And yes I am serious...
Just wanted to put it down, as those infected by Marketing seem to think that the liddle bit of time in an AB amp that both sides are conducting constitutes Class A operation.
1. Full 360 degree conduction angle of the amplifying device
2. No.1 at full power to what ever the distortion spec happens to be.
3. Effective idle point in the middle of the curves across which the load line lies.
It excludes X watts Class A, XX Watts B. As the AB definition covers a bit of both sides conducting for a full 360 degrees.
For a PP amp, the two phases are acting like a bridged pair of SE amps. SE having to be full circle conduction angle...LOL
Any points I missed?
cheers,
Douglas
Just wanted to put it down, as those infected by Marketing seem to think that the liddle bit of time in an AB amp that both sides are conducting constitutes Class A operation.1. Full 360 degree conduction angle of the amplifying device
2. No.1 at full power to what ever the distortion spec happens to be.
3. Effective idle point in the middle of the curves across which the load line lies.
It excludes X watts Class A, XX Watts B. As the AB definition covers a bit of both sides conducting for a full 360 degrees.
For a PP amp, the two phases are acting like a bridged pair of SE amps. SE having to be full circle conduction angle...LOL
Any points I missed?
cheers,
Douglas
OT post removedYour AB amp spends 100% of its time in AB. It is not a bit of A and a bit of B; that condition is AB. Nothing more, nothing less.
The point being is that, to which you are referring is *NOT* Class A. There is more to the definition that both conducting 360 degrees. It is bias a little bit past the middle of the curves through which the load line passes.
If there is 50 mA available to a maximum positive grid swing of X volts, a swing of -X volts from idle must not cut off that phase. It is effectively two bridged SE amps working off their respective halves of a CT primary.
cheers,
Douglas
My PP amp is biased so that it can run in AB IF I turn the volume control up high enough, which I rarely do. My PP amp is not biased on the center as so it would only run in A. That would create balanced clipping and cuttoff if the drive signal is too high. It will go into cuttoff if I desire and turn the volume control up high enough. The negative portion of the drive signal rarely drives the tube into cuttoff so the amp runs in A.... 99% of the time.
Nelson Pass..
...who ought to know a thing or two about this topic, puts it like this:
"The bias current runs through the amplifiers at a minimum value, determining the class of operation
– Class B, Class AB, or Class A.
Class B has no bias current, Class AB has a moderate bias current, and Class A has a high bias current. Class AB push-pull amplifiers are hybrids between Class B and Class A. Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias."
...who ought to know a thing or two about this topic, puts it like this:
"The bias current runs through the amplifiers at a minimum value, determining the class of operation
– Class B, Class AB, or Class A.
Class B has no bias current, Class AB has a moderate bias current, and Class A has a high bias current. Class AB push-pull amplifiers are hybrids between Class B and Class A. Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias."
And yes I am serious...
1. Full 360 degree conduction angle of the amplifying device
2. No.1 at full power to what ever the distortion spec happens to be.
3. Effective idle point in the middle of the curves across which the load line lies.
It excludes X watts Class A, XX Watts B. As the AB definition covers a bit of both sides conducting for a full 360 degrees.
For a PP amp, the two phases are acting like a bridged pair of SE amps. SE having to be full circle conduction angle...LOL
Any points I missed?
cheers,
Douglas
4. no grid current i.e. grid never goes positive relative cathode
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
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
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?
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
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
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.
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
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.
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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.
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.
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.
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.
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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