True Class A Amplifiers

[Smilzo]

You flatter me. I would call myself an ever-learning electronics enthusiast and hobbyist. I know enough to be dangerous. Far from a trained engineer (my math skills are not good enough ).

The way I understand it, the line between class A and class A/B can blur depending on how hot the amp is biased. If your definition of class A is that the tube is always in conduction, push-pull amps can be biased to operate in class A. I believe cathode biased amps like the Vox AC 30 and Tweed Deluxe can walk that line. With small input signals, the bias point can be high enough that the power tubes may not enter cutoff and it's effectively running in class A. But as the input signal gets bigger the tubes start to enter cutoff and class A/B operation happens.

I remember discussing class with Mr. Aiken. It's just a label for early days of tubes, when input signal were known. Class A is when tube conduct for all the cycle. Class B only half. Class AB between. So when we apply a guitar signal that goes into cutoff or interdiction, we are in class AB or B. Even with SE topology. Class doesn't mind if there's another tube, that's topology.

 

[Smilzo]

Most "Class-A" guitar amps are really cathode-biased Class-A/B.

Class-A means that the tubes never go into cutoff. This can be accomplished with both single-ended designs and with push-pull designs.

A push-pull Class-A tube amp is completely possible although the efficiency is poor and keeping the tubes in the SOA can be challenging.

Negative feedback is irrelevant although most amps marketed as Class-A have no feedback. That 20W Marshall Plexi is cathode-biased and has negative feedback.

As far as touch sensitivity I dunno. One aspect of a single-ended design is that the output transformer has a DC bias. This means you're never operating in that region where the magnetic domains are "sticky". The BH curve at the origin is quite nonlinear and many consumer grade transformers exhibit a type of crossover distortion at low excitation levels. A single-ended design won't do that because of the bias. That's part of what makes a Marshall sound the way they do though.

I think SE amps are "touch sensitive" because of resistance cathode bias. There's a squish. Also most the amps marketed as Class A are resistance cathode bias. Hence "touch sensitive".
I have lots of SE amps, and built lot's of them, cause I like the "vintagey" rock n roll vibe, even with hi gain preamps (Peters Amp, for example). Mr Peters told me to pull out a tube in his PP amps, rebias to taste, and see if I Like the sound: the OT should be capable of handle currents.

I would like to see some more SE topology amps...

 

[Chromatizm]

I remember discussing class with Mr. Aiken.

After reading https://www.aikenamps.com/index.php/the-last-word-on-class-a I'm not sure anything in guitar amps with more than one tube is class A ) So right now we surely have only 5F1 and 5F2 as real Class A
But he also says that two tubes can work in Class A mode if you bias them correctly.

The relatively low output rating of 20 watts from a pair of EL34's in the SV20H makes me think it could possibly be class A (less efficient)

After a few hours of googling I found someone mentioning that SV20H works as a Class A in 5w mode. Somehow it uses only one tube in this mode, rendering it "A". We don't have this model and reducing the wattage doesn't seem to be a proper replacement.

I'd wager those are all hot biased class A/B much like the AC 30, but I don't know for sure.

This is the most confusing part for me.
So these "A -> A/B" class amps operate as "A" under some load and then go into "A/B". When this transition happens? They are "A" until Gain at 3? 5? 7? Then they go into A/B? What if we push the input with the pedal or engage Boost or Saturation in the amp?

I'm so excited to try it all and hear the difference myself but really need someone's guidance on which amps a REAL "Class A".

 

[laxu]

Thanks for the clarifciations! The overcoming of crossover distortion is one of the reasons that Gjika outlined as to why he went with a Class A design. Another one being the "always ready" mode that Class A amps are constantly in. He used the frying pan metaphor: Class A is always hot and ready to cook even before the butter hits the pan, whereas Class A/B and Class B amplifiers are likened to a cold frying pan, always waiting for the butter to hit the pan before getting hot. I could potentially believe that this might be beneficial for an "immediate" tonal response.

