# Class A v. Class AB Amps



## Younggun (Jul 2, 2008)

I don't know much about tube amps. Can anyone explain to me in "simple terms" the difference between Class A and Class AB amps or a combination of the same? Thank you for your help.


----------



## Greg Ellis (Oct 1, 2007)

"Class" in this sense refers to the class of tube operation - i.e. it's a description of the electrical operating characteristics of the tubes in the amp. There's lots of info out there if you're curious - try google.

Class AB amps typically used a matched pair of output tubes (or two matched pairs) with a circuit that inverts the signal phase so that one tube is working on a mirror image of the signal. Figuratively, one tube is "pulling" while the other is "pushing", so these are often called "push-pull" amps.

Class A is typically a single-ended design, where a single tube operates on the entire signal. More than one tube can operate in parallel this way, but that's fairly uncommon.


----------



## Wild Bill (May 3, 2006)

Younggun said:


> I don't know much about tube amps. Can anyone explain to me in "simple terms" the difference between Class A and Class AB amps or a combination of the same? Thank you for your help.


+1 on what Greg said!

Just to add a bit, you can run push-pull in any class with pairs of tubes. Or you can run only one tube in any class. The difference will be in how it sounds and how much power you can get out of them.

Class A with a single tube is the warmest and cleanest tone. This is the classic Champ tone. The exception is the Champ 600. They get more power by running a pair of output tubes.This sounds different. With a single tube, class A is really your only choice with audio, 'cuz any other class would have too much distortion.

Most amps of more than 10 watts or so run pairs of tubes in class AB1. This gives much more power with acceptable levels of distortion. 90% or more of the guitar amps out there are running push-pull class AB1. Using a pair of tubes lets you run each tube for less than the entire cycle of the signal waveform, giving some time for a tube to stop heating from delivering power, cool down and let the other tube finish the work during that time interval.

Another kicker is that Champ style class A amps normally get their bias from having a large cathode resistor on the output tube. It doesn't matter how the tube gets its bias voltage. Some manufacturers try to tell you that their amp runs class A simply because it uses these cathode resistors and any tech knows that's just crap! You can use a bias supply and a trimmer control just like in any typical class AB1 amp and run class A, as long as all the other factors are considered. Vox AC30's are not truly class A even though they use the cathode resistor method. For the power that comes out trying to run class A would fry those EL84's! They run class AB1, using cathode resistors to bias the tubes.

However, cathode biasing has other differences. The sound has a bit of compression and a lot more cool harmonics. The usual example is Neil Young's "Rockin' in the Free World". Myself, I really, really like it!

:food-smiley-004:


----------



## hollowbody (Jan 15, 2008)

Greg Ellis said:


> "Class" in this sense refers to the class of tube operation - i.e. it's a description of the electrical operating characteristics of the tubes in the amp. There's lots of info out there if you're curious - try google.
> 
> Class AB amps typically used a matched pair of output tubes (or two matched pairs) with a circuit that inverts the signal phase so that one tube is working on a mirror image of the signal. Figuratively, one tube is "pulling" while the other is "pushing", so these are often called "push-pull" amps.
> 
> Class A is typically a single-ended design, where a single tube operates on the entire signal. More than one tube can operate in parallel this way, but that's fairly uncommon.


Yup, this is gospel truth. But in an effort to further simplify things for the OP, it comes down to efficiency. A Class A amp is less efficient, as the tubes are running full tilt regardless of volume, etc. The amp generates more heat and is more wasteful in terms of how it converts energy to sound. Class A/B amps are designed to more efficiently turn energy to sound. This is why A/B amps tend to be more powerful than Class A as well.

Long story short, Class A is theoretically a more wasteful design, but many guitarists and audio geeks will swear that Class A amps sound better.


----------



## guitarman2 (Aug 25, 2006)

hollowbody said:


> Long story short, Class A is theoretically a more wasteful design, but many guitarists and audio geeks will swear that Class A amps sound better.


Don't know about better but for sure different. If its the sound your after it for sure is going to be better.


----------



## bcmatt (Aug 25, 2007)

Wild Bill said:


> Vox AC30's are not truly class A even though they use the cathode resistor method. For the power that comes out trying to run class A would fry those EL84's! They run class AB1, using cathode resistors to bias the tubes.


Sorry, I know this topic is always done to death, but I never have fully wrapped my head around it.
So, would you say then that EL84s are never really run in Class A because they wouldn't be able to handle it?

Also, I've heard the 5E3 Tweed Deluxe is a class A amp, and I know it runs 2 6V6s. So, are they running in parallel as a single ended amp? Is it truly class A?


----------



## Wild Bill (May 3, 2006)

bcmatt said:


> Sorry, I know this topic is always done to death, but I never have fully wrapped my head around it.
> So, would you say then that EL84s are never really run in Class A because they wouldn't be able to handle it?
> 
> Also, I've heard the 5E3 Tweed Deluxe is a class A amp, and I know it runs 2 6V6s. So, are they running in parallel as a single ended amp? Is it truly class A?


Not exactly. As Mr. Hollowbody pointed out, in class A the tube is constantly "on" and drawing power. Any tube can be run in any class as long as you understand how to keep it within its ratings. With an El84 it's no different. If you want to run it in class A then you would use a much lower plate voltage and pay attention to how much current the bias is letting flow through the tube. That's why you get less power from the tube. A pair of EL84's might deliver an honest 20-25 watts in class AB1 but with one tube in class A you get maybe 6-8 watts or so.

