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Definition of Class A?
Definition of Class A?
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Old 4th December 2017, 08:46 PM   #11
scottjoplin is offline scottjoplin  Wales
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So it's a game? Class A amps are only and always Class A amps, big deal.
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Old 4th December 2017, 08:46 PM   #12
thehoj is offline thehoj  Canada
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Originally Posted by petertub View Post
4. no grid current i.e. grid never goes positive relative cathode
Given a match of conditions 1 - 3, What class of operation would you call it when the grid also does go positive with respect to the cathode?
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Old 4th December 2017, 08:48 PM   #13
Bandersnatch is offline Bandersnatch  United States
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Peter, on 4, what is stopping me from running a power tube who's idle point is set at 0 volts on the grid, and for half the waveform grid current flows, and the other...not. There is idle current to both cut off on the downward swing of the grid, and double on the upswing.

this is what is labeled A2, the 2 denoting grid current.

Scott, the agenda is to clean up the imprecise terminology. Nothing more, nothing less. Engineering requires distinct definitions, and this is a perfect case( one of many you'll likely find ).
cheers,
Douglas
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Old 4th December 2017, 08:52 PM   #14
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Bandersnatch View Post
Scott, the agenda is to clean up the imprecise terminology
Good luck. As you can see below I too am on a mission
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Old 4th December 2017, 08:53 PM   #15
Miles Prower is offline Miles Prower  United States
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Originally Posted by 20to20 View Post
Doesn't it? My AB amp spends 99% of the time in A.
That may be true of audio amplification where the signal levels are usually at some low levels most of the time with the occasional excursion to peak values. Doesn't apply to "brick on the key" modes such as FM or packet (RF amplification) or something like TV deflection that's always max power with a signal that's full RMS.

The only reason to calculate the proportion of signal that keeps both PP devices conducting is to determine how deep into Class AB you are, as this affects x-over distortion. It's still 'AB regardless, and that argument is something Marketing thought up. Same as all those various weird definitions of power to make the product seem more powerful than it really is.
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Old 4th December 2017, 08:55 PM   #16
marco_gea is offline marco_gea  Italy
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Originally Posted by Bandersnatch View Post
as predicted by DF96, there are people who would disagree.

The point is that it is more than just a single quantity definition. The examples cited by marco and 20to20 are exactly single quantity. Class AB amps are only and always AB amps. The definition is wider than 'both tubes conducting for 360 degrees'.

Class A requires more than this...
cheers,
Douglas
I think it's a semantics issue.

Rather than saying "Amplifier X IS class A", I consider it more meaningful to say "Amplifier X OPERATES in class A".

If the former definition is sought, then sure, a class AB amp will only and always be class AB...

but ultimately, who cares? It will still operate in class A up to a given output power, depending on how high its bias is set. And if the bias is set high enough for the intended application, then for all intents and purposes I'd say that one can legitimately call it a class A amp, up to that power level.

For instance, Pass Labs and Accuphase "class A" amps work this way, as I'm sure do most if not all push-pull class A amps on the market.

Marco
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Old 4th December 2017, 08:56 PM   #17
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Bandersnatch View Post
Class A requires more than this...
cheers,
Douglas
An AB amp can become a full time A amp by just lowering ( more (+) ) the bias to the point that the drive signal can't create a condition of cuttoff. But if you raise the drive signal high enough it will clip instead. Overdrive it and it clips instead. If you want to say an amp biased for for AB is an AB amp, go ahead. But it will run in A if the bias voltage is above the cuttoff voltage. And if it's biased for A only operation then it's an A amp. PP or SE doesn't matter.

Last edited by 20to20; 4th December 2017 at 08:59 PM.
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Old 4th December 2017, 09:07 PM   #18
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4. no grid current i.e. grid never goes positive relative cathode

As stated there is class A1 where #4 is true, and class A2 where #4 is not true. I have built several class A2 amps. A single ended amp must be class A unless you like distortion.....a lot. There are plenty of SE class A amps that draw grid current. Just about any SE amp that uses an 811A, 833a or 845 will enter A2 if driven hard enough. The amp must be properly designed to handle grid current without distortion, which is possible.

Quote:
Engineering requires distinct definitions, and this is a perfect case
Agreed. And it's marketing's job to obfuscate those definitions for maximum profit. Consider the Vox AC30 guitar amp and its siblings. They are routinely sold as "class A" amps squeezing 30 watts out of 4 EL84's in Class A. Try that and see how long those poor tubes last. It may be class A up to a watt or two, but those details never make it to the ad copy.

As stated an amp sold as a "class A" amp should be in class A for its entire range of expected operation. A fair example of marketing copy could say "class A up to 4 watts, class AB beyond that up to XX watts." I have seen only two examples of this that I can remember.
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Old 4th December 2017, 09:23 PM   #19
Bandersnatch is offline Bandersnatch  United States
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Originally Posted by 20to20 View Post
An AB amp can become a full time A amp by just lowering ( more (+) ) the bias to the point that the drive signal can't create a condition of cuttoff. But if you raise the drive signal high enough it will clip instead. Overdrive it and it clips instead. If you want to say an amp biased for for AB is an AB amp, go ahead. But it will run in A if the bias voltage is above the cuttoff voltage. And if it's biased for A only operation then it's an A amp. PP or SE doesn't matter.
Taking an AB amp and increasing idle current far enough to stop either of the finals from cutting off at full power is quite possible. Under the elevated idle current conditions it is a Class A amp.

This condition is usually precluded by either power supply capacity or plate dissipation limits...or both.
cheers,
Douglas
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Old 4th December 2017, 09:39 PM   #20
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Bandersnatch View Post
This condition is usually precluded by either power supply capacity or plate dissipation limits...or both.
cheers,
Douglas
Yep, usually so. Those danged engineers....
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