Another Tubes 101 question
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Another Tubes 101 question
Okay, I'm not wrapping my head around a very basic concept. What determines what class (A, AB1, AB2, etc.) a set of tubes is running in? I know (?) that Class A means that the entirety of the waveform is being reproduced by all the tubes, but what I don't understand is what parameters does one vary that determine this behavior? Bias?
-g
Re: Another Tubes 101 question
> What determines what class (A, AB1, AB2, etc.) a set of tubes is running in?
And why do you care? Some writers make much of "that great Class A sound!" but some then mention amps which are well into AB. It seems they really mean the older simpler less overdesigned amplifiers and their fairly simple sonic signatures, not any idealized "class".
But anyhow.....
Audio swings both ways. Acoustically we have positive and negative air pressure variations.
A single tube (or transistor) can only pull one way. It can flow current one way, or not, but current never flows backward.
How do we resolve this problem?
In class A, we bias the tube "halfway ON", then wobble it both ways from there. We start with, say, 50mA no-signal. We wobble from 50 to 75 to 50 to 25 and back to 50mA. Or 50..100..5..0..50. Then we ignore the steady 50mA to get the +/-25mA or +/-50mA variation.
I described the case for one tube, but you can run "Push-Pull" class A. Two devices working equal but opposite signals. We wire the inputs and outputs through levers so one tube pushes cone forward and the other pulls cone backward. Doubling the tubes doubles power, but also the tube gain changes with current, so a single tube in class A tends to distort the signal. Two tubes push-pull tend to cancel each other's distortion. So for a specified THD number, push-pull can give more than double the power.
A single tube working Class B and idled at "OFF" can only reproduce half the waveform, but two tubes can reproduce both halves and therefore the entire wave. On signal, each one sucks current only one way and only on its side of the wave. Ideally we can idle the tubes at zero current. For push, one rises from zero to 50mA. For pull, the other rises from zero to 50mA. The tube that is not working on a given half-cycle stays at zero.
In real devices, going near zero current is problematic. Gain falls, it becomes hard work to push down to very low current, on small signal the tube may fail to come up from "zero" smoothly. Class A has no such problem: the idle current is "halfway" and gain is pretty good.
The simplest amp is of course the One Bottle. One power-tube audio amps MUST work Class A. Any other "class" implies a gross clipping of some part of the wave before the other, and that's not right for best audio at minimum cost.
I said the class A amp must ignore the steady current to extract the useful audio variation. The most efficient way to do this is to store that 50mA in a large choke (which is normally also used to transform impedance). Above a few Watts, the weight and cost of this iron tends to be greater than the cost of a second power-tube. With two tubes, there is no need for the steady energy storage, the iron is much lighter and cheaper. So above around 5 Watts, 2-big-tube push-pull amps predominate.
In push-pull, you can run Class A but you can push a little harder into rich-AB and get a bigger Watts number without unsightly THD numbers. Everybody does.
Self-bias tends to be cheaper, but can't get far past class A without messing up its own bias.
Fix-bias isn't bothered by increase of average cathode current in hard work, so can be idled cooler and pushed harder than self-bias. Before 1950 the cost of the extra power supply for bias was more than it was worth; after 1960 diodes got cheap and everybody went fix-bias for bigger Watts numbers on the same bottles.
Note that, the way we build amps, all amps work in class A up to a point. A single tube works A until it just won't do any more. In push-pull, at small signal, both tubes are wobbling signal. If signal is increased and/or load impedance is decreased, the wobble increases to the point that one tube hits cut-off, zero current (or so near that it may as well be zero), and the amp works like B. Some 18W amps stay A up to around 15 Watts, and some 18W amps have quit class A by 1 Watt and are working like B for all typical stage sounds. And I'm thinking of two amps at these extremes, and there is not such a ton of difference in sound.