With the 5E3 Tweed it's really class AB1. The dead giveaway is the rated 15 watt output. That's more than a pair of 6V6's could put out and expect to get more than a month of tube life.They run pushing 400 volts on the plates of the output tubes.

The 5E3 is cathode biased but as I said, HOW you get your bias has got NOTHING to do with what class the tube runs in! If the tubes were running in true class A I would expect about 10 watts or so of output power, maybe. They are not running in parallel. They are in push pull. One tube will conduct for half the signal wave and then the other. Both tubes feed into the output transformer which will combine the two halves and deliver the complete waveform out to the speaker.

Mind you, if you accept the lower power push pull class A has a golden tone! I built a class A amp once with a pair of 6L6's, running about 300 volts on the plates and cathode biased. What a sound! Great for vintage or rockin' blues!

You do need a beefier power and output transformer as well. I've got stuff for a pair of 6V6's in my junkbox. I should maybe build another class A push-pull amp! 'Course, I'm so damned far behind on service and build jobs it probably won't happen for a while.

:food-smiley-004:


----------



## bcmatt (Aug 25, 2007)

Thanks Wild Bill! That helps a lot.
My confusion was stemming from an article that I read earlier this year in Guitar Player about class and for some reason I thought they said that the Tweed Deluxe was real class A (maybe I read too fast and they only said tweed champ).
Anyways, this was causing my confusion as I looked at what I thought was a phase inverter circuit but wasn't allowed to believe.


----------



## Greg Ellis (Oct 1, 2007)

Wild Bill said:


> A pair of EL84's might deliver an honest 20-25 watts in class AB1 but with one tube in class A you get maybe 6-8 watts or so.


Some examples might help the original poster...

The Blackheart Little Giant is a good example of EL84 in Class A. There's only one tube, so it's got to be class A (well, not literally "got to be", but it would sound pretty awful in class B).

Traynor's YCV20 uses a pair of EL84's, and claims 15 watts output using 'Class "A" cathode bias'. Whether the tubes actually run in class A or not is an open question. It's not impossible.

The Peavey Classic 50 uses a quad of EL84s and outputs 50 watts. Clearly a Class AB implementation. There's no way you could get that much power in Class A.


----------



## Guest (Nov 18, 2008)

Greg Ellis said:


> ... Traynor's YCV20 uses a pair of EL84's, and claims 15 watts output using 'Class "A" cathode bias'. Whether the tubes actually run in class A or not is an open question. It's not impossible.


There are a lot of amp manufacturers out there claiming their amps are class A when they can't possibly be. A true class A amp has one output tube. As soon as you run more than one output tube you are no longer true Class A. Mesa's LoneStar Special comes to mind. The regular LoneStar runs a pair of 6L6's and the Special runs a quad of EL84's that can be toggle switched from 30 watts (4 tubes) down to 15 (2) and down to 5 (1). In 5 watt mode it's possible the amp is running in true class A mode but not in the other modes.


----------



## Greg Ellis (Oct 1, 2007)

Yoda said:


> A true class A amp has one output tube. As soon as you run more than one output tube you are no longer true Class A.


I don't think that's true - see Bill's response, above.

Multiple tubes running Class A in parallel or even in push-pull can still be Class A.


----------



## Guest (Nov 18, 2008)

This question has been hashed and rehashed on the FDP over the past 8 or 9 years. I don't pretend to be an amp circuit guru but the guys who are amp gurus have explained it to me on more than one occasion and I still don't fully understand it but they insist true class A can only be done with a single output tube. One of the guy's over there who is best at explaining brain surgery to a layman is Willie The Geezer. I can usually glean at least something from his explanations.


----------



## guitarman2 (Aug 25, 2006)

Yoda said:


> This question has been hashed and rehashed on the FDP over the past 8 or 9 years. I don't pretend to be an amp circuit guru but the guys who are amp gurus have explained it to me on more than one occasion and I still don't fully understand it but they insist true class A can only be done with a single output tube. One of the guy's over there who is best at explaining brain surgery to a layman is Willie The Geezer. I can usually glean at least something from his explanations.



So if this is true does that mean that only very small powered amps could be class A? I can't imagine anything too powerful with one power tube.
I don't understand it much either. I'm just asking.


----------



## guitarman2 (Aug 25, 2006)

Here is an excerpt from the Dr Z site that describes the amp I own.



> Using the classic setup of a quartet of el84's biased close to true Class A at 30 watts, the new Stang Ray tips its hat to classic British tones, but like all Dr.Z amps, has its own unique voice. The preamp uses an EF-86, and it has a 12AX7 phase inverter.


So its saying my amp is biased close to true Class A. I thought an amp would either be Class A or not.


----------



## Gene Machine (Sep 22, 2007)

*electronics def.*

The definition I always used in electronics was:

Class A: the device conducts current 100% of the duty cycle, generally only was used (with transistors mind you) with 1 transistor. 

Class B: the devices conducts current 50% of duty cycle, generally with two.

