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Post subject: Re: Tube Question
Posted: Sun Sep 07, 2014 10:37 pm
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Plainly and simply, it is mainly for slow warm up and slow cool down, very beneficial for tubes (that's electrically speaking BTW :wink: ). If the Hi-Fi dudes don't get it, so what? No one is going to push their Hi-Fi into distortion and really heat up the tubes like someone with a guitar amp will, so yes, the basic operations, as well as set-up, are very different between the two types of amps. Also, for instrument amps, when changing guitars, it is necessary to switch to standby and avoid the noise of unplugging the cable from one guitar and inserting to another guitar.

Notice that not all guitar amps have a standby? Typically the smaller amps do not.

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Post subject: Re: Tube Question
Posted: Mon Sep 08, 2014 4:41 am
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Arth, you're right about the the Class A lie except for one thing, you can get plenty of umph and breakup with a Class A amp. I run my 74 Champ through either one or two 1x12 cabs with a Weber Z-Matcher for impedance match and it not only sounds huge, but the breakup is amazing. Remember a lot of the old classic rock recordings were made with small Class A Champs, Dano's, Supro's and Silvertones.

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Post subject: Re: Tube Question
Posted: Mon Sep 08, 2014 9:30 pm
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Keep in mind that "Class A" simply means that the tubes are conducting at all times during the signal swings, according to the RCA Receiving Tubes manual. There are a great many Class A amps, the majority of which are single-ended, single output tube amps, but there are several that are push-pull that run in Class A. Most push-pull amps run Class AB.

What is the "Class A lie"? Are there amp manufacturers that claim their amps are Class A when they are not?

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Post subject: Re: Tube Question
Posted: Tue Sep 09, 2014 2:25 pm
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Usually it's the private sellers that use the Class A designation as being "the best" like some school student's report card. That's where the lie comes in. But Lou, you're exactly right with your explanation. Tonally it means nothing. You can get great tones from both Class A and AB. I remember years ago some oddball manufacturers tossing around the Class A designation for everything to use as some sort of selling point claiming Class A amps sound better. I believe it was Guitar Player magazine that called them out on it and actually explained it.

Others have do so too.
http://www.myrareguitars.com/class-a-tu ... vs-reality
http://www.guitarplayer.com/miscellaneo ... s-ab/14204 (not the one I was looking for)
http://www.guitarplayer.com/miscellaneo ... iers/15242

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Post subject: Re: Tube Question
Posted: Wed Sep 10, 2014 8:23 am
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63supro wrote:
Usually it's the private sellers that use the Class A designation as being "the best" like some school student's report card. That's where the lie comes in. But Lou, you're exactly right with your explanation. Tonally it means nothing. You can get great tones from both Class A and AB. I remember years ago some oddball manufacturers tossing around the Class A designation for everything to use as some sort of selling point claiming Class A amps sound better. I believe it was Guitar Player magazine that called them out on it and actually explained it.


Correct, but keep in mind that many of the big guys have sold class ABs as class A too - not because class A is inherently better than class AB, but because many buyers perceived it to be.

My Orange Rocker 30 is a good example. With its two EL34s there's no way it could be Class A and deliver 30 watts. But does it sound good? Oh, yeah. I still wish Orange hadn't called it Class A. It stands on its own as a great Class AB amp, and there's no need to be misleading.

For headphone amps, i strongly recommend Class A. You really don't want to drive those to saturation and beyond. But for guitar amps, I personally prefer Class AB, as long as they break up in nice even harmonics. They take booster pedals better, and you can have a decent wattage without having a boatload of power tubes. They do seem to burn through tubes much faster, though.


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Post subject: Re: Tube Question
Posted: Wed Sep 10, 2014 10:47 am
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Why do you say that it can't be Class A and deliver 30 watts? Two EL34s can easily deliver 30 watts or more in Class A or Class AB, and they can deliver even more if used ultralinear. It is certainly possible, and some MB amps are good examples of push-pull running in Class A. Again, to be Class A, the tubes do not stop conducting at any time during the cycle. Being class A does not limit the wattage out, and quite the contrary, usually have high idle wattage.

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Post subject: Re: Tube Question
Posted: Wed Sep 10, 2014 10:58 pm
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shimmilou wrote:
Why do you say that it can't be Class A and deliver 30 watts? Two EL34s can easily deliver 30 watts or more in Class A or Class AB, and they can deliver even more if used ultralinear. It is certainly possible, and some MB amps are good examples of push-pull running in Class A. Again, to be Class A, the tubes do not stop conducting at any time during the cycle. Being class A does not limit the wattage out, and quite the contrary, usually have high idle wattage.