Nearly all not-small practical audio amplifiers run AB. The less overdesigned ones run rich-AB, and actually work as Class A up to nearly full rated power. The more overdesigned ones idle quite cold, do small signals in Class A but medium to max signals are effectively working like a class B amp.
In tube amps, you may also see a number 1 or 2. "AB1" means AB with "no" grid current. You can use a small cheap tube like 12AX7 to drive your power tube. "AB2" means you must be able to supply non-zero current to the grid. Nearly all power tubes can make more output power if you force current into the grid. However that means you need a far fatter tube in front of your power tube: I sketched a 40W amp which needs a 5W amp (like a 6V6 Champ) to drive the big tube. This is almost always a Bad Idea if you can avoid it. And since it is easy to build 100W-300W amps working AB1, it is always a bad idea until you prove otherwise.
All this has been about AUDIO amplifiers, where every detail of the input wave must be reproduced faithfully, or at least recognizably.
And this assumes "undistorted" operation. In real life, there may be a smooth bend and few-percent distortion. But you can put "too much" signal into an 18 Watt amp, so that you would need a 36 Watt amp to reproduce faithfully. In "good" audio, we don't do that. In electric guitar, the clean sound of the naked string is boring, and overdrive is a useful musical emphasis. So if you are whacking the tar out of your amplifier, all this talk of "Class" has gone out the window.
There are non-Audio amplifiers where waveform details do not have to be preserved. In many radio systems, all you need is the height and the pitch. In some only the pitch is needed. A single tube in class B clips half the wave, but the other half is reproduced well. A simple filter reshapes the full wave. A single tube in Class C will chop off 80% of the wave, but the part that gets through is in sync with the input, and a simple filter can shape-up the pulses. The advantage is very much cooler operation for the same output power (or more output power in the same bottle).
And why do you care? Some writers make much of "that great Class A sound!" but some then mention amps which are well into AB. It seems they really mean the older simpler less overdesigned amplifiers and their fairly simple sonic signatures, not any idealized "class".
But anyhow.....
Audio swings both ways. Acoustically we have positive and negative air pressure variations.
A single tube (or transistor) can only pull one way. It can flow current one way, or not, but current never flows backward.
How do we resolve this problem?
In class A, we bias the tube "halfway ON", then wobble it both ways from there. We start with, say, 50mA no-signal. We wobble from 50 to 75 to 50 to 25 and back to 50mA. Or 50..100..5..0..50. Then we ignore the steady 50mA to get the +/-25mA or +/-50mA variation.
I described the case for one tube, but you can run "Push-Pull" class A. Two devices working equal but opposite signals. We wire the inputs and outputs through levers so one tube pushes cone forward and the other pulls cone backward. Doubling the tubes doubles power, but also the tube gain changes with current, so a single tube in class A tends to distort the signal. Two tubes push-pull tend to cancel each other's distortion. So for a specified THD number, push-pull can give more than double the power.
A single tube working Class B and idled at "OFF" can only reproduce half the waveform, but two tubes can reproduce both halves and therefore the entire wave. On signal, each one sucks current only one way and only on its side of the wave. Ideally we can idle the tubes at zero current. For push, one rises from zero to 50mA. For pull, the other rises from zero to 50mA. The tube that is not working on a given half-cycle stays at zero.
In real devices, going near zero current is problematic. Gain falls, it becomes hard work to push down to very low current, on small signal the tube may fail to come up from "zero" smoothly. Class A has no such problem: the idle current is "halfway" and gain is pretty good.
The simplest amp is of course the One Bottle. One power-tube audio amps MUST work Class A. Any other "class" implies a gross clipping of some part of the wave before the other, and that's not right for best audio at minimum cost.
I said the class A amp must ignore the steady current to extract the useful audio variation. The most efficient way to do this is to store that 50mA in a large choke (which is normally also used to transform impedance). Above a few Watts, the weight and cost of this iron tends to be greater than the cost of a second power-tube. With two tubes, there is no need for the steady energy storage, the iron is much lighter and cheaper. So above around 5 Watts, 2-big-tube push-pull amps predominate.
