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Old 30th December 2012, 11:56 PM   #1
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Default Class switchable amplifiers

So I know that the Fender Prosonic has a three modes of operation. Class AB/1 Solid State, Class AB/1 hollow state, and "class A" hollow state. I do know that in the class A mode they switch to a cathode bias instead of fixed bias, which I believe is not class A at all.
Okay so with all these sale gimmicks of "class switching" does anybody actually make a true class AB to class A switchable amp. I was asked to quote somebody on building a class switchable amp but wasn't sure because some people think that they are switching classes but really just switching two tubes off (parallel push pull topology) or fixed bias to cathode bias topology.
True class A would want a different plate to plate impedance not to mention much lower plate voltage.
To sum up my dilemma building a true class switchable amp would be involved and costly! But I am thinking that most people are familiar with the sound of a cathode bias amp and think it is "class A" like the familiar argument of "my ac30 is a class a amp". The customer also was reading up on this topic from a company in southern California that claims class switchable operation in their amps. I am mainly wondering if this all a sale gimmick and these amps are really just switching off two of four power tubes or just switching from fixed to cathode bias.
All opinions and experiences are welcome except for company bashing etc.....
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Old 31st December 2012, 03:57 AM   #2
DUG is offline DUG  Canada
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Quote:
Originally Posted by famousmockingbird View Post
...
True class A would want a different plate to plate impedance ...
Why?
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Old 31st December 2012, 04:19 AM   #3
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Class of operation has nothing to do with cathode or fixed bias- both, or neither, can be class A.
Most companies provide a lower power setting by strapping the output pentodes as triodes for the lower wattage setting, which most of the time seems to take a little top end out- no bad thing if you are playing in a small venue and the top end can be a little too piercing.
BTW, at any point where you are driving an amp into output stage distortion or compression, you will have left class A operation.
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Old 31st December 2012, 07:12 PM   #4
Enzo is offline Enzo  United States
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As Bernie says, the class of operation has nothing to do with fixed versus cathode bias.

If you want to know how it works, the Prosonic schematic is on the Fender web site for download. Fender Support

The rotary switch on this amp selects rectifier, changes bias voltage, and switches in the cathode resistor for the outputs. The class A setting uses only the tube rectifier, which indeed results in lower plate voltage than the solid state. The bias voltage is reduced about 10% for the tube rectifier AB over the solid state rectifier AB to compensate for the B+. In the class A mode, the lower B+ of the tube rectifier is used, plus the bias is switched over to cathode resistor. The net result of that is of course lower voltage across the tube at the same B+. I don't know what the tube current is at that setting. Switching to cathode bias by itself does not make an amp class A. But in this amp that may well be sufficient to do so. It is a matter of operating point.

SInce the Prosonic only has two power tubes, it sems to me they probably are not just switching a pair of them out. I could be wrong.

Class A is class A, nothing cosmic about it. ANyone can make a Class A amp.

The only reason a class A amp needs lower B+ is because it conducts fully all the time, and you need to stay within tube dissipation limits. This is a pair of 6L6s, and at about 30 watts per tube dissipation as the data sheet max, this amp B+ on tube recto is low eneough you can get a ton of current through them without lowering it further.

Can you drive this class A setting out of class A? Probably. Does normal playing do that? Don't know, probably not much. Are they just making things up out of thin air at fender? Not really, no. Can you catch them on a technicality? Probably.
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Old 31st December 2012, 07:31 PM   #5
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Thanks for the replies but I think I must have mislead my question.

I know what class A operation and I also thought I made clear that I know that this has nothing to do with cathode bias or not so thanks anyway! I was just stating that most amps that claim class A operation are just cathode bias class AB1.


Like I said a customer was on divided by 13 website and saw a KT66 amplifier that puts out 29 watts in class AB1 and 13 watts in "Class A". I was just curious if anyone had any experience with these amps or any other amps that are switchable from Class AB1 to Class A.

With say the Mark III from Mesa Boogie it has four power tubes, two 6L6GC's and two EL34's that you can lift the ground at cathodes of 6L6GC's and only operate the two EL34's in "class A mode". I think the Mesa Boogie might actually run the two EL34's in Class A because it only puts out 15 watts in this mode. I did notice that the output transformer is of special design with separate primary taps for the EL34's.


So to restate my question of does anybody else make a class AB1 to true class A? And as far as the divided by 13 amp if they are switching to a true class A with the KT66's how is that possible? They must run the class AB1 conservatively in respect to plate voltage so that when switched to class A it doesn't over dissipate.
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Old 31st December 2012, 07:37 PM   #6
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Quote:
Originally Posted by famousmockingbird Click the image to open in full size.
...
True class A would want a different plate to plate impedance ...


Why?
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Click the image to open in full size.


I just noticed on published datasheets that most power tubes that operate in a class A operation have a larger plate to plate load than in class AB1 operation.
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Old 1st January 2013, 05:38 AM   #7
DUG is offline DUG  Canada
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Quote:
Originally Posted by famousmockingbird View Post
Quote:
Originally Posted by famousmockingbird Click the image to open in full size.
...
True class A would want a different plate to plate impedance ...


Why?
__________________
Doug We are all learning...we can all help
Click the image to open in full size.


I just noticed on published datasheets that most power tubes that operate in a class A operation have a larger plate to plate load than in class AB1 operation.
Hate to sound like a 3 yr old who knows one word, but:

Why?
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