BUT, I haven't done any design with tubes. So if two tubes could be set up where they are both biased to be conducting current, and the output is the sum of both currents, then yes it would be class A. however, if one tube was pushing one cycle while the other was pulling the other cycle, it would still appear to be a class B operation. This is a wild a$$ guess on my part.

I have an OLD engineering handbook from the 50's, I think I'll go see how the curcuit is set up on a tube amp.


----------



## Younggun (Jul 2, 2008)

Wow! Thanks very much for all the helpful responses. I think I'm sort of getting the hang of it but it certainly isn't as simple as it may at first appear. Great Forum people!:wave:


----------



## Wild Bill (May 3, 2006)

Yoda said:


> This question has been hashed and rehashed on the FDP over the past 8 or 9 years. I don't pretend to be an amp circuit guru but the guys who are amp gurus have explained it to me on more than one occasion and I still don't fully understand it but they insist true class A can only be done with a single output tube. One of the guy's over there who is best at explaining brain surgery to a layman is Willie The Geezer. I can usually glean at least something from his explanations.


Well, every site has its gurus but that doesn't mean they're always right!

I still say that class has nothing to do with how you bias or how many tubes are in the circuit. You might want to point out to Mr. Geezer that many 6L6 data sheets, manufacturers' tube manuals and the ARRL Ham Radio Handbook has published tables since the dawn of tubes that show typical voltages, currents and power ratings, including pairs of 6L6's in push-pull class A.

So if I'm wrong then I've got a LOT of company and about 80 years of history wrong with me! 

I'm not familiar with FDP. Is it a real techie site?

:food-smiley-004:


----------



## Guest (Nov 19, 2008)

FDP = Fender Discussion Page

With some 60,000 members it's the largest Bulletin Board type resource on the net for Fender products. Fender amp gurus abound there.


----------



## Wild Bill (May 3, 2006)

Yoda said:


> FDP = Fender Discussion Page
> 
> With some 60,000 members it's the largest Bulletin Board type resource on the net for Fender products. Fender amp gurus abound there.


6L6-GC Application Data
Class Va Vg2 Vg1 Ia Ig2 Ra S Rk Zout Pout THD Notes
A S/E 350 250 -18.0 54.0 2.5 33,000 5.2 300 4,200 11.0 15 
A S/E (triode) 250 - -20.0 40 - 44 1,700 4.7 5,000 1.4 5 
A P/P 270 270 72.5 8.5 125 shared 5,000 18.5 4.0 
A P/P 360 270 50.0 9.5 250 shared 9,000 24.0 4.0 
A P/P (triode) 325 42.0 375 shared 8,000 6.0 0.6 
AB2 P/P 360 270 -22.5 69.0 6,600 26.5 1.8 
AB1 P/P 450 400 -37.0 116 - 210 5.6 - 22.0 5,600 55.0 1.8 
RF, C (CW) 750 250 -45 100 6 50 
RF, C (AM) 600 275 -90 100 6.5 42.5 
RF, AB1 750 300 -35 15-70 3-8 72 
RF, B 750 0 15-240 120 Input on g2, g1 connected to g2 via 20k ohms

Please note "A P/P", about a third of the way down.

This was copied from here:

http://tdsl.duncanamps.com/show.php?des=6L6GC

Here's a data sheet from an old book:

http://www.triodeel.com/6l6gc_p3.gif

Don't any of those guys at the FDP ever read a textbook? It ends a LOT of technical arguments!

:food-smiley-004:


----------



## guitarman2 (Aug 25, 2006)

Wild Bill said:


> 6L6-GC Application Data
> Class Va Vg2 Vg1 Ia Ig2 Ra S Rk Zout Pout THD Notes
> A S/E 350 250 -18.0 54.0 2.5 33,000 5.2 300 4,200 11.0 15
> A S/E (triode) 250 - -20.0 40 - 44 1,700 4.7 5,000 1.4 5
> ...


It definitely ends any argument from me. I don't understand a dam thing in those numbers. I'm glad that all I have to do is plug in and if it sounds good I like it regardless of Class.


----------



## Wild Bill (May 3, 2006)

guitarman2 said:


> It definitely ends any argument from me. I don't understand a dam thing in those numbers. I'm glad that all I have to do is plug in and if it sounds good I like it regardless of Class.


Quite right! Sounding good is the real goal. I didn't expect anybody but true techies to understand the data sheet stuff but the point is easy - a data sheet on the tube from when it was invented clearly shows specs for class A push-pull, or 'A P/P'.

More importantly, if these guys on FDP are supposed to be techies then to be unfamiliar with such basic information is like some guy claiming to be a mechanic who never heard of Chilton's manuals, or a computer geek who never heard of Windows!

THAT'S how basic the idea of two tubes in Class A push-pull is to anybody with even a basic tube techie education!

That's why I avoid such sites. It always seems that none of the loudest voices actually have read an electronics text.

:food-smiley-004:


----------



## guitarman2 (Aug 25, 2006)

Wild Bill said:


> More importantly, if these guys on FDP are supposed to be techies then to be unfamiliar with such basic information is like some guy claiming to be a mechanic who never heard of Chilton's manuals, or a computer geek who never heard of Windows!


I've heard of the Chilton manuals but couldn't gap a spark plug if I had to and I'm a computer geek that wished he'd never heard of Windows.


