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Class A definition is simple: conducts full cycle. However in real life, when an amplifier manufacturer claims his product is Class A, how do we know it is true or false. In particular, I am thinking of an ultralinear, push-pull, KT88/6550/EL34 tube amp.
Some people will go by static plate dissipation alone, but I am not sure if that is right or not.
Follow Ups:
Easiest way is to use an oscope, looking for crossover distortion.
...put a DMM on Adc in the OPT CT feed. When the DMM dances, you're in class B.
...Or just listen, the crossover distortion can make your ears bleed.
But 1 watt is 0 dbW, 2 watts is 3dbW, 4 watts 6dbW, and 8watts is 9dbw. Tthe 2 extra Class-A watts is less than 1 dbW.
16 watts is 12dbW, 32 is 15 dbw, and 40w is <1 dbW more.
The 1st 9dbW of loudness your amp gives is in class A. that's more than half its loudness.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
You could feed in a sine wave, and use a scope to look across anode or cathode resistors.
There's a whole ton of bad information around about this.
Most push-pull tube amps are going to slip out of class A and transition into class B at some point. When this happens, the static current draw of the amplifier will rise, and this is measurable.
If your amplifier has a way to measure and set the bias, say to measure a low voltage across a 1 Ohm resistor, you could run your amplifier with no signal, then jot down that voltage. Put signal in and resistors on the outputs (8 Ohm 100W), then start turning it up until the voltage across that biasing resistor magically jumps up. The power you are making right at that point is where the class A power ends. If you are using a 60 Hz signal and an 8 Ohm resistor, you can use a $3 multimeter to measure the voltage and calculate what power that actually is.
If you can get all the way to full power without the cathode current/voltage moving noticeably, then you have all class A power.
Do note that the tube amp designer can position that cutoff power wherever they want. There isn't really a solid distinction between what a class AB vs. class B push-pull tube amp actually is, as you can't run an output valve with no current at idle.
It also takes some special design techniques to make a push-pull amp that is just class A, but it's possible.
nt
- The Fisher made an amplifier, the A-50, as an example. I have seen people experimenting with this on the SET forum as well.
If you know what the grid current is at a given power level then you can factor that out and all of what CB says should still apply.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/09/14
Switch it on with no input signal.
A ClassA amp will draw substantially more power than its rated max output without a signal present.
It should also get quite hot at idle, hotter than when it actually amplifies something.
"A Class A amp will draw substantially more power (plate dissipation) than its rated max output without a signal present." is also true for Class B amplifiers. A class A amp will have a MAX effiency of 50%. A class B amp will have a MAX efficiency of 78.5%. So by definition both Class A and Class B amplifiers will "will draw substantially more power than its rated max output without a signal present."
Let me cite some numbers. Amp rating: 40W per channel. Plate voltage = 480V. That implies for each tube, the static dissipation is 20W and plate current, 41.6mA. What would be the plate current for this amp to be operating Class A?
According to Caucasion Blackplate's definition above (which is correct BTW)
The current will not exceed your 41.6 ma. When it does rise is when the amp comes out of Class A. In your example the MOST you can expect is about 10 watts Class A at the speakers (given an estimated 50% efficiency)
Dan Santoni
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