Not usually. Always. A Class A will draw the exact same wattage all the time, either converting it to heat when there's no signal, or part of it to the output stage and speakers when there is a signal. Whether it's push-pull or not doesn't matter - it will always have the same voltage and current draw (well, pretty sinusoidal, but over more than milliseconds, the same draw).

As for why 30W with a Class A dual EL34 isn't possible, the max rated EL34, Mullard old stock, could do 11W clean for one single-ended tube (25W power draw at max bias, where at most half of that can go to output, and in reality somewhat less).
Thus the theoretical max output for two tubes in class A on clean would be 22W, and in class AB it would be 44W. Given that the amp doesn't run at max plate voltage, and doesn't require the highest ranked tubes in the world, 30-35W is realistic for Class AB on clean, non-distorting. Somewhat higher if allowing distortion, but the rating is generally the max for clean, minimal distortion.

The amp is also "self-biasing", which is impossible with a Class A, because there would be no difference at all to bias on - it would always draw the same current.

If in doubt, measure how much juice the power amp section draws. If it varies with the signal you feed it from the preamp, it's not Class A. Which is just fine. Class AB rocks.

The variable current draw of AB can lead to shorter tube life, though - much as a lightbulb tends to last longer if you don't use a dimmer. The higher temperature differences in an AB configured tube causes more internal mechanical stress, if I understand the explanation correctly.


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Post subject: Re: Tube Question
Posted: Thu Sep 11, 2014 1:14 pm
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Yeah, apparently your understanding of how tubes work, is a little convoluted. You can have "self biasing" for Class A, it's called Cathode biased, and in fact most single ended Class A amps are Cathode biased. There are also Class AB amps that are Cathode biased. You can also run in Class A with fixed biasing. There is no need to limit the the wattage out.

And no, the tubes will not always draw the same current, that can't happen or there would be no sine wave out, the current fluctuates with the signal, it has no choice. :wink:

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Post subject: Re: Tube Question
Posted: Thu Sep 11, 2014 2:05 pm
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shimmilou wrote:
Yeah, apparently your understanding of how tubes work, is a little convoluted. You can have "self biasing" for Class A, it's called Cathode biased, and in fact most single ended Class A amps are Cathode biased. There are also Class AB amps that are Cathode biased. You can also run in Class A with fixed biasing. There is no need to limit the the wattage out.

And no, the tubes will not always draw the same current, that can't happen or there would be no sine wave out, the current fluctuates with the signal, it has no choice. :wink:


"the current fluctuations in a proper Class A amplifier are always centered around the mid-point, that 50% of maximum which is the same as the idle current. What this means is that there is no net increase in the current flow like there is in Class B or AB, no matter how loudly you play. In a single-ended configuration, the increases and decreases in current flow are momentary (at the signal frequency), equal and opposite around that 50% mid point. At one instant of the A-440 there will be more current flowing, but in the next instant there will be an equal amount less flowing. Thus the total over any period of time remains constant."
Source: http://mesaboogie.com/US/Smith/ClassA-WebVersion.htm

"In a class A amp at idle, with no signal present, each tube is drawing (ideally) an amount of current halfway between cutoff and saturation. For example purposes, let's assume this is 100mA, and assume the plate voltage is 250V. The tube is therefore idling at 250V*100mA = 25W. Now, if a signal is applied to each of the two output tubes from the phase inverter, one tube's current will increase and the other tube's current will decrease by the same amount (because the phase inverter generates two drive signals that are 180 degrees out of phase).Let's assume that the input signal amplitude is enough to drive the first tube to 150mA - in this case the second tube is now at 50mA, because it decreased by the same amount as the increase of the other tube. Taken to the limit, if the input voltage is enough to drive the first tube to 199mA, the second tube will be driven to 1mA, which is right at the limit of cutoff. If this is the maximum clean output of the amp, and the saturation point occurs at 200mA, the amp is operating in perfect class A, because neither tube has ever hit cutoff or saturation up to the maximum clean output of the amp. The output transformer sums these two complete, unclipped, out-of-phase sine waves to generate a sine wave of twice the equivalent level of one side (this is how you get twice the output power of a single tube running class A single-ended).

If you average the current draw over the full sine wave, the increases and decreases cancel each other out and the average current is the same as the idle value, or 100mA. Therefore, there is no change in the current drawn from the supply."

Source: http://www.aikenamps.com/index.php/technical-q-a

End of discussion.


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Post subject: Re: Tube Question
Posted: Thu Sep 11, 2014 2:30 pm
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The push-pull amp most often thought to be Class A is probably the Vox AC30.

Randall Aiken did a thorough debunking of that. Skip to the second last paragraph if you don't want to digest the technical explanation ...

http://www.aikenamps.com/index.php/is-the-vox-ac-30-really-class-a

however I don't think anyone's about to stop using their AC30 just because of an oscilloscope trace.

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