----------



## Guest (Nov 19, 2008)

Like I said, I don't pretend to be an amp tech. That alpha-numeric string means nothing to me but it does occur to me that definitions are at the root of this argument and definitions are often difficult to agree upon in many industries. I'm gonna let the matter drop. You can declare victory if you like. It makes no difference to me one way or the other. I just thought folks here should know there are a lot of experts out there who disgree. Now that I've done that my work here is done.


----------



## mhammer (Nov 30, 2007)

Yeesh, so few tubes and so much room for misunderstanding.

1) If it has only one output tube (and it is meant to), then it can't be anything *BUT* Class A, since a single tube covers the entire wave.

2) If it has MORE than one tube, it is _possible_ it could be Class AB push/pull, dividing the labour up between one tube for the positive half and another for the negative half of the wave. But it is also possible that it uses two tubes in parallel, both in Class A mode, to produce more output current than a single tube would. Indeed, the venerable Vox AC30 used *four* EL84 output tubes to produce 30 watts in Class A mode. The Jim Kelly amp used a quartet of 6V6 tubes in the same manner from what I understand. And I just finished reading about a pair of Matchless amps this morning, one of which uses a pair of EL34 tubes in parallel in Class A mode.

The splitting of output work across multiple devices run in parallel is quite common. Owners of solid-state amps can often see multiple parallel power transistors mounted to the back of the amp. And, of course, those tube powerhouses in the 100W+ range also use parallel power tubes, except in the AB configuration. So, two in parallel for the positive half, and two in parallel for the negative half.

As has been noted several times in this thread, Class A configuration assumes the tube is "on" all the time, which results in greater current consumption and lower efficiency. Accordingly, the GP review of the Matchless King Cobra amp notes that it weighs 85 pounds. Wouldn't surprise me a bit if 25 or more of that was the power transformer.


----------



## guitarman2 (Aug 25, 2006)

mhammer said:


> Yeesh, so few tubes and so much room for misunderstanding.
> 
> 1) If it has only one output tube (and it is meant to), then it can't be anything *BUT* Class A, since a single tube covers the entire wave.
> 
> ...



Holy Crap Now there is an explanation I understood completely. Thank you mhammer.


----------



## mhammer (Nov 30, 2007)

No problem. Explanations are what I am known for. Well, that and my homemade salsa.


----------



## greco (Jul 15, 2007)

So...can you please post the salsa recipe, now that the Class A/B stuff is more or less "done".:food-smiley-004:

cheers

Dave


----------



## mhammer (Nov 30, 2007)

About 8 ripe (or close enough) medium-sized plum/Roma tomatoes, diced
One medium onion, diced, although the equivalent in Vidalia sweet onion is better
A tablespoon or two of commercial pickled jalapenos, chopped up fine (and a little of the brine; I use Old El Paso because its easy to find)
The juice of one decent-sized lime, or two slightly smaller or dried up limes
half a cup (before chopping) of chopped up cilantro
Couple of cloves of fresh garlic (although for some reason dried minced stuff tastes better to me)
A level teaspoon or so of salt (I use Kosher salt, which is pure NaCl)
A level teaspoon or so of sugar (I have to use Splenda)
A small can of tomato paste

The tomato paste is used for thickening, so you can put in as much as you need to adjust consistency. The key elements are the salt-to-sugar balance and the lime juice. Using tomato paste as thickener also makes the difference between it and commercial because commercial thickening agents have no taste to them, so this stuff just tastes more intense. Happily, you can continue tweaking the salt/sugar/lime balance until you get it right without destroying the batch. 

I find the stuff needs to sit for a couple of hours in the fridge for the garlic to do its thing. It also tastes better cold. Lasts about 3 days or so in the fridge before quality starts to rapidly decline, although if you make it right you'll find yourself eating it up within a day or so anyways. 

Traditionally, I bring a batch and a bag of tortilla chips to the 2nd meeting of any working group I'll be participating in over the course of several weeks. I haul it out around 11:20 before the food arrives. People are usually reluctant to try it at first, but by 11:45 everyone is talking with their mouth full and holding one hand under the other to stop the dripping on their clothes while they shovel it in their mouth. Doesn't matter how high up they are in the management chain. It's the great equalizer. Once everybody has seen everyone else eat like a pig, THEN we can get down to work and accomplish things.


----------



## jcayer (Mar 25, 2007)

I think some people are mixing the terms *class AB *with *class B*

*Class B* will use a push-pull output type of circuit where one half will conduct for the positive half cycle, and the other half will conduct fot the negative cycle. 1/2 - 1/2

*Class AB* is close to Class B, with the two output devices that works largely on a single alternating signal. Thanks to a polarization of low value, each component is also working on a small portion of the other alternation, which "facilitates" the connection of signals, output, thus solving the problems of crossover distortion.

As for the Class A, If the output device (s) conduct for all the cycle, then it will be class A.


----------



## mhammer (Nov 30, 2007)

Correct. And since I am on a roll here, "crossover distortion" is that distortion created when the handoff or changeover between one half cycle and the other is not 100% perfectly seamless. Imagine two tubes in B or AB mode and one of them says "Huh? Oh, is it my turn? Sorry, I zoned out there for a moment." and rushes to catch up to where it should be. At the point where the exchange of duties takes place, the output will not accurately reflect the original waveform being reproduced.

