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Definition of Class A?
Definition of Class A?
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Old 4th December 2017, 08:40 PM   #21
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Tubelab_com View Post
As stated an amp sold as a "class A" amp should be in class A for its entire range of expected operation.
How often is this not the case?
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Old 4th December 2017, 08:45 PM   #22
Bandersnatch is offline Bandersnatch  United States
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Originally Posted by scottjoplin View Post
How often is this not the case?
I believe marco pointed out that Pass does this in an earlier post.

Douglas
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Old 4th December 2017, 08:48 PM   #23
Bandersnatch is offline Bandersnatch  United States
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Originally Posted by 20to20 View Post
Yep, usually so. Those danged engineers....
Why? They got told to build an amp. That given the whole deck of requirements, it was going to be an AB amp that could not afford a solid supply. I believe the term used to deride the appropriate folks would be, 'bean counter'.

Now the bean counter may have been right, and such an amp would not have been sale-able at the required price point. For that I think you can judge who is to blame, yes?
cheers,
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Old 4th December 2017, 08:52 PM   #24
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Bandersnatch View Post
I believe marco pointed out that Pass does this in an earlier post.

Douglas
Good point, but Nelson admits to being in the entertainment industry
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Old 4th December 2017, 09:34 PM   #25
Merlinb is offline Merlinb  United Kingdom
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A class AB amp is class-A for small signals, i.e. both tubes are conduction 360 degrees. That's class-A by definition. Drive them further, to cut off, and it becomes class AB.
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Old 4th December 2017, 09:40 PM   #26
john_tracy is offline john_tracy  United States
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For a PP amp, the two phases are acting like a bridged pair of SE amps.
Not the case if the output stage is configured as a LTP with a CCS tail. In this case the signal current can only circulate plate to plate and cathode to cathode. It will not work with one tube cut off. With class B prohibited, it will only operate in case A. In a normal PP amp as above, when operating in class A there is no guarantee of AC balance between the two halves of the circuit. The stronger tube will send an imbalance current from its plate through the PS picking up distortions and colorations there on its way back to its cathode. A LPT circuit enforces AC balance limited by the impedance of the CCS.

As an aside I have PPP KT-120 mono-blocks that output 37W of class A triode power (and a whole lot of heat).
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Old 4th December 2017, 09:59 PM   #27
Bandersnatch is offline Bandersnatch  United States
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Originally Posted by Merlinb View Post
A class AB amp is class-A for small signals, i.e. both tubes are conduction 360 degrees. That's class-A by definition. Drive them further, to cut off, and it becomes class AB.
and there in lies the problem. Class A is not defined only by 360-degree conduction. Add at full power, at its distortion limit and you approach completeness.

An AB amp operates in AB...the likely marketing derived, incomplete definition of Class A is not the whole story.
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Old 4th December 2017, 10:13 PM   #28
Tubelab_com is offline Tubelab_com  United States
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Quote:
How often is this not the case?
Quite often.

Quote:
Those danged engineers....Why? They got told to build an amp. That given the whole deck of requirements.....the appropriate folks would be, 'bean counter'.
Unfortunately I spent about 20 years of my engineering career dealing with, and trying to balance the marketing guys, the bean counters, and other engineers against the laws of physics.

It is impossible to tell a marketing person that you can't design a VHF police radio that puts out 5 watts of RF in a cell phone sized form factor with a cell phone battery. We actually had to waste half a year to build them a prototype. The battery lasted about an hour max, and the antenna would burn you from the RF.....new marketing team.....new problems. After several years of this, it became mandatory for the marketing people, and several other "committee" members to spend some time in the field with cops, firemen, and paramedics to see the real world where the products get used.

It's a fine line to walk trying to live in the world of "design by committee." The results will ALWAYS be a compromise, and so will the ad copy. The rules are far stricter with "mission critical" radio equipment where lives are at stake if it doesn't work right. Fortunately many of the critical specs are determined by the FCC and APCO (Association of Police Communications Officers).

The world of high end HiFi have no such watchdogs.
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Old 4th December 2017, 10:21 PM   #29
6A3sUMMER is offline 6A3sUMMER  United States
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The definitions of class A, class AB, class B, and class C can be found in any number of documents from the 1930s onward. Try checking Western Electric, Bell Labs, ARRL, RCA, Eimac, etc. for these definitions.

Whatever 'audio' marketing tells you, the original definitions apply.
Changing the definitions does not allow for a level playing field for all players
(Oh, that IS what marketing often is).
The class of an amplifier is defined by the angles of conduction, regardless whether the circuit is single ended, push pull, etc.

Using anything but class A for single ended will not be pleasing to most listeners (*).

2 anti-phase single ended that a combined at the output in another phase reversal,
may be defined as a special case push pull, whether they use a push pull transformer, or 2 single ended transformers. This will allow the tubes to go into cutoff without causing the case listed above (*)

Take a push pull amplifier that can operate in class A at lower signal levels (no tube in cut off), but that can also operate at higher signal levels in AB (tubes alternately in cut off at signal extremes).
To argue over whether that amp is a class A or a class AB amplifier seems pointless.
Actually, there have been marketing firms in the past that have rated Both the class A power out, and the class AB power out of a push pull amplifier model.

As to grid current, 1 is no appreciable grid current (only capacitive current), and 2 is appreciable grid current (conduction current, not just capacitive current).

Normally the 1 is assumed when no number is present (unwritten):

A =A1 A2
AB = AB1 AB2
B = B1 B2

C is the exception. C is most often used for RF. Most of those RF tubes that are used in C Do draw grid current in most real applications. So although there is no written 1 or 2 suffix, the 2 is assumed for class C.
The Control Grids of some RF tubes are rated for Hundreds of Watts dissipation. And also the Screen Grids of some RF tubes are rated for Hundreds of Watts dissipation.
Try that with your favorite audio tube (KT88, etc.)

I once built an audio amplifier using the screen grid of a 4-65 as the "plate". The real plate was disconnected.
The screen of the 4-65 RF tube is rated for 10 Watts, the same as a type 45 triodes 10 Watt plate dissipation rating.

Class C audio will not be enjoyed by most listeners, with or without grid current.

I will let others talk about class D. That is a whole other subject.

Last edited by 6A3sUMMER; 4th December 2017 at 10:31 PM.
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Old 4th December 2017, 11:13 PM   #30
Wavebourn is offline Wavebourn  United States
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Definition of Class A?
The class first appeared when first PP stage was designed. It was called "Class B", while previous amps were called "Class A", to stress that 2 devices work alternately, delivering more power and saving battery life.
Later "Class AB" was defined, for higher idle current, with gm doubling on low volume that is most critical for listening. Later A2 was defined, that assumed presence of control grid current.
Class C was used in transmitters, when power tubes were biased to cut off at idle.
Class D was called an amp that used pulse width modulation of a carrier signal, with saturating output devices, that gave the highest possible efficiency due to low power drop on power devices that switch from cut-off to saturation.

However, some other possibilities exist. I developed class A+B and class A+C amps. In class A+B amp output devices were always conducting the current, but upon demand their current amplification factor was increased. Class A+C works in class A, but 2 additional class C devices gradually add current upon demand. The last approach was also patented by Peter Walker and called "Current Dumping".
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