"Class A" nonsense

[Rhodesplyr]

A friend of mine proudly me today that he had a "Class A" guitar amp that got 18W from a pair of EL84s in push-pull.

Now, as far as I know, that's not possible. It's possible to get 18W from a pair of EL84s running Class AB1 like most push-pull tube output sections.

Who started all this marketing nonsense about "Class A" amps? I can see the motivation to impress buyers.....

David

[Chuck H]

This hardly reqires a response (based on your original premise of marketing and appearant understanding), but I can understand your frustration. Fact is, most el84 amps that are cathode biased idle at, or close to 100% dissapation. MFG's like to call this class A because the term has mojo. But when these amps are pushed the cathode voltage rises disproportionate to the sag in the B+ supply and the amps end up in AB1 (more +V at the cathode is the same as more -V at the grid). So... Even though these amps are technically biased class A they end up in AB1 during operation. A distinction that has long been ignored, probably for marketing purposes. Even the Vox AC30 ends up in AB1 when overdriven. But try telling a non amp building original AC30 owner that his amp isn't class A.?. Be ready to fight about it

Chuck

Edit: Just to clarify... In order to be an actual class A amp the AC30, most 18 watters, some Matchless amps, etc. would need to reduce the power tube grid signal voltage to avoid pushing these amps into AB1. Technically these amps are biased class A. But IMHO that hardly counts since an amp being played is not idling.

I recently designed an amp that may go into production. It is el84 based, cathode biased and idles at or near 100% dissapation... We may decide to call it class A for marketing, even though it's really not IMO.

[Robert M. Martinelli]

Hi,
you're absolutely correct, 17-18 W is about the most you can get out of a couple of EL84s ( AB1 P-P ).

This subject has been dealt with very thoroughly in the past, so I encourage you to do a little search and read all that's been said about it.

Unfortunately, some people don't know what they're talking about when they mention the various classes of operation. The misunderstanding about "class A" most likely comes from the fact that people thinks that biasing a tube at 100% ( idle - no signal ) automatically means we are in class A. This is not the case - the class of operation is solely determined by looking at how long the tube(s) conduct with respect to the signal's period.

If the SIGNAL flows through a tube for a whole period, 360 degrees ( or 2*Pi ) we are in true class A, and only one tube can suffice in reproducing the input waveform ( more tubes can be paralleled to increase the power output ).

If the SIGNAL flows through a tube exactly for 180 degrees ( or Pi ) we are in class B, and at least two tubes are required to reproduce the input waveform ( but zero-crossing distortion will be present ).

If the SIGNAL flows through a tube for more than 180 degrees but less than 360 degrees ( or between Pi and 2*Pi ) we are in class AB1, and as above, at least two tubes are needed ( the tubes are biased to get rid of cross-over distortion )

If the SIGNAL flows through a tube for more than 180 degrees but less than 360 degrees AND some ( control ) grid current is flowing we are in class AB2.

Part of the above misunderstanding is related to the "cathode bias" method, that, over the years, has come to be ( erroneously ) considered synonymous of "class A", but, as you rightfully noted, some manufacturers have been guilty of claiming their amps as "class A" for marketing reasons, thus generating more confusion on an already confused matter.

The Vox AC30 is the best example of this misunderstanding I can think about.

It' s often referred to as being a "class A" amp, while it's a cathode-biased "class AB1" amp. The four EL84s are biased ( by means of a 51 Ohm cathode resistor ) at about 125% ( idle ) ( some 47 mAmps with some 310 VDC dropping through the tubes ), still , looking at the waveforms, it appears clear that the amp operates in "class AB1".

Should the skeptics need further proof to confirm the above statement :

1 - The output power of an AC30 is between 34 and 36 Watts RMS. ( four EL84s operating in "class A" would yield only some 20 Watts RMS ).

2 - In a true "class A" amp the current swing is almost non-existent ( current through the output tube(s) is almost constant ), while in an AC30 the current swing through the output tubes is significant and very visible.

Cheers

Bob

[Enzo]

It isn't of course all the manufacturer's fault, as Bob mentions, a lot of people find out their amp is cathode biased and think that means ClassA.


In fairness to an amp, I don;t consider an amp NOT class A just because it is possible to drive it into AB land. Just as my mom's car doesn't become a race car just because I tried to drive faster than the guy next to me. If the amp is designed to be used in the class A mode, and it functions in class A most of the time when used normally, that seems good enough. Sometimes my amp functions as a doorstop too.

Your basic class AB amp functions part time in class A by that 360 degree standard. What if I cram 200v of signal into my amp's input, does that make it a class C amp? Surely it can;t pass the majority of that signal.

I guess I draw a distinction between the class of an amp - as in its design - versus the class it operates in moment by moment.

OOps, you managed to push a grid to cutoff... can;t call it a whatever anymore. That seems too hard core. I am not disagreeing with Rhodes' premise, but I am not fanatical either.

[PaoloJM]

It isn't of course all the manufacturer's fault, as Bob mentions, a lot of people find out their amp is cathode biased and think that means ClassA.


