Examples of Class A push/pull

[Retrovert]

Well, please enlighten me Sir. I have reworked many of the original versions, and some lesser work on the reissues. And every piece of audio gear that comes across my bench sees a scope/Tek THD+N/IMD distortion analyzer.

Not to step on Douglas' toes, but as he's likely still sleeping here's a brief response.

The Vox AC30 is notorious for being fast-and-loose with the Class A specifications, and being marketing Class A (i.e. actually Class AB) instead of engineering Class A (i.e. actually Class A).

I would point to Randall Aiken's excellent analysis, complete with plots:

https://www.aikenamps.com/is-the-vox-ac-30-really-class-a
Is the Vox AC-30 Really Class A?
by Randall Aiken

There is a lot of discussion on the topic of class A amplifiers. The Vox AC-30 is always brought up as an example of a class A amplifier, but is it really class A?

...

The Vox AC30 has a 4K p-p OT, and if you look in the RCA receiving tube manual you will see that 8K is recommended for the plate-to-plate load in class AB at 300V for two tubes, so the optimum class AB loadline would be 4K for four tubes as it is in the AC-30.

Here's a plot of the 8K loadline for two tubes (which is the same as 4K for four tubes as in the AC-30). As you can see, it is a perfect textbook class AB loadline right through the "knee" of the plate curves, and right along the max dissipation curve (the green curve) as it would be for maximum power in class AB without exceeding the plate dissipation. This proves that the AC-30 was designed for the textbook class AB output transformer specifications as outlined in the tube manuals of the day.

The blue line shows the class B loadline, the red line shows the class A loadline. Note that the transfer from "class A" to "class B" occurs at the crossover point of the two lines, but due to the way push-pull class AB works, it would actually be a smooth, curved transition.

...

In order for an amplifier to be "true" class A, the output must be biased in the center (the most linear portion) of the operating region of the output tubes. However, you can't just jack up the bias on a class AB amplifier to make is class A, because the output tubes will exceed their dissipation limits, and the amplifier will fail. The only way to make the amplifier true class A is to reduce the plate voltage and readjust the bias voltage/current to properly center the output stage operation in the correct portion of the transfer characteristics of the output tubes. It turns out that EL84 tubes can take up to around 250V maximum and remain in class A operation. If this plate voltage is exceeded, the dissipation will have to be set too high to keep the tubes out of cutoff at all times, and they will fail.

The circuit below shows the same AC-30 output stage, but instead of running at 320V, it is running at 250V. The common cathode resistor was changed from 50 ohms to 31 ohms in order to bias the amplifier back up to near the midpoint of it's range (49mA per tube at this plate voltage). This rebiasing resulted in a change in quiescent cathode voltage from 10.5V to 6.8V, which means that the amplifier will clip much sooner, so the input signal had to be reduced from 0.86V p-p to 0.0.51V p-p to drive the output stage to a point just prior to clipping. This results in a quiescent plate dissipation in the output tubes of (250V-6.8V)*49.2mA) = 12Watts (right where it should be for max power in true class A operation).
​

 

[Retrovert]

This sort of complexity is, again, why I suggested the OP read some original sources.

This will turn into very complex analysis and without the basics it will be difficult to make heads or tails of it.

I suggest awaiting further analysis from Douglas when he is properly caffeinated. I know I'm certainly far enough from that point to feel like Staten Island Chuck being held by Mayor de Blasio on Groundhog Day.*


* A New York joke. The mayor, in an impressive burst of his typical incompetence, dropped the groundhog, always named Charles G. Hogg (dimunitive Chuck) by convention, but actually Charlotte in this case, on the ground, causing her demise the following week. His PR team then lied about it because of "bad optics". Nearly all New Yorkers feel like Charlotte.

 

[Bill Ferris]

Not to step on Douglas' toes, but as he's likely still sleeping here's a brief response.

The Vox AC30 is notorious for being fast-and-loose with the Class A specifications, and being marketing Class A (i.e. actually Class AB) instead of engineering Class A (i.e. actually Class A).