In push-pull, you can run Class A but you can push a little harder into rich-AB and get a bigger Watts number without unsightly THD numbers. Everybody does.
Self-bias tends to be cheaper, but can't get far past class A without messing up its own bias.
Fix-bias isn't bothered by increase of average cathode current in hard work, so can be idled cooler and pushed harder than self-bias. Before 1950 the cost of the extra power supply for bias was more than it was worth; after 1960 diodes got cheap and everybody went fix-bias for bigger Watts numbers on the same bottles.
Note that, the way we build amps, all amps work in class A up to a point. A single tube works A until it just won't do any more. In push-pull, at small signal, both tubes are wobbling signal. If signal is increased and/or load impedance is decreased, the wobble increases to the point that one tube hits cut-off, zero current (or so near that it may as well be zero), and the amp works like B. Some 18W amps stay A up to around 15 Watts, and some 18W amps have quit class A by 1 Watt and are working like B for all typical stage sounds. And I'm thinking of two amps at these extremes, and there is not such a ton of difference in sound.
Nearly all not-small practical audio amplifiers run AB. The less overdesigned ones run rich-AB, and actually work as Class A up to nearly full rated power. The more overdesigned ones idle quite cold, do small signals in Class A but medium to max signals are effectively working like a class B amp.
In tube amps, you may also see a number 1 or 2. "AB1" means AB with "no" grid current. You can use a small cheap tube like 12AX7 to drive your power tube. "AB2" means you must be able to supply non-zero current to the grid. Nearly all power tubes can make more output power if you force current into the grid. However that means you need a far fatter tube in front of your power tube: I sketched a 40W amp which needs a 5W amp (like a 6V6 Champ) to drive the big tube. This is almost always a Bad Idea if you can avoid it. And since it is easy to build 100W-300W amps working AB1, it is always a bad idea until you prove otherwise.
All this has been about AUDIO amplifiers, where every detail of the input wave must be reproduced faithfully, or at least recognizably.
And this assumes "undistorted" operation. In real life, there may be a smooth bend and few-percent distortion. But you can put "too much" signal into an 18 Watt amp, so that you would need a 36 Watt amp to reproduce faithfully. In "good" audio, we don't do that. In electric guitar, the clean sound of the naked string is boring, and overdrive is a useful musical emphasis. So if you are whacking the tar out of your amplifier, all this talk of "Class" has gone out the window.
There are non-Audio amplifiers where waveform details do not have to be preserved. In many radio systems, all you need is the height and the pitch. In some only the pitch is needed. A single tube in class B clips half the wave, but the other half is reproduced well. A simple filter reshapes the full wave. A single tube in Class C will chop off 80% of the wave, but the part that gets through is in sync with the input, and a simple filter can shape-up the pulses. The advantage is very much cooler operation for the same output power (or more output power in the same bottle).
Re: Another Tubes 101 question
PRR,
Excellent write up !!. You got me thinking...When I was a kid my dad made a optical transmitter using an op amp to modulate a light bulb. In those days the buld was a 3v bulb and I remeber that he told me that he set the bulb to run at 1.5v and then vary the signal up or down to match the input. This to me sounds exactly like class A. so I guess my question to you is how do you determine if the amp is running on class A or AB?
Excellent write up !!. You got me thinking...When I was a kid my dad made a optical transmitter using an op amp to modulate a light bulb. In those days the buld was a 3v bulb and I remeber that he told me that he set the bulb to run at 1.5v and then vary the signal up or down to match the input. This to me sounds exactly like class A. so I guess my question to you is how do you determine if the amp is running on class A or AB?
