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Post subject: Re: Class A amps
Posted: Tue Sep 28, 2010 5:31 am
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shimmilou wrote:
After reading the Sheldon Amps link that you provided nn, I can see where he is confused about class A. Remember, class A only means that the output tubes are biased such that they never stop conducting current during operation. This statement in particular (among others):

"Class A amps are at the very most 50% efficient so to get say, 25W of output they have to have valves capable of dissipating at least 50W of power. It's not altogether good news for those of us who like a lot of 'Sag' in our amps but more on that later."

Yes, the class A AMPS are inefficient, not the tubes. The tubes are the same efficiency regardless of how they are biased or what class that they are run. So if you want 25 watts of power out, you only need 25 watts from the tubes. How ridiculous to say that 50 watts out of the tubes will only give you 25 watts out of the tubes. So, 25 watts is disappearing? Nonsense! Some of the power is "dissipated" as heat, but it doesn't change because of the class it's running. :? The inefficiency of the class A AMP means that the AMP needs to supply more power to the tubes than they would in a class AB amp, because none of the tubes turn off in class A and that means more power wasted by the amp. The tubes don't somehow become less efficient because they are run class A. :idea:


Thanks for that explanation Shmmlou. I poke and prod ask questions, get corrected and hopefully learn a bit. :wink:
Makes sense.

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Post subject: Class of amp
Posted: Tue Sep 28, 2010 7:04 am
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Guys, this may be a tough question, but does anyone know how to tell whether an amp is class A or AB or a combination of the two? Of course you could tell by using an oscilloscope, but is there any other way to tell? It doesn't seemed to be standard practice to list the class of operation as one of the specs. For instance, kineman karma was told that his Pro Tube Twin is a Class A amp at GC, how can we tell whether it is or not? :?

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Post subject: Re: Class of amp
Posted: Tue Sep 28, 2010 8:40 am
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shimmilou wrote:
Guys, this may be a tough question, but does anyone know how to tell whether an amp is class A or AB or a combination of the two? Of course you could tell by using an oscilloscope, but is there any other way to tell? It doesn't seemed to be standard practice to list the class of operation as one of the specs.


It's easy. Count the number of output tubes. If there is only one output tube then it is Class A. For example a Fender Champ is Class A. If there are 2 or 4 output tubes it is Class A/B.

Amps with more than one output tube that are advertised as "Class A" are afflicted by marketing hype (I'm pointing my finger at you Vox!). Now every amp maker that has an amp that has cathode biased output tubes and no negative feedback advertises that the amp is Class A. Cathode bias and no NFB do not make an amp Class A. My cathode biased no NFB Mesa-Boogie Blue Angel says "Class A" right on the control panel...what a bunch of hogwash.
http://www.aikenamps.com/VoxAC30classA_2.html
"These findings prove that the Vox AC30 circuit design is indeed, a class AB amplifier, although, to be fair, it is biased a bit hotter than most fixed-bias class AB amplifiers. Now, does this make it any less desirable? No! Does it make "true" class A amps any better? No! Use your ears when deciding whether or not you like an amplifier, not some marketing buzzwords. However, you should question any and all claims made by amp manufacturers (including me!) - it keeps them honest, and we all benefit in the end"

I can't think of any current off the shelf production line amps with more than one output tube that are true Class A. Sure plenty of boutique amp makers can do a true Class A amp, but it will cost you.

shimmilou wrote:
For instance, kineman karma was told that his Pro Tube Twin is a Class A amp at GC, how can we tell whether it is or not? :?


Never assume ANYTHING a sales dudes says at GC is the truth. Do your research first. The GC sales dudes have no idea what they are talking about becuse they get no training and make up answers instead of asking for help. And when they do know about the merchandise they are selling, they will tell you what you want to hear in order to make the sale.

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Posted: Tue Sep 28, 2010 8:51 am
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So you cant get say 4 6v6's to all operate in class A together?

I thought the hifi guys had been doing that for years.

You can have cathode biased class A or class A/B can't you?

Truth be told I trust Mark at Sheldon amps a fair bit (despite his bad 330 sales reputation :lol: ). He certainly knows enough about Vox's to make that statement, he worked for em for long enough.

Am I now getting confused between ClassA and Single ended?

Bare with me guys, I'm picking up a good bit here.

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Posted: Tue Sep 28, 2010 9:11 am
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nikininja wrote:
So you cant get say 4 6v6's to all operate in class A together?

I thought the hifi guys had been doing that for years.

You can have cathode biased class A or class A/B can't you?

Truth be told I trust Mark at Sheldon amps a fair bit (despite his bad 330 sales reputation :lol: ). He certainly knows enough about Vox's to make that statement, he worked for em for long enough.