I would point to Randall Aiken's excellent analysis, complete with plots:

https://www.aikenamps.com/is-the-vox-ac-30-really-class-a
Is the Vox AC-30 Really Class A?
by Randall Aiken

There is a lot of discussion on the topic of class A amplifiers. The Vox AC-30 is always brought up as an example of a class A amplifier, but is it really class A?

...

The Vox AC30 has a 4K p-p OT, and if you look in the RCA receiving tube manual you will see that 8K is recommended for the plate-to-plate load in class AB at 300V for two tubes, so the optimum class AB loadline would be 4K for four tubes as it is in the AC-30.

Here's a plot of the 8K loadline for two tubes (which is the same as 4K for four tubes as in the AC-30). As you can see, it is a perfect textbook class AB loadline right through the "knee" of the plate curves, and right along the max dissipation curve (the green curve) as it would be for maximum power in class AB without exceeding the plate dissipation. This proves that the AC-30 was designed for the textbook class AB output transformer specifications as outlined in the tube manuals of the day.

The blue line shows the class B loadline, the red line shows the class A loadline. Note that the transfer from "class A" to "class B" occurs at the crossover point of the two lines, but due to the way push-pull class AB works, it would actually be a smooth, curved transition.

...

In order for an amplifier to be "true" class A, the output must be biased in the center (the most linear portion) of the operating region of the output tubes. However, you can't just jack up the bias on a class AB amplifier to make is class A, because the output tubes will exceed their dissipation limits, and the amplifier will fail. The only way to make the amplifier true class A is to reduce the plate voltage and readjust the bias voltage/current to properly center the output stage operation in the correct portion of the transfer characteristics of the output tubes. It turns out that EL84 tubes can take up to around 250V maximum and remain in class A operation. If this plate voltage is exceeded, the dissipation will have to be set too high to keep the tubes out of cutoff at all times, and they will fail.

The circuit below shows the same AC-30 output stage, but instead of running at 320V, it is running at 250V. The common cathode resistor was changed from 50 ohms to 31 ohms in order to bias the amplifier back up to near the midpoint of it's range (49mA per tube at this plate voltage). This rebiasing resulted in a change in quiescent cathode voltage from 10.5V to 6.8V, which means that the amplifier will clip much sooner, so the input signal had to be reduced from 0.86V p-p to 0.0.51V p-p to drive the output stage to a point just prior to clipping. This results in a quiescent plate dissipation in the output tubes of (250V-6.8V)*49.2mA) = 12Watts (right where it should be for max power in true class A operation).
​

Well, it would appear that I`m incorrect in that the Vox AC 30 is a class A tube amplifier, though that amplifier draws considerable current at idle when operating correctly, much more than any quad configured EL 84`s amplifier that had ever crossed my bench, both musical instrument, and home integrated tube amplifiers.

My information about it`s class of operation was first presented to me back in the mid seventies by respected people long term employed as technicians in the music equipment repair business, that at that time in my early days of being employed as a bench tech. (the public "interwebs" were not yet a twinkle in anybody`s eyes, so when multiple seasoned( decades of experience) guys convey these statements as fact, who was I to challenge them, I was trying to learn all that I could in the audio world and never thought to explore in depth that particular amplifier later on .
Oh well, such is life.
Thanks for debunking that for me retrovert, and Douglass, you, as usual, were correct Sir. and I`m humbled ..

Blind Faith, I guess !?!?

Take care, Billy "Ferrous Oxide"

 

[Retrovert]

An easy way to spot deception about Class A amplifiers is to examine the size and weight of the output transformer.

A true Class A output transformer must cope with DC in the winding, and the core is consequently considerably larger and heavier, and the overall transformer is thus more expensive, than a Class AB — or Class B for that matter — output transformer.

I'm sure ChiFi will soon begin placing output transformers in oversized sheet-metal cans filled with concrete to deceive customers, but until that point the size and weight are a reasonably good test.

 

[PakProtector]

If y'all want to take on some analysis, examine the Atmasphere claims. 42 6AS7G channel. 500W delivered to between 2 and 16 Ohms. At a 2 Ohm load that is nearly 16 Amperes delivered. That is the rms value...multiply by root2 for its peak( 22.3 A ).