-
Darkbluemurder
- Posts: 584
- Joined: Tue Jan 03, 2006 7:28 pm
Re: Another Tubes 101 question
The term "Class A" is widely used as a marketing method for amps which are cathode biased. What strikes me is that while the VOX AC 30 and its clones are always marketed as Class A, I have never seen the Tweed Deluxe or its clones or an 18W Marshall or its clones marketed in this way, even though they are cathode biased as well.
Most surprisingly at all I have never seen the Tweed Champ marketed as Class A. Isn't a single ended amp Class A by definition?
Most surprisingly at all I have never seen the Tweed Champ marketed as Class A. Isn't a single ended amp Class A by definition?
Re: Another Tubes 101 question
Yes, class A is used as a marketing ploy, as if it's somehow "better" than other classes of operation.Darkbluemurder wrote:The term "Class A" is widely used as a marketing method for amps which are cathode biased. What strikes me is that while the VOX AC 30 and its clones are always marketed as Class A, I have never seen the Tweed Deluxe or its clones or an 18W Marshall or its clones marketed in this way, even though they are cathode biased as well.
Most surprisingly at all I have never seen the Tweed Champ marketed as Class A. Isn't a single ended amp Class A by definition?
No, SE does not mean Class A. The Champ is a good example of Class AB SE as you are driving the tube partially into cutoff. This is also characteristic of "that" Champ distorted tone.
Here's Randall Aiken's white paper on Class A.
http://aikenamps.com/ClassA.htm
Great reading and explains a lot. I highly suggest reading this over a few times.
Eardrums!!! We don't need no stinkin' eardrums!
Re: Another Tubes 101 question
Thanks for some good responses...especially PRR's! You're a good writer, and should think about writing a book. Your explanation of the differences between classes is as good as any I have read. Why do I care? I'm curious, and I know when I don't know something. I don't know the answer to my original question.
I still don't have a feel for what parameters determine what class you're running in. Maybe an example with fill-in-the-blanks would help.
I have a power stage with a pair of EL-34's in it.
To operate it in Pure Class A, I would wire it as follows:_____
To change it to Rich Class A, I would change the following components:____
To run it in AB1, I would________
That's the kind of thing I'm looking for. I'm still gathering that it has to do with how the tubes are biased, but I don't know if that is all there is to it, or how bias changes determine the class if that's the way it works.
I still don't have a feel for what parameters determine what class you're running in. Maybe an example with fill-in-the-blanks would help.
I have a power stage with a pair of EL-34's in it.
To operate it in Pure Class A, I would wire it as follows:_____
To change it to Rich Class A, I would change the following components:____
To run it in AB1, I would________
That's the kind of thing I'm looking for. I'm still gathering that it has to do with how the tubes are biased, but I don't know if that is all there is to it, or how bias changes determine the class if that's the way it works.
-g
Re: Another Tubes 101 question
> This to me sounds exactly like class A.
Right. A lamp can make more light or less light, but can't make negative light. To carry a signal which swings both ways, with one light you bias it half-bright and wiggle from there. Or use two lights in see-saw.
> how do you determine if the amp is running on class A or AB?
For simple fixed-bias push-pull amps with the now-popular 1 ohm cathode current-sense resistor: put a 'scope on the resistor and watch the wave. Ideally use a 2-ch 'scope in DC mode and note the zero position, but you can get a clue with one AC-coupled input. Apply audio at various levels. When the wave reproduces the top and bottom faithfully, it's class A. When the bottom of the wave is cut-off (touching zero current), it has gone into AB. If all signals at any amplitude are half cut away, it is class B (that practically can't be done with tubes, though some transistor amps get too close). As confirmation: watch both cathodes, when one is cut-off the other is conducting hard.
> Isn't a single ended amp Class A by definition?
Not really by definition. Non-Audio amplifiers which need not reproduce every detail of a signal can run SE in B or C.
Audio amplifiers are generally expected to reproduce "faithfully" within limits of noise and maximum power. Starting from scratch, it can be hard to find the "happy zone" of a tube and the other parts, and to optimize the power against cost. The A/B/C definitions define the good working limits for the common uses of amplifiers. And yes, "faithful" audio in SE must be working class A.
