Class A & Class AB difference?

brendansmith

Lad what the difference between the above, thinking about the line 6 spider valve 2 x 12.

And one that note what benefits are there to 2 x 12 spekers over just the 1 x 12?

Thanks guys!

Paolo_M

Class A has signal current flowing in the output valve for the entire 360* of the wave cycle.
Class AB has current current flowing for less than 360* but appreciably more than 1/2 the cycle.
Class A tends to be cathode biased which, even with a bypass cap, still has a slighly varying bias level that fluctuates with the signal. This causes a natural compression and warmth to the sound. This is opposed to Class AB which tends to be fixed bias and whose bias does not vary when passing signal. Hense the term fixed bias. I say tends because there are exceptions, this is by no means a rule.
When a Class A power amp distorts it tends to produce more 2nd order harmonics than Class AB. Even order harmonics are generally considered to be more musical than odd order which a Class AB tends to produce under distortion.
There is a common misconception that Class AB cancels 2nd order harmonics. This is not true. Class AB is usually in push pull configuration which cancels even order harmonics produced in the power amp only, in the same way that it cancels noise. This is a function of the architecture and not the Class of operation. Even order harmonics produced by the preamp (almost always Class A) pass unscathed. Many Class A amps are also push pull (Vox AC30) and so cancels some of the even order from it's power amp.

In short, Class A will be lower powered for the same valves and sound warmer/softer than Class AB. I'd go for Class AB for metal, hard rock etc. and Class A for blues, jazz etc.

A 2 X 12 will sound fuller/bigger than a 1 x 12.

Flesh Gorden

well in basic language

you wont need to worry about class a or a/b in a line 6 or any other digi amp - you need output tubes for that

the biggest advantage of a 2x12 is you get a lot more projection and spread of your sound - especially if you take off that back panel

But:they can be a nightmare to mic up - not as bad as a 4x12 but tricky - sound great in a big room though and in any band situation


Class A and A/B refers to the draw of the output transformer - class A is less efficent - A/B are more

Marshalls are normally A/B

Fenders are normally Class A


replacing tubes on a Marshall or any other A/B amp is like micro surgery - you need to be good with a multimeter - and you need tubes that are all rated the same (matched) and to be extra safe you'd more then likely have to replace the resistor across the base of the tubes - and to be extra extra safe a variac to bring up the voltage slowly to make sure there are no problems from the change

And if you don't like the sound of them tough! unless you want to spend another 4 hours or around 80 quid for someone to do it your stuck with them till they blow


With a small Fender? its the same as replacing a lightbulb - 20 secs maximum - they bias (adjust) themselves to the difference in voltage and current draw - so you can use anytype of similar rated tube once it doesn't exceed the maximum rated plate voltage


so - with a class A/B you get more distortion and quicker
Class A - a lot sweeter sound and more options

felim

Flesh Gorden wrote: »

replacing tubes on a Marshall or any other A/B amp is like micro surgery - you need to be good with a multimeter - and you need tubes that are all rated the same (matched) and to be extra safe you'd more then likely have to replace the resistor across the base of the tubes - and to be extra extra safe a variac to bring up the voltage slowly to make sure there are no problems from the change

And if you don't like the sound of them tough! unless you want to spend another 4 hours or around 80 quid for someone to do it your stuck with them till they blow

I learned to replace and rebias the valves in my Marshall in around 10 mins. It took me around 20 minutes to actually do the work. I had to replace nothing but the valves. It's really not close to as hard a task as you make out.

progsound

Flesh Gorden wrote: »

But:they can be a nightmare to mic up - not as bad as a 4x12 but tricky

WTF??? This makes no sence

Paolo_M

Flesh Gorden wrote: »

well in basic language

you wont need to worry about class a or a/b in a line 6 or any other digi amp - you need output tubes for that

The spider valve uses 2 or 4, depending on the model, 6L6/5881 power valves.
Most of the rest of what you're saying is either not true, mixed up or misleading.

brendansmith

Flesh Gorden wrote: »

well in basic language

you wont need to worry about class a or a/b in a line 6 or any other digi amp - you need output tubes for that

https://www.billyhydemusic.com.au/shop/index.cfm?action=view&id=4388


"40watt class AB tube amp" ??? Should i worry now??

brendansmith

progsound wrote: »

WTF??? This makes no sence

*Sense

Also i think what he means is that 1 speaker needs 1 mic, 2 speakers needs 2 mics, and 4 speakers needs 4 mics.

boycey

Paolo_M wrote: »

In short, Class A will be lower powered for the same valves and sound warmer/softer than Class AB. I'd go for Class AB for metal, hard rock etc. and Class A for blues, jazz etc.

