Eeyore wrote:

Multi-grid wrote:

maxhifi wrote:

Stating that "the amplifier is Class A until XXX watts", really, is
telling you how hot the tubes are biased relative to the two extremes of
pure class A (full dissapation), and pure class (cut off).- Hide quoted text -


Class A has nothing to do with dissipation either. Just because some
marketing group noticed that class A means something good, does not
make it right either. Just because it seems to make sense is no reason
to *******ize the definition. Find some other way to describe it.

The definition of Class A is very simple. It requires that the output device(s)
never cease conducting under any signal condition.

It means slightly more than this because tubes don't cut off as sharply
as other devices.

So class A means where PP power tubes do not have Ia on either side of
the PP circuit ever
having Ia fall below 10% of the idle value.
Below about 10% of the idle value, the cut off behaviour becomes
substantially non linear, as opposed to substantially linear prior to
cut off.

Gradual cut off in triodes makes them particularly nice to use in AB
amps.
Minimal switching artifacts are generated. And the gm rises with Ia, so
the
load change reduction happening after cut off of one tube means the
triode compensates for the sudden drop in gain you see in pentodes and
beam tubes.
Its the internal triode NFB at work to give the low Ra.

And if the first 1/2 of output power maximum is class A by my
definition, then is its deemed A,
and the remaining 1/2 is B, and the total action is AB.

Most ppl will not hear any difference if the bias is increased to ensure
all power
up to clipping is pure class A.

The load experienced by one of the tubes in an AB PP pair is NOT a
straight line load
as one would draw across the anode curves of a given tube.
While in class A, each tube sees an almost straight line load, but
always of slightly
different ohm value because the gm of each tube isn't the same.
But the RL for each tube while in class A is approximately 1/2 of
RLa-a.1
Once cut off has begun in earnest during each wave cycle, the load each
tube sees
become its "class B load" = 1/4 RLa-a, and one has to draw a CURVED LOAD
LINE to describe the loading
of each PP output tube to describe what each is really doing.
The load of one tube is affected by the load on the other until cut off
happens,
and then each tube is on its own to deal with the load of 1/4 RLa-a.


Patrick Turner.



Graham