With a Class A amp, idle current is effectively the same as full power current. I call BS on Class A.
cheers,
Douglas

 

[Bill Ferris]

An easy way to spot deception about Class A amplifiers is to examine the size and weight of the output transformer.

A true Class A output transformer must cope with DC in the winding, and the core is consequently considerably larger and heavier, and the overall transformer is thus more expensive, than a Class AB — or Class B for that matter — output transformer.

I'm sure ChiFi will soon begin placing output transformers in oversized sheet-metal cans filled with concrete to deceive customers, but until that point the size and weight are a reasonably good test.

That the large and heavy power, output, & choke pieces of iron in those AC 30`s made me believe that they are class A, a long with their higher idle current, that didn`t seem to vary much when load testing !!
And they ran very hot, as well..
Maybe biased at AB2 + ? Of my rather large vintage guitar/amplifier collection, I never acquired a AC 30 for it, they were always too pricy for my purchase consideration, when available so to explore !!( being partially left side paralyzed, can`t play a guitar !)

I became so busy later on with 5 music store`s repairs(10:OOAM ~ 3:OOAM self employed personal bench time) that there was no time to explore/confirm the validity of the class A claim, as reliable repair productivity( quick turnover) was paramount..

 

[Retrovert]

Randall's explanation, including math and plots, makes it clear the Vox AC30 is a Class AB amplifier.

All AB2 would mean is grid current for higher output. That wouldn't fundamentally change from actual Class AB configuration to the claimed Class A.

 

[dcgillespie]

And the grid current that Retrovert mentions can only be created by a driver stage that can supply that current, which the driver stage in the AC-30 cannot.

Dave

 

[Pio1980]

Well, it would appear that I`m incorrect in that the Vox AC 30 is a class A tube amplifier, though that amplifier draws considerable current at idle when operating correctly, much more than any quad configured EL 84`s amplifier that had ever crossed my bench, both musical instrument, and home integrated tube amplifiers.

My information about it`s class of operation was first presented to me back in the mid seventies by respected people long term employed as technicians in the music equipment repair business, that at that time in my early days of being employed as a bench tech. (the public "interwebs" were not yet a twinkle in anybody`s eyes, so when multiple seasoned( decades of experience) guys convey these statements as fact, who was I to challenge them, I was trying to learn all that I could in the audio world and never thought to explore in depth that particular amplifier later on .
Oh well, such is life.
Thanks for debunking that for me retrovert, and Douglass, you, as usual, were correct Sir. and I`m humbled ..

Blind Faith, I guess !?!?

Take care, Billy "Ferrous Oxide"

What was the idle current?

 

[Bill Ferris]

What was the idle current?

Well Pio1980, the last AC 30 that crossed my bench was almost 2 decades ago, and I`m a little hazy, but I suspect that at 120 volts, it idled around 1.4~.5 amps as indicated on my variac`s amp meter.
I don`t know any owners of Vox AC 30`s anymore, or I would invite them to bring their amp by for a free checkup to refresh my memory..

Back in the early to mid nineties, there was some kind of a pilot exchange program going on at a nearby Naval Air station, and several British pilots who played guitar & owned original AC 30`s were recommended to me for going through their guitar amps by a local Navy Pilot who I knew, who played Fender amps.
And the last reissue AC 30, that I recall, that I worked on, was in the early 2001. and I retired from public electronic repair in 2002.

Maybe 20 Vox amps have crossed my bench total, and infrequently at that, in my 44 years of active repair, they didn`t seem to be a popular, or common tube guitar amplifier in FL. both north and south, I never felt the need to keep service notes, just receipt copy`s for warranty reference.

 

[Audiovet]

I consider it better to study a more complete treatise on something, at first skimming over 'unattractive' mathematical details so as to get the overall picture, than to study a more brief and possibly incomplete version and never have the chance to progress into the 'unattractive' parts for a full understanding..

 

[arts]

What was the idle current?