In some respects, it is a kind of sideways clipping. The clipping most of us are most familiar with occurs when the actual signal should continue higher than some point but is cut off at a given amplitude. So, a kind of artificial ceiling is created. In the case of crossover distortion, the "ceiling" is replaced with a "wall", as the tube or transistor suddenly races to catch up to where the actual signal is at that moment. The input signal might have an upwards slope, but the nonseamless crossover results in a brief flat part followed by a sudden rise, producing audible distortion.

As jcayer points out, in pure B, there is a kind of clear midpoint and division of duties between the tubes/devices. The handoff has to be *perfect*, like an Olympic 4x100m relay. With AB there is a little more leeway because of some overlap. My sense is that one sees class AB most often because the design requirements are less stringent than for pure class B, and the power requirements are less expensive or heat generating than pure class A.


----------



## greco (Jul 15, 2007)

mhammer said:


> About 8 ripe (or close enough) medium-sized plum/Roma tomatoes, diced
> One medium onion, diced, although the equivalent in Vidalia sweet onion is better
> A tablespoon or two of commercial pickled jalapenos, chopped up fine (and a little of the brine; I use Old El Paso because its easy to find)
> The juice of one decent-sized lime, or two slightly smaller or dried up limes
> ...


Thank You, Thank You, Thank you :bow::bow:

Cheers

Dave


----------



## guitarman2 (Aug 25, 2006)

mhammer said:


> Correct. And since I am on a roll here, "crossover distortion" is that distortion created when the handoff or changeover between one half cycle and the other is not 100% perfectly seamless. Imagine two tubes in B or AB mode and one of them says "Huh? Oh, is it my turn? Sorry, I zoned out there for a moment." and rushes to catch up to where it should be. At the point where the exchange of duties takes place, the output will not accurately reflect the original waveform being reproduced.
> 
> In some respects, it is a kind of sideways clipping. The clipping most of us are most familiar with occurs when the actual signal should continue higher than some point but is cut off at a given amplitude. So, a kind of artificial ceiling is created. In the case of crossover distortion, the "ceiling" is replaced with a "wall", as the tube or transistor suddenly races to catch up to where the actual signal is at that moment. The input signal might have an upwards slope, but the nonseamless crossover results in a brief flat part followed by a sudden rise, producing audible distortion.
> 
> As jcayer points out, in pure B, there is a kind of clear midpoint and division of duties between the tubes/devices. The handoff has to be *perfect*, like an Olympic 4x100m relay. With AB there is a little more leeway because of some overlap. My sense is that one sees class AB most often because the design requirements are less stringent than for pure class B, and the power requirements are less expensive or heat generating than pure class A.



What you're describing here with the one tube trying to catch up with the other is that is what is meant by "sag" or is that a term relating only to the rectifier and is something different all together?


----------



## jcayer (Mar 25, 2007)

guitarman2 said:


> What you're describing here with the one tube trying to catch up with the other is that is what is meant by "sag" or is that a term relating only to the rectifier and is something different all together?


http://www.aikenamps.com/Sag.html

Happy reading :smile:


----------



## guitarman2 (Aug 25, 2006)

Thanks for the article.

Hmmm. I wonder about this statement.



> A true class A amplifier has no sag because the current draw at full power is the same as the current draw at idle.


I originally believed that sag mainly occurred from the rectifier. I recently changed out an NOS Valvo rectifier with an NOS Mullard. I immediately felt the difference in feel with the Mullard, seeming to get way less sag. The amp was cleaner, bass tighter seemed to have more headroom.


----------



## mhammer (Nov 30, 2007)

Good link. Crossover distortion has to do with whether someone/something is actually reproducing the input signal at that moment where it changes from the upper to lower half-wave. Sag has to do with the amount of power that can be applied to the signal.

The degree of sag is affected by the properties of what is supplying the power. If I swap out the 5Y3 in my tweed Princeton for a solid-state rectifier, you find an easy-to-detect "hardness" to the sound, even though the amp is still Class A.

Sag is also observed with battery-powered pedals, too. My sense is that the near-mythic preferences for old Eveready carbon-zinc batteries when using some germanium fuzzes is because when carbon-zinc batteries get older, they respond differently to instantaneous current demands from the fuzz, in comparison to alkaline and other 9v batteries. My sense is that this results partly from the compositon of the 6 individual cells in a 9v, but also because of the different physical shape of the cells (carbon-zinc has flat slugs with lots of surface area, while alkaline have sub-AAA-sized mini-batteries packed into the shell).

JC Maillet has some interesting thoughts on the series resistance between rectifier and tubes in older amps in his book on Fender/Marshall amps ( http://www.lynx.bc.ca/~jc/ ), and notes that some have considerably more resistance in that path than others, especially when the "others" have chokes. So the quality of the tone, is not just in amp class, and not just in tube rectifier type, and not just in tube-vs-SS-rectifier, but in how that current is fed to the tubes as well.


----------



## Wild Bill (May 3, 2006)

guitarman2 said:


> Thanks for the article.
> 
> Hmmm. I wonder about this statement.
> 
> ...


