surfsup wrote:Thanks guys, great posts! FunkyE, i thought on a SE you could still bias the tube hot to achieve AB. I guess i read that wrong elsewhere...thanks, i didnt think to mention the single tube because i didnt think it mattered.
In a single ended circuit there is no "B".
SE is Class A by definition.
FunkyE9th wrote: just trying to understand how it will fall out of class A and what class it ends up in. Maybe a class of its own?
There is no "Class" definition for a misadjusted or poorly designed circuit.
It's still Class A, it's just not working right.
The correct adjustment (or design) places the full output at the place where it will clip or begin to distort evenly on both top and bottom peaks of the signal.
Firestorm wrote:You can bias (misbias) an SE amp so it falls out of Class A; but you sure don't want to. I can't think of any advantage in trying to set this amp up with fixed bias (although you could). Just stick a 2-watt 470R on the cathode, bypass it with a 25/25 or 25/50 and you're done.
How does it fall out of Class A and what class does it fall into?
You can bias it cold and all it will do is go into cutoff sooner so your max clean output swing is reduced (i.e. rated output power reduced). If you bias it hotter, then the grid conducts sooner and max swing is reduced again. Does that change the class of operation?
Even if you biased it smack in the middle of the load line, if you drive amp hard enough (i.e. outside of max clean output swing) it will go into cutoff or the grid will conduct.
Not trying to be argumentative, just trying to understand how it will fall out of class A and what class it ends up in. Maybe a class of its own?
In Class A, the tube must conduct for 360 degrees of the waveform. Part of the definition. If you bias too close to cutoff for the amplitude of the max input signal, the tube will cutoff for part of the 360 degree cycle so it's no longer Class A. Shall we say it falls into Class B? Or Class C? Depends on how "technical" you want to be.
How about "It falls into the Class of Sounding-Like-Crap?"
When a SE output tube reaches cutoff, the output is distorting. That is the point at which there is no class. The class is defined only by the behavior that occurs before output distortion.
Keep in mind that PP outputs reach cutoff (on one tube) while the other is still conducting. So the output is not automatically clipping/distorting just because the "other" tube has stopped conducting.
My '74 VC reaches cutoff before saturation on the 6v6. It is class A no matter what volume, distortion level or time of day.
Except that single-ended Class B amplifiers as used in radio clearly do cut off, thus distort (but it doesn't matter) and are still considered Class B. The nomenclature is limited. What are we to call the transition point in an amp like the AC-30 where it stops being Class A?
Firestorm wrote:Except that single-ended Class B amplifiers as used in radio clearly do cut off, thus distort (but it doesn't matter) and are still considered Class B. The nomenclature is limited. What are we to call the transition point in an amp like the AC-30 where it stops being Class A?
The Vox AC30 doesn't fall out of Class A because it's not Class A to begin with. <hint: it's a push-pull design>
It's a hotly biased ClassAB circuit.
To bias an AC30 so hot that it doesn't change the output tubes' current from idle to full output would exceed the limits of the EL84s at the voltages used, and the tubes would quickly fail.
What terrible thing did EL84s do in a previous life that led them to be so mistreated. Most amps just beat the crap out of them. The Vox has them above their rated dissipation at idle; so does the DM25; the Fender Blues Junior crosses the line once you turn it up to "3." Poor little bastards. But their history does show that the 12W rating is way conservative.
Yes, the AC30 is a hot Class AB amp, which I think extremely odd. Not so much because they could have really made it Class A with a slightly different PT (there was no Class A mystique at the time). But because this was an era where output power was beginning to matter and if they had simply biased it colder they could have got 35W out of it instead of the measly 23W they wound up with.
