"Class A" nonsense

[Jazz P Bass]

Conner
What was the application on the TPS11165?
Quote: "The TPS61165 is available in a space-saving,
· 2mm ´ 2mm QFN package with thermal pad."
Thats a small IC!

[Robert M. Martinelli]

Hmmm.....
Let's put it in another way....
You'll all forgive me if I keep on using my '64 AC30 as an example, but, having scoped the waveforms in the past, and having kept the results, I can give some real-life figures.

With a signal generator set to 1 Khz ( Sine ) and adjusting the signal's amplitude so that the amp doesn't clip, I've verified that, over the 2*Pi period, each couple of tubes stays some 6/5*Pi on, some 3/5*Pi off ( IDLING at some 48 mAmps, but OFF signal-wise nonetheless, and that's the thing to consider while evaluating the class of operation ) then on for the remaining 1/5*Pi. Being the tube(s) on for more than Pi, but appreciably less than 2*Pi, the class of operation is undeniably "AB1".

To keep the tubes out of the ( signal ) cutoff region at all times so that the SIGNAL flows continuously through the tubes ( thus achieving true "class A" operation ) the tubes' operating point should be considerably moved, and in these new conditions it would only be possible to get some 20 W RMS out of four EL84/6BQ5 ( BTW that's exactly four times the power an EL84 SE output stage yields with a single tube, as ZnI noted ).

Cheers

Bob

P.S. ( off topic ) I'd like to build a fixture for my digital camera to take snapshots from the screen of my TEK2335, has someone tried to do something similar already? I suck as a tube amp tech, but I confess I'm even worse as a photographer....

[hasserl]

...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..."

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.

The "maximum claimed power output" should probably be "maximum undistorted power output". That would disqualify the AC30, and would prevent the situation of just claiming lower power output.

[hasserl]

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.

Well, that may work for you, but I think you're the only one it works for. That's not the definition of operating class.

[Robert M. Martinelli]

....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?

Hi Paolo, I was re-reading this post of yours, and I think that by saying that your amp went into CUT OFF ( not conducting ) you actually meant that your GA5 was CLIPPING ( squashing/squaring the waveform ) with the volume above 7.

Being a SE design, your amp is "per se" "class A", as a single tube ( or more than one tube in parallel ) can't go into cutoff at any time ( it has to be on during the whole period to amplify the entire waveform appearing at its grid ).

Cheers

Bob

[Albert Kreuzer]

Hi Paolo, I was re-reading this post of yours, and I think that by saying that your amp went into CUT OFF ( not conducting ) you actually meant that your GA5 was CLIPPING ( squashing/squaring the waveform ) with the volume above 7.

Being a SE design, your amp is "per se" "class A", as a single tube ( or more than one tube in parallel ) can't go into cutoff at any time ( it has to be on during the whole period to amplify the entire waveform appearing at its grid ).

I think he is talking about asymmetrical clipping. But as we are speaking of undistorted output, this doesn't apply here.

You'll all forgive me if I keep on using my '64 AC30 as an example

I forgive you

Cheers,
Albert

[Robert M. Martinelli]

I think he is talking about asymmetrical clipping. But as we are speaking of undistorted output, this doesn't apply here.



I forgive you

Cheers,
Albert

Yeah, that's what I thought, I definitely think he was talking about clipping and not cutoff.

Thanks for your forgiveness, Albert....I' m always afraid of boring people by keeping on talking 'bout my beloved AC30

OK, now let's start talking 'bout my '66 Vox UL4120 instead

( just kidding )

Cheers

Bob

[Albert Kreuzer]

OK, now let's start talking 'bout my '66 Vox UL4120 instead

I'd like to, but that would be worth a thread of its own

Cheers,
Albert

[Old Tele man]

The "maximum claimed power output" should probably be "maximum undistorted power output". That would disqualify the AC30, and would prevent the situation of just claiming lower power output.

...ah, not really true, because almost ALL musical instrument amplifiers are rated with around 5% THD because of the "nature of the (rock-n-roll) beast" and the fact that 5% THD is the amount that is "...barely perceptible..." to most listeners.

...so, the wording "...maximum undistored power output..." cannot really apply to musical amplifiers, which is our subject. They (manufacturers) know there's 5% THD in the mix, which lets them: (a) claim slightly more output power and (b) account for the manner in which most (but certainly not all) amplifiers are actually used and operated.

...(tongue-in-cheek observation: ah, but that is a very good kind of 5% THD in the hands of most blues players!)

[Enzo]

And by that undistorted standard, there would be plenty of guitar amps that would have to have a zero watt rating, as there is no level at which the signal is undistorted.

[mooreamps]

It's also interesting to consider single-ended amps. Class-B operation requires two devices in push-pull,

Class B does "not" require two devices in push-pull.
Class B is the condition of bias, such that, the tube is biased right at cut-off. This is a bias condition much lower in voltage, than a stage biased for Class A.

so a single-ended amp goes straight from class-A to class-C when it's overdriven.

Again, Class C is the condition of bias, such that, the power tube is biased far below cut-off, which again the power tube is not operating at the same condition as being biased for Class A.

I don't know why this is just so hard for some to understand ; and ; has absolutly "nothing" to do with the level of the input drive signal.

-g

[balijukka]

[QUOTE=mooreamps;119174
has absolutly "nothing" to do with the level of the input drive signal.

-g[/QUOTE]

A direct quote from IRE standards 1938:

1E69. Class A Amplifier.* A class A amplifier is an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows at all times.
1E70. Class AB Amplifier. A class AB amplifier is an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows for appreciably more than half but less than entire electrical cycle.

according to this the input signal does matter.
The class would be a step funtion of the input signal.

At least we all do know what sells best.

jukka

[balijukka]

That asterisk after class A, refers to 1 or 2 according to grid conduction.
too lazy to type it all.

jukka

[Chuck H]

We need to consider that input levels on amps such as guitar amps can vary greatly. Evidenced by the fact that Fender never intendeed their tweed to BF era amps be overdriven, but you can't turn them above four on the volume control without distortion when using a humbucker. So lets assume the input signal is the responsibility of the user and, for most designs, is attenuated before clipping by the amps volume control. Then we have to fall back to the amps indicated (rated) output power in RMS. No consideration of any other parameter including input signal needs to be examined in this case. If an amp does not drive the tubes into cutoff prior to when it's rated output power is achieved we can consider the amp class A regardless of weather it is pushed into AB at a greater output.

Just for giggles... Robert, how much does your AC30 put out before cutoff???

Chuck

[mooreamps]

I'm going to bury this once and for all.

Classes of bias for EL-84 {approx}

Class A -7.5 volts vdc
Class A/B 2 tubes in push/pull -11.7 volts vdc
Class B 2 tubes in push/pull -14.7 volts vdc
Class C 1 tube in grounded grid -30 volts vdc


These classes of operation were written ; as a tube operates within a radio frequency transmitter ; and not how they are used in a FKG guitar amp.'



-g