Sag works this way, Terry. There's always a voltage drop across a tube rectifier. The tube rectifier delivers the voltage to the tube plates. When the tubes work harder 'cuz the signal is louder there's more current drawn from the power supply. The voltage drop across the tube rectifier increases as the current draw increases. 

So the voltage to the tube plates drops a bit as the amp is cranking hard. This drop reduces the power a bit and acts like compression. The lowering of sound is not enough to be obvious to the ear but the compression is noticeable as "thickening".

This is sag! Some rectifier tubes have more voltage drop than others. A 5U4 has a LOT more than a 5AR4/GZ31! Fender switched between the two types over the years and that accounts for some of the difference in tone between different years of the same amp.

Since a true class A condition means the tube is always drawing the same steady current, the voltage drop from the rectifier will not change. This means there's no sag.

Just an inside for my buddy Mike, I'm not sure but I got the inference from your answer about class A that if you use more than one tube they have to be in parallel. 'Tain't true at all, as I'm sure you'd agree after a coffee or two. I've got two class A push-pull amps ripped out of some old 50's hifi's staring me in the face right now!

:food-smiley-004:


----------



## Bevo (Nov 24, 2006)

Lots of great info guys but now my head hurts..

Staying on topic, can you let us know what are the details we want to look at, or for, in a metal, blues, clean and jazz amp.

Not really looking brand to brand but when I go to the shop I want to say..I am looking for a class X amp with a X preamp and a X transformer.

Make sense?

Thanks
Bev


----------



## Wild Bill (May 3, 2006)

Bevo said:


> Lots of great info guys but now my head hurts..
> 
> Staying on topic, can you let us know what are the details we want to look at, or for, in a metal, blues, clean and jazz amp.
> 
> ...


Well, all this stuff is really more for techies who might be modding or building their own amps. If you go to a music store few amp makers put all this stuff into their sales brochures. Few salesfolks would know about it, either.

If I were you, I'd play 'em all! Then I'd buy the one that sounded best to me.

:food-smiley-004:


----------



## mhammer (Nov 30, 2007)

Actually, no, it doesn't make sense. And I don't say that in a dismissive way. The thing of it is that the suitability of the amp for a given style of music has very little to do with the factors you mention, once you factor in/out things like speaker complement, cab style/size, power, gain structure, voicing, and a whole whack of other things. Certainly people DO design amps for a particular purpose or musical style. Marshall and Mesa have never really aimed for the CMT Awards crowd, where Peavey has made a point of designing for country players for nearly 4 decades, and Polytone made it clear they were designing for the clean jazz player. But this targetting isa really more a question of features, gain structure, and tone tweaking than broad classes of design, or even tube type. As well, musical style tends to follow design, historically. I mean, what type of music WAS the AC30 designed for when it was first produced? What sort of music was the Bassman designed for, or the Deluxe Reverb? I mean, it would certainly be nice if things DID line up with the sort of request you made. It would sure make amp shopping a whole lot easier. One might say that a Class A amp is "better" for blues, but then whose blues are we talking about: Johnny Guitar Watson or Gary Moore?

So, it's an honest and sincere request you make, but one that I feel only fools would attempt to honour.


----------



## Younggun (Jul 2, 2008)

Okay, its official, I'm now more confused than ever! LOL.:wave:


----------



## Bevo (Nov 24, 2006)

Sounds good to me, I will shop by ear and the amps reputation.

Most of the shop guys don't have a clue about what they have to sell. I will do my own homework.

Back to your chat techies:banana:

Bev


----------



## RHGraham (Nov 13, 2008)

You can have a single tube in class A operation, and that's all you can do with it.

You can have as many of those tubes running parallel as you have power and output transformer to run them. Two tubes is common in audiophile gear, three and four appear occasionally. Biased cathode or Fixed, either way.

You can have a pair, quartet, six, eight, ten, as many tubes as you have power to operate running in push-pull, In class A, class AB, class B, and other classes of operation as well.
whether the amp is class A, or AB, the most common for guitar amps, is determined by the voltages presented to the tubes and the current they draw. Thats it.

Class A usually isn't seen in push-pull amps simply because class AB operation makes a lot more power for the same amount of juice consumed.

The method of bias has a great deal of influence on the amps sound and response, but alone does not indicate class of operation. 
The AC30, Matchless DC-30, and most other amps that have a reputation of being class A, are not. They are class AB, although some of these ARE cathode biased amps which has a distinctive sound and response, that through miss-understanding, and miss-information, have come to be believed as being class A.

The Traynor amp mentioned is also NOT class A.
Tweed deluxe is NOT class A

They all *could* be if you wanted, the mods to do it are fairly simple... you will give up a lot of power. They would sound cool.

I have two stereo tube amps running a pair of 6550's on each side, ( so a quartet in total, 2 for the left channel, 2 for the right), they are fixed-bias push-pull class A amplifiers, they sound REALLY good, and they put out a whopping, stunning, 12 watts per channel or so.
Came out of an old theater. They make my nearfields all warm n fuzzy.