A 2 X 12 will sound fuller/bigger than a 1 x 12.

I always wondered was class A higher powered rather than lower powered for the same valves, purely from comparing my 15 & 30 watt class A Laneys to my mates 50 watt class A/B Randall. The Laneys just seem to get REAL loud real quick whereas the Randall has a more even volume range. Maybe thats just a manufacturer difference though rather than anything technical?
BTW great (and very informative) answer to Brendans original question Paolo.

progsound

brendansmith wrote: »

*Sense

Also i think what he means is that 1 speaker needs 1 mic, 2 speakers needs 2 mics, and 4 speakers needs 4 mics.

Thats the thing you generaly only mic one speaker if you miced all 4 speakers in a 4 x 12 you would run into phase hell. His post was to suggest that a 2 x 12 is harder to mic up than a 1 x 12 which is of course quite untrue

brendansmith

progsound wrote: »

Thats the thing you generaly only mic one speaker if you miced all 4 speakers in a 4 x 12 you would run into phase hell. His post was to suggest that a 2 x 12 is harder to mic up than a 1 x 12 which is of course quite untrue

Ok, fair enough. Never had to mic an amp before tbh.
Poor flesh gordon, no one will give him an inch.

brendansmith

Paolo_M wrote: »

In short, Class A will be lower powered for the same valves and sound warmer/softer than Class AB. I'd go for Class AB for metal, hard rock etc. and Class A for blues, jazz etc.

A 2 X 12 will sound fuller/bigger than a 1 x 12.

Listen man, thanks very much for the detailed response. Very informative.

I do play metal and am interested in the line 6 but i would prefer a class AB without the build in effects which im told are supposed to be terrible. Any suggestions keeping in mind that budget etc??

progsound

brendansmith wrote: »

Listen man, thanks very much for the detailed response. Very informative.

I do play metal and am interested in the line 6 but i would prefer a class AB without the build in effects which im told are supposed to be terrible. Any suggestions keeping in mind that budget etc??

What kind of metal are you playing i.e what type of tone are you going for?

For a little extra cash you could get this http://www.thomann.de/ie/framus_fr212_cb.htm

and this http://www.thomann.de/ie/krank_revolution_jr_pro.htm

which would be a step up from the line 6

Paolo_M

boycey wrote: »

I always wondered was class A higher powered rather than lower powered for the same valves, purely from comparing my 15 & 30 watt class A Laneys to my mates 50 watt class A/B Randall. The Laneys just seem to get REAL loud real quick whereas the Randall has a more even volume range. Maybe thats just a manufacturer difference though rather than anything technical?
BTW great (and very informative) answer to Brendans original question Paolo.

To give an example:
Take a 380Vdc power supply and a push pull output transformer rated for 50 Watts with a primary impedance of say 3.5k ohm. Take two EL34 power valves. I could design a typical Class AB power amp, like a Marshall 1987 type, that would produce 50 Watts. If I wanted Class A operation I would only be able to get about 20(ish) Watts from the exact same set up.
However it get's a bit more complex as Class A would require less signal to get to clipping point than Class AB, and remember that power rating should be given at the on set of clipping. My Class AB might have 40Vdc bias giving a max input signal swing of 80V before clipping whereas my Class A might only have 18Vdc bias giving a max input signal swing of 36V.
It's even more complex than that given that there is no standard for power rating in guitar amps. In Hi-Fi power rating is given as the max output power @ 1% THD (total harmonic distortion, in simple terms max clean output power) for a set period of time (which I can't remember how long, 30 mins I think). No such standard exists for the guitar amp industry. It's basically up to the marketing department to decide what power rating an amp should be given.

Originally Posted by brendansmith
Listen man, thanks very much for the detailed response. Very informative.

I do play metal and am interested in the line 6 but i would prefer a class AB without the build in effects which im told are supposed to be terrible. Any suggestions keeping in mind that budget etc??

Why not check out the Orange Tiny Terror. It's comes as a combo and a head version and with a little push from a boost pedal get's into metal territory. Check out Rob Chappers vids on uTube. They're fairly reasonably priced.
Prog recommended the Framus and Kranks which also have a good reputation, though I haven't played one myself.
You may also be able to pick up a second hand Boogie (MK III, Roadster etc.) for that kinda money, that's the way I'd go I think.

Eoin Madsen

Well, Paolo explained it better than I would have.

Only thing I'd (contentiously :pac:) add/amend is that I hear the AC30 is technically a class AB amp and doesn't operate in class A despite the cathode bias, and despite being marketed as class A. There's a good article about it somewhere on the intermalarky. It even has graphs and scopes and such. Real class A amps are almost exclusively single-ended, as I understand it, and most of the EL84 cathode biased push/pull amps are really class AB.

