Class A detection

[aclempoppi]

Just wondering if a ham&egg tech, like me, can determine if an amp is truly operating in Class A, without the use of an oscilliscope ? If there is a thread on this subject, from previous discussions, that would be fine. Thanks, Art

[KT66]

The Aiken article on this is pretty informative : http://www.aikenamps.com/ClassA.htm

[tubeswell]

Well if the output tube is biased to maximum dissipation at idle, and it can't run any hotter than that under signal conditions, it must be running in Class A - right? (Implies cathode-biasing I think)

[Bob-I]

Well if the output tube is biased to maximum dissipation at idle, and it can't run any hotter than that under signal conditions, it must be running in Class A - right? (Implies cathode-biasing I think)

Yes and no. Yes if it's running correctly but no if it's just incorrectly based.

The Aiken article is pretty good.

[Alexo]

Well if the output tube is biased to maximum dissipation at idle, and it can't run any hotter than that under signal conditions, it must be running in Class A - right? (Implies cathode-biasing I think)

Max dissipation does not mean that the tube is incapable of dissipating any more power. You can run a single 6V6 at 30 watts if you want, it'll just burn out a lot quicker.

You can also bias to 100% and/or have a class A power stage in cathode or fixed bias.

You can also have a pure class A stage that is not biased to 100%, and vice versa.

If you work out a load line for your power stage, that can tell you what class you're running (in theory, anyway). Check out valvewizard.co.uk for some good reading on the topic.

[Andy Le Blanc]

You have to find the conduction angle.

You need the zero signal plate current.

And you need the max signal plate current.

so its your bias reading, and a max power test with a dummy load.

CA = 2*ACOS ( - I pq (dc)/ Ip (rms) )

By strict definition class A is 360deg and class B is 180deg,

in between is AB.

DONT PANIC.... putting a 0 in the equation gives 180 and 1 gets 360.

So.... the total bias current for the amp divided by the max current
will give you a number, say 0.xxxx, the closer to 1 it is the closer to class A.

Its a good way to compare bias settings and relative efficiency.

[20to20]

Just wondering if a ham&egg tech, like me, can determine if an amp is truly operating in Class A, without the use of an oscilliscope ? If there is a thread on this subject, from previous discussions, that would be fine. Thanks, Art

It's not mysterious.

First, decide if your amp has a "single ended" type OPT. If so, it can only operate as Class A.

If the amp has "push-pull" OPT's then it will be biased under normal conditions to operate as Class AB. It will operate as A with small drive signals and go into Class AB operation as the signal (volume) rises and takes the tube into the cutoff region on the negative signal swing. Can you know precisely where the signal level enters the cutoff region without a scope? Nope. The negative bias voltage and the negative half of the signal combine to take the tube toward the cutoff point. So you have to know the drive level and the bias level and then find that operating point on a data sheet to estimate if the tube is operating outside of Class A for a given volume level and bias setting.

[tubeswell]

the total bias current for the amp divided by the max current will give you a number, say 0.xxxx, the closer to 1 it is the closer to class A.

Yep - That's what I was saying, albeit in simpler terms

[tubeswell]

deleted

[Andy Le Blanc]

Its not too bad to test for, but you need a large wattage load resistor.
You said O-scope, its the same set up as scope bias.
Getting ma reading (bias) over a known resistance is preferred,
because it gives you "data", a test result, a hard number.
Combine that with V/R over the load to get a Pout, and your in business.

You have a method to find plate current and power out which will get
you most of the data you need to define the operation of your amp.

You still need a signal source to test OPT to find effective plate to plate
load resistance.

The only missing piece is a DIY method to measure screen grid current.

Any way...... You'll be surprised what the math results for operating class
will tell you. There's quite a bit of fudge. Even a SE power side operating
at max plate dissipation might not meet this strict definition of class by a
percentage margin. Run the numbers out of a RCA tube manual for the
various examples for 6l6, then try to generate your own data set with an
amp on your bench.

The number given dividing the zero and max signal current is a test result.
Its a hard number that can be repeated and compared to other results.
Say for different OPT (plate load), for power, efficiency, and harmonic distortion (tone).

You need data to make valid statements and comparisons.
To hell with tube mojo, A hard test result wins out every time.
Especially when some bone head starts going on about "class".

Amplifiers are classified by frequency, voltage/power, and operating level.
All three are general classifications. The third is the "description" of bias.
The Eg-Ip curve demonstrating class is pretty well known.
The point of class A is to get linear gain, a larger signal out with minimal amplitude distortion.
Keep the signal between the knees, the wave form of the signal in is
centered on the most linear portion of the transfer curve.
It's an instantaneous transfer of a known grid voltage variation,
but its only a description of a known set of conditions and how the tube is operating.

I like the number given by Ipq / Ip, It gives you a valid number within the
AB description that can be placed in a range between "1" and "0",
that repeatable conditions can be easily tested and designed for.
That godlike trainwreck can be biased with these tubes with this plate load
to 0.1234 (whatever it may be) and no one has to BS around what class it is.

[aclempoppi]

Appeciate all of the info. I'm familiar with the Aiken article and a handful of others. Been repairing and restoring the older Fender amps, as a side job for well over 25 yrs. But never invested in a scope. At any rate, guys have been asking me whether or not this Mesa, Matchless, Badcat, etc is actually operating in Class A. So this is the reason I brought the question to the forum. I was thinking, with enough basic measurements, I should be able to construct a loadline. So I'll try to process the info you given me. Thanks, Art

[Merlinb]

guys have been asking me whether or not this Mesa, Matchless, Badcat, etc is actually operating in Class A.

Single ended amps work in class A all the way up to clipping by necessity. Almost every push-pull amp you will ever see is designed for class AB at clipping (but at low volumes they will technically be operating in class A). The chances of a Mesa/Badcat being an all-out class A amp are zero, unless the amp is single ended. Not sure about Matchless, as they sometimes come up with novelties...

You'd be well to answer your customers' questions with "do you know what class A actually means? or are you just asking to sound knowledgeable, like when you're fixing your car and random men slowly gather around and shout names of parts they know? (could be the sprocket / have you checked the ball cock etc.)"

[aclempoppi]

Thanks Merlin, I refer to your website periodically, to help with my understanding of amps. Just re-read the tutorial on single ended output sections. Yeah, sounding knowledgable around most folks that come in, wouldn't be tough at all. I was just interested in being able to demonstrate the difference, without all of the equipment. Thanks again for all the help! Art

[Alexo]

...one other way you can (sorta) tell is by output power.

For instance, a single ended 6V6 amp can only put out about 5 watts of power at most (generalizing here).

A true class A push-pull 6V6 amp, continuing with the example, can only get, at most, double the power of a SE 6V6 amp, since there is no point where one tube cuts off and the other enters class B operation.

But an AB amp using 2 6V6's can easily get more than 2X the SE output.

Therefore a 15 watt push-pull 6V6 amp kind of has to be AB. If it were A, it would be hard to get more than 10 watts out of it. A 40 watt PP EL34 amp has got to be AB, but if it's only 20 watts, class A is a strong possibility.

This method is not 100% accurate, but generally if you see a PP amp getting more than what you would get out of 2 combined SE amps, it's in AB.

[aclempoppi]

Used a Clubman-35 in the blues band for 8-9 years, just kind of made that leap of faith, that it was indeed what Matchless said it was. Talk about energy spent on heat, at the end of the night the guitar's input plug, at the amp, was almost too hot to hold on to. On another note, that amp sure had some moments of sag ! But otherwise, it was just a great amp. Art