Randal


----------



## guitarman2 (Aug 25, 2006)

Very confusing subject Class A, B or AB amps. But thanks to mhammer and wildbill I understand all I need to as a player. I find that as a player I'm more apt to judge what I might find in an amp based on whether they are EL84, El34, 6L6, 6V6 etc. I know enough about different rec tubes (GZ34, 5Y3, Solid state) and preamp tubes (12Ax7, EF86) to know what to look for if I'm looking for an amp for a specific application. But when it comes down to it, as a player, I need to get out there and try all these amps. When looking for an amp I'll go to the music stores and try as many different ones that I can. I have a good relationship with the music store I deal with and can take any amp out on a job for a good trial. I then make my decision based on my needs and how the amp performed for me in a real life situation. About all I really need to know about tubes is which ones I need to replace when their done.
Its nice to know a little about how they work but as a player not necessary. It doesn't make my amp any less appealing to me whether its True Class A or Class A/B. If the good Doc says my amp is true Class A, then thats good enough for me.


----------



## Canadian Charlie (Apr 30, 2008)

I have a Peavey Valveking 112 with the texture controll knob and I've tried it on A and A/B. 

So far I love the tone I get from the knob turned all the way to A/B


:smilie_flagge17:


----------



## RHGraham (Nov 13, 2008)

guitarman2 said:


> Very confusing subject Class A, B or AB amps. But thanks to mhammer and wildbill I understand all I need to as a player. I find that as a player I'm more apt to judge what I might find in an amp based on whether they are EL84, El34, 6L6, 6V6 etc. I know enough about different rec tubes (GZ34, 5Y3, Solid state) and preamp tubes (12Ax7, EF86) to know what to look for if I'm looking for an amp for a specific application. But when it comes down to it, as a player, I need to get out there and try all these amps. When looking for an amp I'll go to the music stores and try as many different ones that I can. I have a good relationship with the music store I deal with and can take any amp out on a job for a good trial. I then make my decision based on my needs and how the amp performed for me in a real life situation. About all I really need to know about tubes is which ones I need to replace when their done.
> Its nice to know a little about how they work but as a player not necessary. It doesn't make my amp any less appealing to me whether its True Class A or Class A/B. If the good Doc says my amp is true Class A, then thats good enough for me.


There ya go.
:0)
Words of wisdom.

Randal


----------



## ssdeluxe (Mar 29, 2007)

RHGraham said:


> Originally Posted by guitarman2
> Very confusing subject Class A, B or AB amps. But thanks to mhammer and wildbill I understand all I need to as a player. I find that as a player I'm more apt to judge what I might find in an amp based on whether they are EL84, El34, 6L6, 6V6 etc. I know enough about different rec tubes (GZ34, 5Y3, Solid state) and preamp tubes (12Ax7, EF86) to know what to look for if I'm looking for an amp for a specific application. But when it comes down to it, as a player, I need to get out there and try all these amps. When looking for an amp I'll go to the music stores and try as many different ones that I can. I have a good relationship with the music store I deal with and can take any amp out on a job for a good trial. I then make my decision based on my needs and how the amp performed for me in a real life situation. About all I really need to know about tubes is which ones I need to replace when their done.
> Its nice to know a little about how they work but as a player not necessary. It doesn't make my amp any less appealing to me whether its True Class A or Class A/B. If the good Doc says my amp is true Class A, then thats good enough for me.
> 
> ...




+100000000

absolutely, I agree completely, as a player we need not have to get confused in the jargon, its gr8t to have a working knowledge, but I think some folks think A anything is better than B. Always best to trust your ears !!

well said above. :food-smiley-004:


----------



## RHGraham (Nov 13, 2008)

ssdeluxe said:


> +100000000
> 
> absolutely, I agree completely, as a player we need not have to get confused in the jargon, its gr8t to have a working knowledge, but I think some folks think A anything is better than B. Always best to trust your ears !!
> 
> well said above. :food-smiley-004:


I see it at our shop quite regularly, we have a studio and a teaching studio so have a lot of folks around all the time, which is cool, but every now and then we gotta pull all the cool stuff out and do the "blind taste test", and very often, it results in a lot of surprised looks and raised eyebrows.
We have a couple of single-ended class a amps, some vintage fenders and a plexi, and a few amps I've modded and my first build, which is a variation on the good old tweed deluxe... everybody likes something different, and almost every time, they aren't what they *think* they were operation wise.

And, like was said, does it matter?
I don't think so. Everything else identical, two guitar players will sound like two different guitar players, and everybody will find thier own rig that suits them.

The joy is the journey... try everything,and groove on the trip.

Randal

...personally, I dig cathode-biased push-pull ab amps myself, they just suit the way I play. Tweed Deluxes and DC-30's are a couple of my absolute faves...


----------



## Wild Bill (May 3, 2006)

RHGraham said:


> I see it at our shop quite regularly, we have a studio and a teaching studio so have a lot of folks around all the time, which is cool, but every now and then we gotta pull all the cool stuff out and do the "blind taste test", and very often, it results in a lot of surprised looks and raised eyebrows.
> We have a couple of single-ended class a amps, some vintage fenders and a plexi, and a few amps I've modded and my first build, which is a variation on the good old tweed deluxe... everybody likes something different, and almost every time, they aren't what they *think* they were operation wise.
> 
> And, like was said, does it matter?
> ...


Yeah, it's the journey! And we share the same taste in cathode biased push-pull tones! One of the amps I most enjoyed was called a Sceptre. It was built by some "little guys" here in Hamilton, Ontario during the mid 60's. Two channels of straight up 12AX7 stages with a pair of 6L6's in push-pull cathode biased. What a great rockin' blues tone!