A pair of EL84s can produce around 15 watts (in tradition amp wattage) in cathode biased push/pull config, while a pair of EL34s produce equiv 50 watts in a grid biased push/pull config. Aside from the difference in maximum power output and how they're normally biased, the two kinds of valves also sound different.

In a single-ended configuration, a single EL84 will produce around 5 watts, while an EL34 could produce as much as 10. Putting several valves in parallel in single ended mode is pretty much directly cumulative.

boycey wrote: »

The Laneys just seem to get REAL loud real quick whereas the Randall has a more even volume range. Maybe thats just a manufacturer difference though rather than anything technical?

Getting loud quicker is probably actually more to do with the circuitry in the input stage of the preamp - the value and taper of the volume pot(s) and also how the first gain stage is set up.

Flesh Gorden wrote: »

But:they can be a nightmare to mic up - not as bad as a 4x12 but tricky - sound great in a big room though and in any band situation

4x12 are the easiest to mic up. There's more places to put your mic.

Flesh Gorden wrote: »

replacing tubes on a Marshall or any other A/B amp is like micro surgery - you need to be good with a multimeter - and you need tubes that are all rated the same (matched) and to be extra safe you'd more then likely have to replace the resistor across the base of the tubes - and to be extra extra safe a variac to bring up the voltage slowly to make sure there are no problems from the change

You would only need those kinds of precautions if you were putting new valves in an amp that hasn't been powered up since the 70s. The old screen resistors sometimes need to be changed, but probably only once every couple of decades and even then, they're probably fine unless some moron has allowed a valve failure to occur.

Flesh Gorden wrote: »

unless you want to spend another 4 hours or around 80 quid for someone to do it your stuck with them till they blow

That sort of implies that you should actually leave the valves in place until they blow, as if they were lightbulbs. That would be incredibly dumb, unless you hate your output transformer and you love paying your tech.

Flesh Gorden wrote: »

With a small Fender? its the same as replacing a lightbulb - 20 secs maximum - they bias (adjust) themselves to the difference in voltage and current draw

They don't actually "bias themselves", the cathode bias is just not adjustable in most amps because there's more leeway than with a grid/fixed bias amp and therefore it doesn't absolutely have to be. If one wanted to be thorough with a cathode biased amp, one could actually adjust the cathode resistors every time one changed the valves.

brendansmith wrote: »

Ok, fair enough. Never had to mic an amp before tbh.
Poor flesh gordon, no one will give him an inch.

There's blood in the water. :pac:

Paolo_M

Eoin Madsen wrote: »

Only thing I'd (contentiously :pac:) add/amend is that I hear the AC30 is technically a class AB amp and doesn't operate in class A despite the cathode bias, and despite being marketed as class A. There's a good article about it somewhere on the intermalarky. It even has graphs and scopes and such. Real class A amps are almost exclusively single-ended, as I understand it, and most of the EL84 cathode biased push/pull amps are really class AB.

No contention, you're spot on. Force of habit refering to the AC30 as Class A. :pac:
It is most definitley, along with it's derivitives/copies (EL84 based Matchless, Trainwreck etc.) Class AB1. The max output power for four EL84s in Class A is 20 Watts.

I think the website you refer to is http://www.aikenamps.com/ in the article "Is the Vox AC30 really Class A". That's a class web site, especially for geeks like me!!

I think you mean all single ended amps are Class A. You can also have Class A biased push pull amps but you can't have Class AB single ended.

I'm looking to design and build a Class A single ended amp of 20 Watts with switchable power levels 20/15/10/5/3 but I can't find a single ended OT to handle the power. Might have to resort to push pull after all, a custom built single ended one would be too pricey :mad:

The Boarder Man

Paolo_M wrote: »

I think the website you refer to is http://www.aikenamps.com/ in the article "Is the Vox AC30 really Class A". That's a class web site, especially for geeks like me!!

Nice. Another article there seems rather relevant to this thread:
http://www.aikenamps.com/ClassA.htm

Eoin Madsen

Paolo_M wrote: »

I'm looking to design and build a Class A single ended amp of 20 Watts with switchable power levels 20/15/10/5/3 but I can't find a single ended OT to handle the power. Might have to resort to push pull after all, a custom built single ended one would be too pricey :mad:

Try Sowter.co.uk, if you haven't already. I'm sure they do one... it's just a matter of how much it costs. SE transformers seem to get extortionately expensive as soon as you start building anything more exciting than a fender champ. I did manage to get a 15ish watt one from some place in the UK for a pretty reasonable amount. It has a UL tap. Cannot remember the url off-hand though.