Would you say his 4xKT77 power amp section is basically a push-pull Class A amp? I know he mentioned that he runs the power tubes at their limit and has to reject a lot of them in order to find a quartett that can put up with the high voltages and currents that he puts them through. Could have something to do with the difficult to achieve safe operating area. The thing that's confusing me is his single ended amplifier label, even though he's in fact using a 4xKT77 power amp section. That would speak against a push/pull design as you make a distinction between single ended and push/pull, no?

Most likely the Gjika is a parallel single ended amp taken to the max. Parallel single ended up amps run multiple tubes in parallel for higher output, but afaik they are way less efficient than a class AB design. So more noisy, hot, less output for the hardware used etc.

Bob Gjika is very vague about his amps' specs and it seems quite a bit of heavy lifting is done simply by the preamp. The Gainmaster box you get with it seems to be basically a pretty barebones tube gainstage and overall it's most likely a pretty simple circuit paired with a unique, high-ish output poweramp.

Based on all the (mostly crappy phone recording) clips I've heard of his amps, the soundwise closest Fractal model we might have is the Komet 60. It's an amp that tends to sound "tubby" unless you are playing single coil guitars, in which case it sounds just the right amount of "fat" and makes for a pretty great lead amp. Note that I'm not saying the Komet is in any way similar design, just that it sounds similar.

 

[Smilzo]

After reading https://www.aikenamps.com/index.php/the-last-word-on-class-a I'm not sure anything in guitar amps with more than one tube is class A ) So right now we surely have only 5F1 and 5F2 as real Class A
But he also says that two tubes can work in Class A mode if you bias them correctly.

Yes, bias them hotter and make sure they conduct all the time. When tube break up you've reached the limit of class A. Every tube amps can run in class A. Push-pull have some harmonics cancellations when driven. No magik!

 

[Seanhanlonguitar]

Well I don’t know anything about this class A A/B talk… but I have gotten to play one of Mr Gjika‘s amps. Not my style, however it was an incredibly unique and responsive amp. I’ve never played anything else that was like that. As to why that is… no clue

 

[Joker]

Well I don’t know anything about this class A A/B talk… but I have gotten to play one of Mr Gjika‘s amps. Not my style, however it was an incredibly unique and responsive amp. I’ve never played anything else that was like that. As to why that is… no clue

What amp in the Axe FX III would you say comes closest to the Gjika amp? Was it the powers of ten amp that you played or was it one of his 3 channel designs?

 

[mr_fender]

This is the most confusing part for me.
So these "A -> A/B" class amps operate as "A" under some load and then go into "A/B". When this transition happens? They are "A" until Gain at 3? 5? 7? Then they go into A/B? What if we push the input with the pedal or engage Boost or Saturation in the amp?

A tube has a certain range of conduction based on the level of the input signal. The upper limit of that range is the saturation point where the tube starts to compress and clip the signal but current still flows. The lower limit of that range is the cutoff point where the tube stops conducting completely. The bias point of the tube is the spot in that range you offset the center point of the input signal.

Found these online that illustrate it nicely:


Class A gives the truest reproduction of the input signal, but is the least efficient due to the wasted power of all of that idle current (no input signal).





Class A/B is a compromise between A and B that is more efficient that class A since less idle current flows but also has less crossover distortion than class B.

All class A/B amps can run in class A as long as the input signal is small enough to not make the tube enter cutoff. How high the bias point is set (how hot it's biased) determines where that transition between class A and class A/B happens. For hot biased amps like the AC 30, more of the input signal range falls in the class A range, but not all of it, so it's still technically a class A/B amp.

 

[Smilzo]

Most likely the Gjika is a parallel single ended amp taken to the max. Parallel single ended up amps run multiple tubes in parallel for higher output, but afaik they are way less efficient than a class AB design. So more noisy, hot, less output for the hardware used etc.

I've built a couple of parallel SE amps years ago. In a guitar amp the poweramp noise is neglettable, the noise come for the first stages of preamp and insane gain amount. A good powered SE EL34 amps could keep up with a loud drummer just at the edge of poweramp breakup. If you want crystal clean sound you have to lower poweramp input level.... If you add a second parallel tube, level hardly goes up (in theory 3 db, to my ears like 1 or 2 db). The only bonus I saw was mixed tubes (a KT88+KT66, or 6L6+KT90, subtle but the feel change is rewarding). The drawaback was the OT... they were HUGE, a 20w SE was like a 100w PP, pricey and lots of hum radiation. So I stopped building them, a couple of PP 6V6 was louder and lighter, and sound great for blues and rock...
The lack of sag in SE probably make them snappier. Lots of hungry attack. They growl, while PP roars....

