amplify half the waveform and the other side amplify the other half? Isn't this why we use a phase splitter? These are common misconceptions. You can, indeed have a true class A amplifier that operates in push-pull mode. Amplifier class has absolutely nothing to do with output stage topology. If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions. As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount. The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding. As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier. Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance.
Another misconception is that of cathode biasing. The method of biasing has nothing to do with the class of operation. You can have a fixed-bias class A amplifier or a cathode-biased class AB amplifier, or vice-versa. The presence of a cathode bias resistor and bypass capacitor is not an indication of class A operation.
...
The main disadvantage of push-pull class A amplification over single-ended class A, is the necessity for a phase splitter stage to generate the oppositely-phased drive signals. Another "disadvantage", in terms of guitar amplification, is that even-order harmonics generated in the output stage are canceled out in a push-pull output stage (hi-fi guys consider this a great advantage, by the way!). This does not mean that the push-pull amplifier generates no even order harmonics, however, because even-order harmonics generated in the preamp stages are amplified by the output stage and will pass right through to the output. Only those even-order harmonics generated in the output stage itself are canceled.
How is class AB defined?
A class AB amplifier is one in which the grid bias is set so that plate current flows for more than half, but appreciably less that the full 360 degrees of the the input cycle, again measured at the full, unclipped output of the amplifier. This increase in idle bias current over class B operation keeps the tubes on a small amount at all times, resulting in reduced crossover distortion, because it keeps the tubes out of the highly nonlinear region near cutoff. Unless the idle bias is set too close to class A operation, efficiency gains similar to class B operation can be obtained, without the unwanted crossover distortion. This is the most popular class of operation for medium to high powered guitar amplifiers.
...
Which is better, class A or class AB?
From a guitar amplification standpoint, neither class of operation is necessarily better, they are just different. You shouldn't get too hung up on the "class A" designation, because most of the push-pull amplifiers that are supposed to be class A aren't really class A at all, they are just cathode-biased, non-negative feedback class AB amplifiers. Operating class is not the reason for the tonal differences between these amplifiers.
The cathode biasing and lack of negative feedback is one of the main differences between the Vox clones and the Marshall/Fender style stuff. The typical Marshalls and Fenders used a fixed-bias output stage with negative feedback from the output back to the phase inverter input, while the Vox clones use a cathode-biased output stage and no global negative feedback. In addition, the output tubes and preamp stage/phase inverter configurations contribute greatly to the tonal signature of these amplifiers.
I never found anything that covered all the bases so well. It will take me a little while to understand the answers to my questions though.
http://www.aikenamps.com/ClassA.htm
First of all, what is a class A amplifier?
For audio amplification, a class A amplifier can be either single-ended or push-pull. Now, you might be thinking, how can a push-pull amplifier be class A? Doesn't one side amplify half the waveform and the other side amplify the other half? Isn't this why we use a phase splitter? These are common misconceptions. You can, indeed have a true class A amplifier that operates in push-pull mode. Amplifier class has absolutely nothing to do with output stage topology. If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions. As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount. The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding. As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier. Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance.
Another misconception is that of cathode biasing. The method of biasing has nothing to do with the class of operation. You can have a fixed-bias class A amplifier or a cathode-biased class AB amplifier, or vice-versa. The presence of a cathode bias resistor and bypass capacitor is not an indication of class A operation.
...
The main disadvantage of push-pull class A amplification over single-ended class A, is the necessity for a phase splitter stage to generate the oppositely-phased drive signals. Another "disadvantage", in terms of guitar amplification, is that even-order harmonics generated in the output stage are canceled out in a push-pull output stage (hi-fi guys consider this a great advantage, by the way!). This does not mean that the push-pull amplifier generates no even order harmonics, however, because even-order harmonics generated in the preamp stages are amplified by the output stage and will pass right through to the output. Only those even-order harmonics generated in the output stage itself are canceled.
How is class AB defined?
A class AB amplifier is one in which the grid bias is set so that plate current flows for more than half, but appreciably less that the full 360 degrees of the the input cycle, again measured at the full, unclipped output of the amplifier. This increase in idle bias current over class B operation keeps the tubes on a small amount at all times, resulting in reduced crossover distortion, because it keeps the tubes out of the highly nonlinear region near cutoff. Unless the idle bias is set too close to class A operation, efficiency gains similar to class B operation can be obtained, without the unwanted crossover distortion. This is the most popular class of operation for medium to high powered guitar amplifiers.
...
Which is better, class A or class AB?
From a guitar amplification standpoint, neither class of operation is necessarily better, they are just different. You shouldn't get too hung up on the "class A" designation, because most of the push-pull amplifiers that are supposed to be class A aren't really class A at all, they are just cathode-biased, non-negative feedback class AB amplifiers. Operating class is not the reason for the tonal differences between these amplifiers.
The cathode biasing and lack of negative feedback is one of the main differences between the Vox clones and the Marshall/Fender style stuff. The typical Marshalls and Fenders used a fixed-bias output stage with negative feedback from the output back to the phase inverter input, while the Vox clones use a cathode-biased output stage and no global negative feedback. In addition, the output tubes and preamp stage/phase inverter configurations contribute greatly to the tonal signature of these amplifiers.
glassman
Electron Herder
Re: About Phase Inverters
Sorry guys...I was assuming that Bognerfan was speaking of a push-pull circuit. I have a tendancy to over simplify my answers so that I don't cause peoples eyes to glass over. Rich_s's explanation is correct and very complete.
Sorry guys...I was assuming that Bognerfan was speaking of a push-pull circuit. I have a tendancy to over simplify my answers so that I don't cause peoples eyes to glass over. Rich_s's explanation is correct and very complete.