I wonder if you can put SE OTs in parallel. Use one OT per pair or something. How is the 3 watt switch going to work?

boycey

@ Eoin & Paolo.

Any recomendations on sites/good books where I could read up and try and learn more about this stuff? A real "Dummy's guide to.." kind of thing as most of what you guys are saying is going over my head:o.

I think I'll make it my New Years resolution to try and figure out some basic electronics, and try and acchieve a working mans knowledge of how amps are put together. That would be cool, and sure the wife will be only thrilled:D.

Paolo_M

Eoin Madsen wrote: »

Try Sowter.co.uk, if you haven't already.

I wonder if you can put SE OTs in parallel. Use one OT per pair or something. How is the 3 watt switch going to work?

Cool, thanks Eoin, haven't been on this site before. Another good one is Majestic Transformer Co. in Dorset. They built me an exact replica OT for a AC50 that was blown. They copied everything down to the interleaved winding patterns. There's a soundclip of the amp up and running on my uTube page. Mad expensive for custom, higher powered SE OTs though. I'll try Sowter for pricing, cheers.
I was gonna install a "triode" switch to get down to three watts. I know they say it goes done to 1.8 Watts but I installed one on the HB GA5 and it came up more about 3 watts. I suppose when you think about it the supressor is still connected as normal and the screen is still at a positive voltage (plate) allbeit fluctuating up and down with the signal.

Any recomendations on sites/good books where I could read up and try and learn more about this stuff? A real "Dummy's guide to.." kind of thing as most of what you guys are saying is going over my head.

Boycey, you can download The Elctric Guitar Amplifier Book by Jack Darr for free if you do a google search. It's old now but top shelf stuff. Tom Mitchell also has an excellent beginners guide to valve amps, it's called How To Service Your Own Tube Amp. They're both great starting points and I've both of them somewhere.
When you get beyond that stage I would then recommend picking up Valve Amplifiers by Morgan Jones (he also has an excellent book on the actual construction of valve amps), The Ultimate Tone by Kevin O'Connor though there are some technical errors, RCA Receiving Tube Manual RC30 (or any of the previous editions) and Radiotron Designers Handbook 4th Edition if you can find it, they usually go for €130+ on eBay (search long enough on google and you'll find it for free). Gerald Weber also has two entertaining books on vintage valve amps if you can look passed the blatent adds (his company sells speakers) and hilarious insights such as the classic "tube shields confuse electrons". This lot is also on my shelf. That should be enough to keep you going!! :pac:

Quattroste

boycey wrote: »

@ Eoin & Paolo.

Any recomendations on sites/good books where I could read up and try and learn more about this stuff? A real "Dummy's guide to.." kind of thing as most of what you guys are saying is going over my head:o.

I think I'll make it my New Years resolution to try and figure out some basic electronics, and try and acchieve a working mans knowledge of how amps are put together. That would be cool, and sure the wife will be only thrilled:D.

We should talk Boycey. I'd love to get into this aswell. I started a thread about building our own amps a while back but the advice from the more experienced posters(thanks lads) was to start small on a pedal kit or similar then move onto the bigger stuff. I plan on the New Year as a start time for this too.

boycey

Quattroste wrote: »

We should talk Boycey. I'd love to get into this aswell. I started a thread about building our own amps a while back but the advice from the more experienced posters(thanks lads) was to start small on a pedal kit or similar then move onto the bigger stuff. I plan on the New Year as a start time for this too.

Talk we shall Ste, then its off to Maplins for a shopping trip. The goal is to make a pedal or amp that makes me sound good.
Its gonna take a while :pac:

brendansmith

What is a solid state rectifier.

After a blackheart now tbh

Eoin Madsen

brendansmith wrote: »

What is a solid state rectifier.

The rectifier is the component that converts the AC into DC - for the plate voltage and etc. This was originally done exclusively with rectifier valves, but since semiconductors were invented it's more common to do it with silicon diodes. Lots of classic amps have used both kinds of rectifier, so it's not really a question of better or worse or vintage vs modern.

Rectifier valves tend to introduce more voltage drop when current is being drawn, resulting in slightly more "sag" or compression.

Zangetsu

Thought I'd post this here, save starting a new thread... Does feedback hurt amps? My guitarist use's a jagstang and loves to fcuk around with feedback, would this hurt a new tube amp?

Eoin Madsen

Not above the normal wear and tear of an amp. It's probably doing more damage to your (irreparable) ears tbh.

Zangetsu

True that.... Thanks!