From Dstor–(at)– Wed Aug 24 00:07:01 CDT 1994
Article: 26396 of
From: Dstor–(at)– (Dr.Distortion)
Subject: Cool Amps & Output Class
Date: 23 Aug 1994 18:55:36 -0400
Organization: Concentric Research Corporation
Lines: 80
Message-ID: <33dup8$kn--(at)>

LJ–(at)– wrote:
LJ>Class A amplification uses a single element (tube, transistor) to
LJ>amplify the complete signal, both positive and negative half-cycles.
LJ>You find 6V6s in Class A amps.

LJ>Class AB uses a “push-pull” scheme that uses separate elements to
LJ>amplify each half-cycle. One for the top (positive half-cycle) and one
LJ>for the bottom (negative half-cycle.) There are variants to Class AB
LJ>amplification like Class AB1 and AB2, but the basics are the same.
LJ>You find 6L6GCs and 5881s in push-pull amps.

LJ>There are also Class C and Class D, neither of which are used in audio
LJ>amplification (as far as I know.)

LJ>[Note to amplifier gurus: Yes, I know it’s oversimplified, but he
LJ>didn’t ask how to build one. I’m just trying to answer Alex’s
LJ>question. By all means, correct me if I’m leading him astray.]

Your simplification is a good one, but assigning certain power tube
types to specific output classes is a bit misleading. You can find
any type of output tube used in either class. There are class-A 6L6
amps and class-AB 6V6 amps, for instance.
Also, class-A doesn’t necessarily mean that only one output device
is used. Class-A can also incorporate multiple devices used in
parallel, push-pull, parallel push-pull (like the AC-30), etc.
No matter how many output tubes/transistors are used, the amp
is class-A if all output devices conduct 360 degrees (or 100%
of the cycle).
Your description of AB is actually more applicable to class-B,
where each device (or set of devices) conducts 180 degrees.
Class-AB is more accurately described as the bias region where
each device or “side” conducts less than 360, but more than
180 degrees.
Class-AB can be broken down further into AB1 and AB2. Grid current
does not flow in AB1, whereas there is grid current flow in AB2. AB2
produces more power and needs a higher driving voltage. Some audio
amps are biased so far into AB2 that they’re virtually class-B; in
fact, I’ve heard some amps biased into class-B that sounded better
than you might suspect. Class-B is VERY efficient and powerful.
Most Fenders using two 6L6 tubes, made from the early ’60s to
the present, are biased in the AB2 region. Most will put out about
50W of clean power, and some pretty serious power (in the hundred
watt region at times) on clipping. That’s why a Fender with a
nominal 40 or 50W rating is plenty loud enough for most gigs; I have
a ’71 Super Reverb that I’ve used to play in loud hardcore punk bands,
and I’ve never had to turn it up past “3”–even during outdoor gigs.

LJ>The practical differences are: Class A is associated with low-power
LJ>amplifiers like Champs and AC-30s (I think.) The sound is generally
LJ>pretty sweet. Class AB is usually thought of in terms of the more
LJ>powerful amps like the Bassman, Super Reverbs, and Marshalls. The sound
LJ>tends toward clean but “gritty”; variations are pretty wide.

The variations are so wide, in fact, that it’s almost impossible
to generalize. Strictly speaking, class-A is in fact more LINEAR than
AB, although there’s a higher percentage of second harmonic present.
That may account for the perception that class-A is “sweeter”, although
it’s my opinion that the characteristic sound of amps such as the
AC-30 results as much from their lack of negative feedback as from
their use of a class-A output stage. In theory, second harmonic tends
to be cancelled out in push-pull outputs, but in practice that’s
not always the case.
A class-AB output stage with little or no negative feedback can
sound surprisingly close to the sound many associate with class-A,
due to the presence of more harmonics. An amp with no negative feedback
will tend to go into distortion more smoothly and gradually, whereas
an amp with a fair amount of feedback (like Blackfaces and most later
Fenders) will tend to stay clean all the way up to a point where
they start distorting suddenly. (“Like a bubble bursting”, my friend
Joe Pampel likes to say).
To close this longwinded note, I’d like to say that I think the
current obsession with class-A is just a fad, perpetrated by boutique
amp makers and facilitated by technically ignorant guitar magazines.
A properly-designed class-AB output stage can sound just as good
while giving you more power with less heat and lighter demands on
the output transformer and power supply.


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