Modern class A

[tube geek]

I have been hearing the term "Modern Class A" being tossed around a lot lately can someone explain the diff between this and the common class a from decades ago?

[TheGimp]

No such thing as modern vs classical class A.

Class A is Class A. It means the tube remains in conduction 100% of the time.

Class AB1 is class A at low levels, and class B at high output levels. Class B indicates the tube is in cutoff part of the time. So, class B is less than 100%conduction time.

AB2 would allow for grid current to flow, whereas AB1 is no grid current (grid drive below 0V at all times).

[dartanion]

No such thing as modern vs classical class A.

Class A is Class A. It means the tube remains in conduction 100% of the time.

Class AB1 is class A at low levels, and class B at high output levels. Class B indicates the tube is in cutoff part of the time. So, class B is less than 100%conduction time.

AB2 would allow for grid current to flow, whereas AB1 is no grid current (grid drive below 0V at all times).

Class A is the condition where all output tubes are biased to where they conduct plate current throughout the 360 degree input cycle. Again measured at max output before the onset of clipping.

Class AB is not class A at low output levels. This goes against how we measure output and class of operation. Again, measured at max output before the onset of clipping.

[TheGimp]

Class A 360degree conduction
Class B 180 Degree conduction
Class C less than 180 degree conduction (Not used for audio for the most part).

Class AB in between class A and Class B. Conduction of each tube less than 360 degrees and greater than 180 degrees.

[Reeltarded]

The "TERM" comes from a Line 6 manual, and is their way of saying "Matchless".

Lose that term, please.

[Reeltarded]

oops

[Zippy]

The "TERM" comes from a Line 6 manual, and is their way of saying "Matchless".

Lose that term, please.

Lose the Line 6 too.

[paulster]

Modern class A, in my experience, seems to mean cathode-biased class AB these days.

Even Ken described the Rocket as a class A amp in his literature when it blatantly isn't.

[Zippy]

Modern class A, in my experience, seems to mean cathode-biased class AB these days.

Even Ken described the Rocket as a class A amp in his literature when it blatantly isn't.

That was an homage to the Vox AC30.

[paulster]

That was an homage to the Vox AC30.

Exactly. That most Class A of amps!

[Zippy]

For those not familiar with the issue see:

http://aikenamps.com/ - Tech Info - Advanced (Aiken) - "Is the Vox AC-30 really class A?"

[dartanion]

I also suggest that folks should grab a copy of an old RCA tube manual and read the introductory chapters. This should be mandatory reading for all budding builders.

[TheGimp]

To correctly phrase what I was trying to say, in class AB neither tube may go into cutoff at low output levels (similar to class A), while at higher levels both tubes will spend part of their time in cutoff.

This distinction is improtant because there is a difference in distortion profile between the two. While both tubes are conducting for 360 degrees at lower output levels the distortion will be fairly low. Once one tube goes into cutoff, the matching of gm becomes critical in that perfectly matched tubes have the same output levels. Miss-match tubes result in an asymetrical output and a significant increase in harmonic distortion.