 

[Seanhanlonguitar]

I’m

What amp in the Axe FX III would you say comes closest to the Gjika amp? Was it the powers of ten amp that you played or was it one of his 3 channel designs?

not exactly sure which amp I played. Nothing in the axe fx reminded me of it; and I only remember playing one channel. It may have been a one channel amp, but I typically gravitate to one medium gain channel on any amp.

 

[Cooper Carter]

Great stuff @2112. Santiago's bicycle analogy is fantastic.

 

[Admin M@]

Santiago Alvarez designed the JVM410HJSB. We've emailed back and forth a bit with him while working with Joe Satriani. Great guy.

 

[Dave Merrill]

So it's settled then, one of you folks is going to lend Cliff your Gjika and get it modeled on the Axe, right?

 

[Joker]

Based on the postings of Smilzo, this seems like a prime opportunity for a truly digital amp model, one that transcends the physical realm. I'm thinking of a 100W true Class A amplifier, not limited by huge and costly physical transformers, failing extra hot biased tubes and exorbitant electricity bills. From what I've read so far, anything Class A above 10W output power gets a bit tricky and very costly in the real world, but since the Axe FX is not constricted to the real world, we could maybe play the first "impossible", yet real sounding Class A amp.

I recognize that Cliff has probably done these "impossible" mods to create the idealized FAS Crunch, Lead etc. models. I wonder if the currently available, idealized FAS Class A amp model can be manipulated enough through the advanced settings to always remain in Class A mode, no matter how hard it's driven...

 

[IronSean]

Would you say his 4xKT77 power amp section is basically a push-pull Class A amp?

I can't speak to that amp, but you can run tubes in parallel instead of opposite (push-pull) also. Any normal Push-pull 100w amp with 4 power tubes it doing this too, 2 tubes are pushing in parallel, 2 tubes are pulling in parallel. No reason you couldn't have all 4 push if you wanted single-ended, other than the effeciency concerns making it typically a bad compromise.

 

[Jonathan Thomas]

Laney Lionheart L50H with its 5x EL34 power tubes is probably the king of Pure Single ended Class A amplifiers, IMO. Let's get one in the Fractal library!

 

[mr_fender]

Oof. Five EL34's running class A for just 50 watts. That's horribly inefficient. All that idle current is going to wear those tubes faster than class A/B would as well. That will get pricy over time.

Luckily in Fractal land we are immune to wasted power and tube wear.

 

[Greg Ferguson]

Oof. Five EL34's running class A for just 50 watts. That's horribly inefficient. All that idle current is going to wear those tubes faster than class A/B would as well. That will get pricy over time.

That’s the downside of Class-A.

The Mesa Lonestar Special has a single EL84 outputting 5 watts for its low-power setting, and it sounds great, but breaks up really quickly. That and it’d burn through tubes keeps me from using the setting, well, and because it sounds really good in the normal 15 and 30 watt settings.

Luckily in Fractal land we are immune to wasted power and tube wear.

Definitely a benefit.

 

[2112]

Great stuff @2112. Santiago's bicycle analogy is fantastic.

You know someone has really mastered their craft when they can break down complex ideas so beautifully!

 

[Smilzo]

Oof. Five EL34's running class A for just 50 watts. That's horribly inefficient. All that idle current is going to wear those tubes faster than class A/B would as well. That will get pricy over time.

Luckily in Fractal land we are immune to wasted power and tube wear.

Strange enough, my experience is the opposite. I never had a SE power tube failure (they wear out, I changed them for the sonic and volume properties...). I had a lot of PP power tube amp failure. It could be the fixed bias, or the peak voltage, or the thermal cycle... I dunno! Guitar tube amps runs at about max dissipation, for SE is about 10w (for an EL34), for PP is 25w. When you are within limits, tube should last their expected life.