"Class A" nonsense

[Guitarist]

Wow, that was a read and a half.

So all tube preamps are class A, unless you crank them above 4. Even until now, I figured triodes were generally Class A, when used as polite preamps. "Class A" simple evokes or communicates something "Top Tier" and "best of the Best"; something covetous guitar need.

If I sell an amp, she'll be called something like "Switchable Class A to Class X Preamps..." We know anyone who knows the difference, like Ampagers, will be building their own anyways The output could be called, "Scaleable Class".

Two feet away, I have the 50 Year Ann. Vox "hand wired" AC15 unit. The company has a good reputation but it seems that they've mislead the public many times. With a few tweaks this amp sounds better but the power amp distorts VERY quickly on both channels. It should be called an ABC15!

[Steve Conner]

having the warmness of Class A for soft tones, and the punch of Class A/B for attacks on the string.

But isn't that exactly what hot biased Class-AB does already?

I heard Randall Smith also has the patents on fire and the wheel.

[Old Tele man]

I heard Randall Smith also has the patents on fire and the wheel.

...and patents pending on Wind, Rain, Lightning and Thunder too.

[mooreamps]

. With a few tweaks this amp sounds better but the power amp distorts VERY quickly on both channels. It should be called an ABC15!

If you look at the print of an AC-15, you can determine the bias on the power tubes at approx -18 vdc. This means when the positive swing of the input single is higher than this, the power tube is in saturation ; compared to an amp using fixed bias with a value more closer to -55 vdc on the power tubes. Perhaps this is the reason you perceive your power amp distorting "very quickly".. .. I would still call this class A bias, and for the same reasons as I have stated as above.


-g

[pdf64]

-18V bias on the el84 pair of an AC15 sounds rather high - what schematic are you looking at to derive that? I would expect around 10V across the cathode resistor.
-55V would be appropriate to 6L6GC with >450V on the plates, but would send el84 (~300V on the plate) way into cut off, surely?

[mooreamps]

You're absolutely right, I'm getting my tubes crossed here. I'm also re-looking at the datasheet for the EL-84. Now it just occurs to me, they spec the grid bias for Class B operation at only -14.7 vdc @ 300 volts on the plate. My initial thoughts were I didn't think a PI could drive the bias of an EL-34 or a 6L6 low enough to drive them into cut-off, if they were biased for Class A. But as I take a second look at the EL-84, if the negative signal swing from a PI could drive it as low -20 vdc or so , then yes perhaps it could be driven into class a/b. It's the EL-34 that has to be driven below -50 volts ; or thereabouts to drive it into cut-off for class a/b. .

[Gollum]

They way I've learned to think about it (thanks to various published articles in books) is that the class of an amp HAS to be determined at maximum output BEFORE clipping (aka maximum clean output). At THAT point, is what class is it operating in? That's the true class of the AMP.

So as well stated, an amp can run in multiple classes, but this is the only way I know of to definitively define the amp itself. So just because an amp might truly BE a class A amp (meaning at max clean volumes it's output tubes are operating 360 degrees) doesn't mean the amp sounds it's best there. This amp might still be well regarded for it's ton beyond that range, once pushed into class AB1.

Now, the WATTAGE rating of an amp, I truly believe should be what the actual attainable output is under the designed conditions. Basically, turn all the knobs to 11 and strum a humbucking guitar. There's your wattage rating, and odds are it's a good RMS figure even, since the amp could do it all day long. Now, I'd bet that 99% of all tube guitar amps, other than single ended, won't be Class A at this point, so rating them there is completely pointless (hence why changing the rating can't change the class of the AMP in my opinion).

[mooreamps]

So, you're still wanting to quantify the definition as it applies to a musical instrument amplifier, and not audio amplifiers in general ; yes ??

[Gollum]

Yes, because musical application is complete apples to oranges compared to most other uses, since they can run a variety of various classes. Therefor, since you can't rate it at idle (since that's not realistic) you need to measure it at peak before clipping. Most "real" class A amps to me fall right in line in this point. Obviously once they're pushed hard enough they'll be operating in class AB1, but so will almost any amp other than a single ended amp.

[mooreamps]

Obviously once they're pushed hard enough they'll be operating in class AB1, but so will almost any amp other than a single ended amp.

Class AB1 is defined as implying a higher quiescent current and therefore more of the properties of Class A. This, sigh ; again is a function of bias, not drive signal.



-g

[wizard 651]

I don't mean to hijack the class A debate in a new direction, but I do have a "class question" for the class, (LOL). I could tell from your exposition, (and those of others), you know what you are talking about. I have an 81' Musicman RD100, that supposedly develops 100 RMS from just a pair of O.P. tubes, (6CA6's I think?). Subjectively it is the loudest sngl 12" amp I have ever heard. I also had an old Traynor YBA1A Mk II that got 90 from the same complement. Not sure if these numbers are cheating by allowing for >% of distortion in the ratings. I do not possess your level of theoretical understanding, but I enjoy trying to grasp the concepts. How do these two amps do this, (other than just upping plate voltage and beefier trnnys), and what class is the MM?

[Gollum]

Class AB1 is defined as implying a higher quiescent current and therefore more of the properties of Class A. This, sigh ; again is a function of bias, not drive signal.



-g

Please forgive my ignorance, but isn't Class A versus AB purely defined by weather or not a tube is operating 360 degrees, or 180 to 360 degrees? I can see how bias can effect how the tube will operate, and thus class, but I simply don't see how bias alone can classify the class of operation.

This is why I'm saying most "class A" amps can be pushed into AB. Once you've got the signal turned up enough the tubes will no longer be operating 360 degrees.

[mooreamps]

This is why I'm saying most "class A" amps can be pushed into AB. Once you've got the signal turned up enough the tubes will no longer be operating 360 degrees.

Well, yes it could. When the signal is at it's most positive peak, and if the tube is well saturated, ya well, it's "still conducting". Then, when the signal is at it's most negative peak, most triodes can not generate a signal strong enough to completely reverse bias the tube, and so it's "still conducting". The plate impedance of a 12ax7 is just too high to drive a power tube hard enough to turn it off, when biased for class A. Now, you could go with a triode with a lower plate impedance, like a 12au7, but then you don't have enough gain. Just my take on what I am seeing........



-g

[balijukka]

The plate impedance of a 12ax7 is just too high to drive a power tube hard enough to turn it off, when biased for class A.
-g

I find this quite untrue.
The downswing at the grid does not load the PI really much at all.
All you need is just big enough signal (100V pp does it mostly in fixed bias), since it's cap coupled.

jukka

[pdf64]

Wizard 651, the philips 6CA7 was a pretty tough tube, see
http://www.mif.pg.gda.pl/homepages/f...127/6/6CA7.pdf
On page 3 you can see the operating conditions for a classB pair with 800V on the plates, which will put out 100watts.
JJ and Svetlana EL34 have suitable spec in theory to replace them, but they may not have a long life.