IIrc,it was something like 50-60mA per tube on the ''original'' version.While it has always been considered a class ''A'' design,it is not.
It is class AB1,but biased rather hot. I think many people erroneously associate cathode biased output stages with class A.

 

[Pio1980]

IIrc,it was something like 50-60mA per tube on the ''original'' version.While it has always been considered a class ''A'' design,it is not.
It is class AB1,but biased rather hot. I think many people erroneously associate cathode biased output stages with class A.

Like EL34s.
How long do they last like that?

 

[arts]

Like EL34s.
How long do they last like that?

They're actually not that bad on power tubes,but you do need quality. While the old tubes generally died of exhaustion,the reissues generally die of shorts or go gassy.Seems like most of the modern ones can't take the heat OR the vibration.

Re the cathode current; The EL84/6BQ5 has (for the size of the tube) a rather large cathode,allowing for a higher current level than would otherwise be considered normal.For the originals,65mA is the stated max,so as long as you respect the max plate dissipation,go for it.The reissues,meh,not so much.

 

[Pio1980]

They're actually not that bad on power tubes,but you do need quality. While the old tubes generally died of exhaustion,the reissues generally die of shorts or go gassy.Seems like most of the modern ones can't take the heat OR the vibration.

Re the cathode current; The EL84/6BQ5 has (for the size of the tube) a rather large cathode,allowing for a higher current level than would otherwise be considered normal.For the originals,65mA is the stated max,so as long as you respect the max plate dissipation,go for it.The reissues,meh,not so much.

So, Blackburn Mullards, if you can find them?

 

[Bill Ferris]

Like EL34s.
How long do they last like that?

The Vox AC 30 employs EL 84, not EL 34 output tubes, Pio1980
Back in the day, with quality tubes, and moderate playing levels, I`m guessing the outputs could last more than a year & half, to 2 years, depending how frequently the amp was played, probably a lot less if cranked up often.

 

[Retrovert]

How long do they last like that?

Back in the day, with quality tubes, and moderate playing levels, I`m guessing the outputs could last more than a year & half, to 2 years, depending how frequently the amp was played, probably a lot less if cranked up often.

That's an eternity in the guitar amplifier world!

Because guitar amplifiers routinely push tubes beyond the design limits—to excessive bias, and beyond!—the owners have become trained — indoctrinated might be a more accurate wording — to generally view tubes as a short-lived consumable, whose lifespan is to be amortized across a small number of gigs. Tube lifespan tends to not be an issue. I have known of guitarists who were proud of the number of tubes destroyed as it somehow proved aspects of playing.

(in best Bill & Ted voice)
"Bill, like, the amplifier has not yet been fabricated that can keep up with the Buddha nature of my most heinous shredding and absorb it's totally massive and awesome power!"
"I know Ted, but, like, output tubes have recently become most heinously expensive, and the Chinese ones, like, only last, like, three days, and my dad's telling me that his HiFi tubes last, like, years, and he's totally cutting back on my allowance, and we're never going to save the world if we can't overdrive a poorly designed Class AB amplifier with excessive bias to achieve distortion of the gods."

 

[arts]

The Vox AC 30 employs EL 84, not EL 34 output tubes, Pio1980
Back in the day, with quality tubes, and moderate playing levels, I`m guessing the outputs could last more than a year & half, to 2 years, depending how frequently the amp was played, probably a lot less if cranked up often.

Actually Bill,I think he was comparing the cathode currents those used with EL34's. Now that I think about it,it's kinda scary

 

[Pio1980]

The Vox AC 30 employs EL 84, not EL 34 output tubes, Pio1980
Back in the day, with quality tubes, and moderate playing levels, I`m guessing the outputs could last more than a year & half, to 2 years, depending how frequently the amp was played, probably a lot less if cranked up often.

Yes, I was making a comparison to a common larger tube. I think my Marantz runs its outputs ca 50ma.
I dunno what something like a ST-35 runs the smaller EL-84. Console amps are pretty easy on them.

 

[arts]

So, Blackburn Mullards, if you can find them?

Yeah,they would be an excellent choice,but I can't think of too many guitar players I would waste them on