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Topic: What does the (A) in Class-A amp mean? |
Andy Volk
From:
Boston, MA
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Posted 3 Jan 2003 11:13 am |
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| When amplifiers are designated "Class-A" what exactly does that mean? |
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Jon Light (deceased)
From:
Saugerties, NY
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Posted 3 Jan 2003 11:33 am
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Andy---the following is copied from VHT Amplifier's website.
There are from a technical standpoint, many things that distinguish Class A operation from Class A/B. For our purposes we'll be discussing only those characteristics which pertain to guitar amps in general and VHT amps in particular.
Generally speaking, the power tubes in a Class A amp are operating at pretty close to full power whether or not a signal is being amplified. As long as the tubes are being operated at reasonable voltage and bias range we're OK. The beauty of this type of operation is that there is no significant difference between the tubes work interval and its rest interval. The distortion products created by this method of operation are very musical sounding and the overall tone has a pure quality. When combined with the right output transformer, the harmonic blend created by the tube distortion characteristic and transformer saturation ( that is to say, just past the point of maximum linear operation), is rich, full and fat sounding. It sounds pretty much like an amp turned up loud even when it isn't because it's operating nearly full out in a sense.
In class A/B operation the tubes are getting a big break. When no signal is present the tube is essentially at rest. When you begin to play, there occurs a transition from "off" state to "on" state between the push-pull pairs of tubes. This transition, known as the crossover region, produces a noticeably different type of behavior typically referred to as crossover distortion. Crossover distortion contributes a harshness to the sound which can give the amp an aggressive personality. This can be interpreted as a good thing depending on what you are trying to accomplish musically. The real up side of Class A/B operation is its inherent efficiency. The simple fact that each tube has a rest interval in its duty cycle allows the tube to operate at higher output during its work interval or "on" state. Thus, a power amp operating in Class A/B will typically produce about 30% more power than a comparable Class A amp. Since the typical output transformer in a Class A/B amp will not be required to operate at high continuous current, it will spend less time in saturation mode. This contributes to the clarity and detail of the power amp sound. An additional advantage of Class A/B is that because they tend to run cooler, tube life can be extended somewhat. For strictly comparative purposes however, the essence of the debate is sound quality. We are attracted to the Class A sound because of its warmth, sonic complexity and rich harmonic content. The Class A/B sound is more articulate, dynamic and gives us the sensation of immediacy.
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Michael Holland
From:
Nashville, Tennessee, USA
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Andy Volk
From:
Boston, MA
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Posted 3 Jan 2003 2:02 pm
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| Good info. Many thanks! |
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Andy Volk
From:
Boston, MA
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Posted 3 Jan 2003 2:06 pm
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| Good info. Many thanks! |
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CrowBear Schmitt
From:
Ariege, - PairO'knees, - France
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Posted 4 Jan 2003 12:51 am
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Thanx Michael 
------------------
Steel what?
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jim milewski
From:
stowe, vermont
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Posted 4 Jan 2003 2:30 am
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| so then the preamp tubes are class A, as they do the whole wave, should they be replaced more often than the power tubes in the A/B power amps |
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Fred
From:
Amesbury, MA
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Posted 4 Jan 2003 4:17 pm
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Jim,
The preamp tubes run at very low current (they are voltage amplifiers) and don't deteriorate nearly as fast as power tubes which are current amplifiers.
If you're having a problem and changing the power tubes doesn't fix it check your preamp tubes.
Fred |
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