In fairness to an amp, I don;t consider an amp NOT class A just because it is possible to drive it into AB land. Just as my mom's car doesn't become a race car just because I tried to drive faster than the guy next to me. If the amp is designed to be used in the class A mode, and it functions in class A most of the time when used normally, that seems good enough. Sometimes my amp functions as a doorstop too.

Your basic class AB amp functions part time in class A by that 360 degree standard. What if I cram 200v of signal into my amp's input, does that make it a class C amp? Surely it can;t pass the majority of that signal.

I guess I draw a distinction between the class of an amp - as in its design - versus the class it operates in moment by moment.

OOps, you managed to push a grid to cutoff... can;t call it a whatever anymore. That seems too hard core. I am not disagreeing with Rhodes' premise, but I am not fanatical either.

To make sense of all of this in my own head it tend to define an amps Class of Operation as the Class it operates in at it's stated power output (Vrms).

Under extremely small drive signals even a Class B amp may meet the criteria for Class A, as it's unlikely that they have absolute zero idle current.
Equally I've scoped my Harley Benton GA5, which is very definitley Class A single ended, and it goes into cut off for part of the signal when the volume is above 7 ish where it's pumping out 7-8 watts. Is it Class AB1 so?

To me Class is only relevent at the amps stated power output.

[mooreamps]

single ended, and it goes into cut off for part of the signal when the volume is above 7 ish where it's pumping out 7-8 watts. Is it Class AB1 so?

I going to say technically, no. Any power amp, in guitar amp land, I would still call that class A. Now, having said that, let me write some comments about classes of operation.

These classes of operation come from radio and transmitter land, where all the signals past the first RF amp gain stage tend to be consistent in amplitude. Therefore, there is "less" confusion if a gain stage is operating in class A, or any of the other classes. If you have a radio frequency transmitter operating, lets say, Class B grounded gird, it will always operate Class B, even regardless of the level of the drive signal.

Now, in guitar amplifier land, the input signal can vary so much, you can have an audio power amp, such as the AC-30, biased as Class A push/pull, but the input signal can become high enough to drive the PA into Class A/B.


-g

[Old Tele man]

...EIA rules specify the class of operation is determined at the condition of maximum claimed power output, not at idle.

...EIA is the standard, any other way is simply "...fudging..."

[balijukka]

are there any class A amps at all ?

Here we go again
We had a huge discussion about this at AX84 last year.
I remember in the end someone found the old operation class specs which say that the parameters include the input signal when determining the class of operation. Meaning that the operation class can change during operation from A to AB if the input changes. The classes are operation classes, not amp classes.
This leads to that there is no such thing that class A amp. (unless you make some kind of limiter into the input that prevents the amp from cutting off)
At least this was the situation back then (1930's)
This was as far we got, then the discussion died.
I'm still not sure if we agreed on anything at all.
Can you point me to that EIA specification.

jukka

[Robert M. Martinelli]

...EIA rules specify the class of operation is determined at the condition of maximum claimed power output, not at idle.

...EIA is the standard, any other way is simply "...fudging..."

A Heart felt +1 ! Succinct and straight to the point!

( much better than I could ever do with my wordy posts )

This is especially true about the Vox AC30 I used as an example in my previous post.....I have heard ( and busted their bubble ) many friends of mine saying "the AC30 is class A" or "it works in class A at lower volumes", just because they don't know neither the differences between the various classes of operation nor the standards to comply with when testing or determining the class of operation.

Cheers

Bob

[PaoloJM]

...EIA rules specify the class of operation is determined at the condition of maximum claimed power output, not at idle.

...EIA is the standard, any other way is simply "...fudging..."

Who is the EIA?
It seems that I am in agreement with them whoever they are.

[Steve Conner]

The Electronics Industry Association (of America) I guess. Other standards apply in other countries, such as DIN or IEC.

I agree with this method of rating, even though it throws up some oddities. For instance, the Vox AC30 could be made into a true Class-A amp by just renaming it to the AC22.

It's also interesting to consider single-ended amps. Class-B operation requires two devices in push-pull, so a single-ended amp goes straight from class-A to class-C when it's overdriven.

[balijukka]

For instance, the Vox AC30 could be made into a true Class-A amp by just renaming it to the AC22.
.

LOL, that's a good one

jukka

[balijukka]

A jewel

Here's a jewel from Rick Campbell :

A couple more definitions for the list:

"True Class A" -- Same as "Class A" but costs twice as much.

"Pure Class A" -- Same as "True Class A" but costs ten times as much and contains components made of Unobtanium.

jukka

[Zoe_N_Iain]

The way I always think of it is simple. See what maximum power you can get out of 2 valves in parallel single ended operation. Now connect them in push-pull instead (with an appropriate load impedance, of course). Does the resulting power amp put out exactly the same power as, and no more than, the PSE one? No matter how hard you drive it? If yes, it's class A. If no, it's AB1 to some varying degree. Simples.

[Steve Conner]

So it should have been the AC12?