> I have never seen the Tweed Champ marketed as Class A.
When new, it was cheap stuff. The price was the marketing. (And pretty soon, the brand: Fender quality was usually a step above the other-brands.)
"Epiphone Valve Junior Head Features: 5 Watts, Class A Single-Ended".
Back when the Champ entered the ring, MOST domestic audio amplifers were Class A, or just-barely AB, so why boast about it? (Actually, several pre-WWII guitar amps used 6F6 or a twin power triode in "Class B", which was more like lean AB2. But the 6L6 and 6V6 would match that power without heavy grid current, and the old hacks vanished.)
Right. A lamp can make more light or less light, but can't make negative light. To carry a signal which swings both ways, with one light you bias it half-bright and wiggle from there. Or use two lights in see-saw.
> how do you determine if the amp is running on class A or AB?
For simple fixed-bias push-pull amps with the now-popular 1 ohm cathode current-sense resistor: put a 'scope on the resistor and watch the wave. Ideally use a 2-ch 'scope in DC mode and note the zero position, but you can get a clue with one AC-coupled input. Apply audio at various levels. When the wave reproduces the top and bottom faithfully, it's class A. When the bottom of the wave is cut-off (touching zero current), it has gone into AB. If all signals at any amplitude are half cut away, it is class B (that practically can't be done with tubes, though some transistor amps get too close). As confirmation: watch both cathodes, when one is cut-off the other is conducting hard.
> Isn't a single ended amp Class A by definition?
Not really by definition. Non-Audio amplifiers which need not reproduce every detail of a signal can run SE in B or C.
Audio amplifiers are generally expected to reproduce "faithfully" within limits of noise and maximum power. Starting from scratch, it can be hard to find the "happy zone" of a tube and the other parts, and to optimize the power against cost. The A/B/C definitions define the good working limits for the common uses of amplifiers. And yes, "faithful" audio in SE must be working class A.
> I have never seen the Tweed Champ marketed as Class A.
When new, it was cheap stuff. The price was the marketing. (And pretty soon, the brand: Fender quality was usually a step above the other-brands.)
"Epiphone Valve Junior Head Features: 5 Watts, Class A Single-Ended".
Back when the Champ entered the ring, MOST domestic audio amplifers were Class A, or just-barely AB, so why boast about it? (Actually, several pre-WWII guitar amps used 6F6 or a twin power triode in "Class B", which was more like lean AB2. But the 6L6 and 6V6 would match that power without heavy grid current, and the old hacks vanished.)
Re: Another Tubes 101 question
I'm not sure you have this completely correct.PRR wrote:>
Note that, the way we build amps, all amps work in class A up to a point. A single tube works A until it just won't do any more. In push-pull, at small signal, both tubes are wobbling signal. If signal is increased and/or load impedance is decreased, the wobble increases to the point that one tube hits cut-off, zero current (or so near that it may as well be zero), and the amp works like B. Some 18W amps stay A up to around 15 Watts, and some 18W amps have quit class A by 1 Watt and are working like B for all typical stage sounds. And I'm thinking of two amps at these extremes, and there is not such a ton of difference in sound.
Nearly all not-small practical audio amplifiers run AB. The less overdesigned ones run rich-AB, and actually work as Class A up to nearly full rated power. The more overdesigned ones idle quite cold, do small signals in Class A but medium to max signals are effectively working like a class B amp.
The class determination is ALWAYS done at full out before clipping. Therefore it doesn't matter what output level the amp is set to as to what class the amp is operating in. Otherwise the class of operation wouldn't mean anything.
It is true however that an AB amp operates "LIKE" class A when at lower output levels, yet is still a class AB. This is probably where most folks get confused.
Eardrums!!! We don't need no stinkin' eardrums!