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Class A - how to tell?

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Posted on September 8, 2014 at 17:35:35

ianm0
Audiophile

Posts: 306
Location: AU
Joined: June 1, 2006

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.

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RE: Class A - how to tell?

wheezer

12:29:35 09/11/14

Just 10 watts out of 40 watts? Doesn't look like much!?

Timbo in Oz

00:01:03 09/11/14

RE: Class A - how to tell?

fredtr

12:44:58 09/09/14

...

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RE: Class A - how to tell?

wheezer

12:29:35 09/11/14

Just 10 watts out of 40 watts? Doesn't look like much!?

Timbo in Oz

00:01:03 09/11/14

RE: Class A - how to tell?

fredtr

12:44:58 09/09/14

RE: Class A - how to tell?

Caucasian Blackplate

08:13:09 09/09/14

RE: Thanks. That's the answer I'm looking for. nt

ianm0

06:33:24 09/10/14

A minor exception is class A2

Ralph

11:38:13 09/09/14

RE: A minor exception is class A2

Tre'

12:52:29 09/09/14

RE: Class A - how to tell?

b.l.zeebub

03:38:57 09/09/14

RE: Class A - how to tell?

Palustris

15:07:50 09/09/14

RE: Can you shed some light on "substantially more"?

ianm0

06:00:11 09/09/14

RE: Can you shed some light on "substantially more"?

Blackdog

05:26:41 09/10/14

RE: Class A - how to tell?, posted on September 9, 2014 at 03:38:57
b.l.zeebub Audiophile Posts: 9361 Location: 52deg 28'N,1deg56'W Joined: April 17, 2006	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.

RE: Can you shed some light on "substantially more"?, posted on September 9, 2014 at 06:00:11
ianm0 Audiophile Posts: 306 Location: AU Joined: June 1, 2006	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?

RE: Class A - how to tell?, posted on September 9, 2014 at 08:13:09

Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004

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.

A minor exception is class A2, posted on September 9, 2014 at 11:38:13
Ralph Manufacturer Posts: 4769 Location: Minnesota Joined: April 24, 2002	- The Fisher made an amplifier, the A-50, as an example. I have seen people experimenting with this on the SET forum as well.

RE: Class A - how to tell?, posted on September 9, 2014 at 12:44:58
fredtr Audiophile Posts: 1987 Location: Phoenix Joined: January 4, 2005	You could feed in a sine wave, and use a scope to look across anode or cathode resistors.

RE: A minor exception is class A2, posted on September 9, 2014 at 12:52:29

Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002

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"

RE: Class A - how to tell?, posted on September 9, 2014 at 15:07:50

Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008

"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."

RE: Can you shed some light on "substantially more"?, posted on September 10, 2014 at 05:26:41
Blackdog Manufacturer Posts: 1505 Location: Ontario Joined: March 20, 2006	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

RE: Thanks. That's the answer I'm looking for. nt, posted on September 10, 2014 at 06:33:24
ianm0 Audiophile Posts: 306 Location: AU Joined: June 1, 2006	nt

Just 10 watts out of 40 watts? Doesn't look like much!?, posted on September 11, 2014 at 00:01:03
Timbo in Oz Audiophile Posts: 23221 Location: Canberra - in the ACT - SE Australia Joined: January 30, 2002	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

RE: Class A - how to tell?, posted on September 11, 2014 at 12:29:35
wheezer Audiophile Posts: 4309 Joined: January 24, 2001	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.

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