I see you're in Truro. My mother's side is from Pictou. I was a navy brat in Shannon Park till 1960 when my dad got out and we moved to Ontario. Still remember as a kid being driven through Truro on the way to visit the grandparents. Lot's of servicemen hitchhiking in those days. Since my dad and my uncles were all servicemen we picked up all the car could hold.

Vaguely remember a caged bear at a gas station/diner stop that used to love being fed Coke through the Frost fencing of his cage. It was either Truro or "Shuebenackidee" - I likely don't have the spelling right as I was about 3-6 years old back then. Mid 1950's.

Guess it's true about east coaster blood. All these years and Ontario still doesn't feel like my true home.

:food-smiley-004:


----------



## nonreverb (Sep 19, 2006)

Wild Bill said:


> Yeah, it's the journey! And we share the same taste in cathode biased push-pull tones! One of the amps I most enjoyed was called a Sceptre. It was built by some "little guys" here in Hamilton, Ontario during the mid 60's. Two channels of straight up 12AX7 stages with a pair of 6L6's in push-pull cathode biased. What a great rockin' blues tone!
> 
> I see you're in Truro. My mother's side is from Pictou. I was a navy brat in Shannon Park till 1960 when my dad got out and we moved to Ontario. Still remember as a kid being driven through Truro on the way to visit the grandparents. Lot's of servicemen hitchhiking in those days. Since my dad and my uncles were all servicemen we picked up all the car could hold.
> 
> ...


Agreed...and possibly the BEST cathode biased amp of all time IMO...the Leslie 147/122/251...beauty and simplicity in design...and that sound!!!:smile:


----------



## Younggun (Jul 2, 2008)

Just wanted to thank everyone for all their help. I appreciate it!


----------



## RHGraham (Nov 13, 2008)

Younggun;
I hope it all really was a help, I think the main thing to come away from this with is that it's probably best to keep an open mind, and try everything that's available to you to try.

Over 35 years ( ouch, that's starting to hurt to admit), being a guitar/amp tech, sound man, musician, in just about every style and genre over that time, I leaned to just listen to stuff and got blind to the name-plates, price, and marketing. Just try it all out. 
I have a couple of "iconic" amps that really turn me on, some specific tweed deluxes, and some specific ( 1 actually) matchless SC-30, I love them enough that if I had to, I could just have one of em, and I'd be good, I could do everything I wanted to do.

BUT, we don't have to have one of anything, or try just one kind of anything, I love old plexis, fender twins, supers, and deluxe reverbs, I love a lot of different garnets, a few traynors, I've played truly awe inspiring Hiwatts, and I've experienced a bunch of fancy one-off amps that were killer.
They're ALL good, cause, they're all good... class, tube types, majical elf-made capacitors wrapped only in the light of a full moon... who gives a crap.

If it sounds good, it is good. ANd what works for you, will just work for you, doesn't mean it'll work that way fro the next guy, so many different ways to play, types of guitars, pickups, pedals, etc.

All that being said, there are some VERY cool single-ended class A guitar amps out there... and I've heard some that sounded crappy too.

When it makes your heart race just a little, or gives you a shiver, when you get that little shimmer of feedback on that bend, or that awesome big pop in the bottom end just when you really wanted it, then that's it, that's one of your amps.

I'm starting work at a new shop now, and they have some newer amps I'm new too, and I'm looking forward to trying them out. There will be some that dig me, some that won't... it's all good. The journey *can* be one of the fun parts of being a musician. You just have to make it that way.

Ahh, the Bear, that was in the Shubenacadie Wildlife Park... man, that was a while ago... Can't feed the bears pop anymore, still a cool park though.
My wife and I go and sit somewhere and watch people... it's as entertaining as the animals. Moreso some days 
:smile:


----------



## RHGraham (Nov 13, 2008)

Wild Bill said:


> Yeah, it's the journey! And we share the same taste in cathode biased push-pull tones! One of the amps I most enjoyed was called a Sceptre. It was built by some "little guys" here in Hamilton, Ontario during the mid 60's. Two channels of straight up 12AX7 stages with a pair of 6L6's in push-pull cathode biased. What a great rockin' blues tone!
> 
> Guess it's true about east coaster blood. All these years and Ontario still doesn't feel like my true home.
> 
> :food-smiley-004:


There was a Sceptre around here when I was a lot younger, I think probably late 70's, I remember it was a screamin amp, but I can't really remember who was playing it.

I'm just starting to scratch-build amps finally after building kits and doing lots of mods on existing amps over the last few decades, I'm building a few straight tweed deluxes cause they're simple and I love em, but ultimately, I want to explore that whole "super-deluxe" thing with cathode 6L6's also.
I did mod an old super one time to cathode bias, and switched out the feedback loop... did the same to a deluxe reverb too... and they were MONSTERS like that, well, for me anyway. I loved them like that.
Ended up un-modding them and selling them, back when they still weren't worth all that much.
Man,I sure wish I had kept em. I pretty sure the super is largely responsible for my mild tinnitus though, so maybe it's just as well.

And the east-coast blood... I used to scoff at that, but after being away for the better part of 15 years... damn, it's good to be home.

Randal


----------