Am I now getting confused between ClassA and Single ended?

Bare with me guys, I'm picking up a good bit here.


Sorry, I should have been more specific and said "in the guitar amplifier industry". Yes I'm sure hi-fi guys have been doing 4 6V6's in Class A for years, as in your example. I just don't see that in the guitar amp world. I don't even know if it would even sound good as a guitar amp.

Single-ended amps are Class A by default since the single valve is always "on", or however Sheldon amps stated it. I just meant that when you walk into your average guitar shop, the only true Class A amps you'll find are single-ended Champ type amps.

I'm picking up a good bit here also. Interesting reading and links.

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Post subject: Re: Class A/B Fender Amps and Effects Pedals.
Posted: Tue Sep 28, 2010 9:37 am
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kineman karma wrote:
As the Twin allows me to run my pedal board through the effect loop, I have the joy of hearing what I believe to be the true tone of the pedals themselve's.


Getting back to kk's original post, I thought certain footpedals should not be run through the effects loop like distortion/overdrive pedals, while others work equally well in front of or in the loop like delay, reverb, etc. What's the scoop on this?

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Post subject: Re: Class of amp
Posted: Tue Sep 28, 2010 9:58 am
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metropolis74 wrote:
It's easy. Count the number of output tubes. If there is only one output tube then it is Class A. For example a Fender Champ is Class A. If there are 2 or 4 output tubes it is Class A/B.


Well, thanks anyway, but that isn't true at all. Mesa Boogie uses push-pull configurations running class A. They even have a patent on "Simul Class" operation, using 4 tubes, 2 tubes running class A, and 2 tubes running class AB. You can run more than one tube in class A in a push-pull configuration, but obviously you can get much better efficiency using class AB, that's why most amps are AB these days. :)

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Posted: Tue Sep 28, 2010 9:59 am
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Bluesky

I don't think any effects should be run through a loop :P

I'm just too much of a neanderthal when it comes to amps. One in, the guitar input. One out, the speaker. :lol:

Metropolis

For sure the little single power valve small amps are by and large my favourite sounding things. It's about the only application I've heard where I find the el84 sound entirely usable.
Luckily there are plenty of other good valve types out there to use instead. Tungsol have made a KT88 with a plate dissipation of 60watts. Be interested to hear one of them in a single ended amp.

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Post subject: Re: Class A/B Fender Amps and Effects Pedals.
Posted: Tue Sep 28, 2010 10:04 am
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bluesky636 wrote:
kineman karma wrote:
As the Twin allows me to run my pedal board through the effect loop, I have the joy of hearing what I believe to be the true tone of the pedals themselve's.


Getting back to kk's original post, I thought certain footpedals should not be run through the effects loop like distortion/overdrive pedals, while others work equally well in front of or in the loop like delay, reverb, etc. What's the scoop on this?


Yeah the OP never answered my question as to what pedals he is using and where he is locating them.

Generally speaking; wah-wah, distortion, and overdrive pedals sound best in front of the amp. Modulation (time based effects) sound best in the loop; like delay, chorus, flanger, and phaser as they can sound muddy in front of an amp especially if it's a high gain amp with lots of preamp distortion. EQ, tremolo, volume pedals can go anywhere.

There are no rules though. What sounds great to me might sound like poo to you. It depends on the amp and guitars being used also. My amp has an effects loop, but I never use it because I like the old school sound of my delay pedal through the front end of my amp. Delay in front instead of the loop wouldn't work for a heavy metal player looking for ultra clean delay repeats while cranking up the gain on the overdrive channel.

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Post subject:
Posted: Tue Sep 28, 2010 10:26 am
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metropolis74 wrote:
Single-ended amps are Class A by default since the single valve is always "on", or however Sheldon amps stated it. I just meant that when you walk into your average guitar shop, the only true Class A amps you'll find are single-ended Champ type amps.

I'm picking up a good bit here also. Interesting reading and links.


Hi metropolis74, I hope that you don't get the impression that I'm picking on you, I'm not. :lol: Yes, class A usually is single ended, but can be push pull also. Remember, single-ended does not mean single output tube, it is just a configuration. You can run multiple output tubes in a single ended configuration also. :)

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Post subject:
Posted: Tue Sep 28, 2010 11:13 am
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shimmilou wrote:
Hi metropolis74, I hope that you don't get the impression that I'm picking on you, I'm not. :lol: Yes, class A usually is single ended, but can be push pull also. Remember, single-ended does not mean single output tube, it is just a configuration. You can run multiple output tubes in a single ended configuration also. :)


Aha! Thanks for the clarification.

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