Re: "Tube watts" vs. "solid state watts"

RichL expounded in news:5cmdnTIiiMHsVSTTnZ2dnUVZ_tydnZ2d@xxxxxxxxxxxxx:

How are amps rated? When a manufacturer says a particular amp is a
"30 Watt" amp, what is meant?
...
But let's get more specific. Most honest manufacturers rate an
amplifier according to the largest output power it can produce
without exceeding a certain level of total harmonic distortion (THD),
and the good ones will specify that THD level (usually expressed as a
percentage). THD is defined here:

http://en.wikipedia.org/wiki/Total_harmonic_distortion

The power rating of an amp depends on the level of THD specified. An
amp rated at 30 W, 1% THD is louder than one rated at 30 W, 10% THD.
That's because the amount of distortion (obviously) depends on how
hard the amp is driven, and for a given amp you reach the 1% THD
point at lower output power than you do the 10% THD point.

By the way, 1% THD is a typical spec.

First of all, in SS terms, 1% THD is horrible! More on that later.

This was precisely what I presented not too long ago. But some folks
"don't like rules". They want to compare results without any defined
method.

So what's all this got to do with "Tube watts" vs. "Solid state
watts"?

It's pretty simple, really. What it comes down to is this: musical
instrument amplifiers can produce power in excess of the power at
which the THD specification is achieved.

No argument as stated but of course implied in this is that the excess
power is also producing excess distortion.

For the audio purist, you
don't want to go past the 1% THD point (some perfectionists would
insist on much lower THD for accurate music reproduction). But
guitarists can tolerate, and in fact many would prefer, distortion
well beyone the 1% level.

How much beyond? It depends on whether it's a SS amp or a tube amp!!

As I was a sayin', this goes _beyond_ the "ratings".

See this is the thing- if you don't care about THD, you could build a ss
amp (or even a tube one) that spews out gobs of crap. If you don't care
about distortion almost anything can deliver more power. Of course, were
talking about pleasing distortion here -- that "artistic element".

For solid state devices, the onset of clipping is abrupt, and most of
us would agree that a SS amp driven beyond clipping sound harsh and
unpleasant; you don't want to take a SS amp there!

One has to be careful about making general statements here. This is
definitely in the realm of general experience (I would think), but does
not necessarily make it generally so. There are several distortion
mechanisms, some soft, some not so.

So what's the THD for a SS amp driven to clipping?

You must take into account is what kind of clipping is going on. First of
all, almost all distortion pedals use semiconductor diodes to create the
distortion (FETs in more exotic ones). Yet people are not objecting to
that tone..

The bad distortion in ss amps can come from a number of sources:

- is it [hard] clipping in the preamp (bad design)?
- hitting the limit of the class B power capability?
- is the SOAR circuit being activated to keep the trannies
in the Safe Operating Area Range?
- is the speaker protection circuit being activated?

Any or all these are factors in the output when you exceed maximum
ratings of a ss amp. There's much more variation in ss power amp designs
than there is in tube amps. It is therefore, much more difficult to
generallize.

Tube amps have no protection systems at all, or I'm not aware of any in
guitar amplifiers. At most a fuse or thermal cutoff. Tube hifi/stereos
may have a small number of special design cases. A tube amp will let you
run it until you melt the tubes, the OPT or the power transformer or it
simply catches on fire.

A SS amp OTOH, must protected the output transistors. They don't just
glow red and more-or-less return to normal when you shut it off. The
semiconductor junction just phzzzsssts instead! Nor are power transistors
as convenient to change. Adjusting a class B ss amp is trickier as well.
So keeping them safe is very worthwhile.

Specialized circuits are added to the ss power amp so that if
voltage+current crosses an eliptical Safe Operating Area Range that they
start removing the drive into the base of the power tranny. So obviously
when this limiting is occurs, the audio "quality" is sacrificed in the
process (but saves the amp).

Then there is the speaker protection circuit. Since any SS worth it's
salt today has _NO_ heaving tone sucking output transformer. The ss power
finals are coupled directly to the speaker (except for some capacitively
coupled amps which tend to be lower power). Since speakers can cost more
than the power transistors, there is a safety circuit there also. This is
in case only one driving side blows and DC current flows continously
through the speaker(s). The design of these circuits vary _enormously_
ranging from crowbar circuits that blow fuses to a signal foldback scheme
similar to the SOAR protection.

What most folks think about is the effect of the power trannies
themselves and this too can be a factor. However, in a well designed SS
amp, this doesn't happen since NFB generally corrects this. It can happen
just before SOAR kicks in where the trannies hit limits that NFB cannot
correct any further.

But here is the single most important thing about ss amplifier ratings,
when it comes to power: they tend to be _absolute_ _maximum_ ratings. A
ss amp will maintain excellent low THD distortion up to its absolute
maximum limit, due the use of fairly high NFB. Tube amps OTOH have
distortion figures climbing quickly as the power level rises near the
limit.

Once you cross that power rating line, NFB might not be able to help
(depends upon design), or a protection mechanism is activated. In any
case, hard limits are involved.

But can you blame this design? The maximum rating is a maximum rating.

With a tube amp, the THD starts to rise quickly but it isn't as sudden
as the SS as it crosses the limit. The tube amp has no protection
mechanisms and nothing to act as a hard limit. But everyone does know the
THD has suddenly risen in the tube amp because it is quite audible.

But this does not mean one amp's watts are different than the other?

Nope.

You can just as easily pit a bigger ss amp against a tube amp and win
that fight. It's just that the ss amp is rated at it's hard limit.

So the comparison problem is that if you ignore the tube amp's THD, it's
power rating is not actually the _maximum_ power rating for that tube amp
(it is actually higher). Of course this means adjusting the audio
quality (THD), which is what musicians do.

The point is, in tube amps you can go way past the ~10% THD limit
characteristic of SS amps and STILL SOUND GOOD! Which in turn means
that for a tube amp rated at 30 W @ 1% THD, you can get well past the
output power limit of 37.5 W for the SS example described above and
still have the amp producing sound that's pleasing to the ear!

So, in essence, for a tube amp and a SS amp that are rated the same
in terms of power at 1% THD, the tube amp is capable of producing
more "USABLE WATTS" than the SS amp!

Again, this is because, if you discard the audio quality requirement, you
then accept that the tube amp's rating wasn't an absolute maximum rating
after all. IOW, if you now agree that 10% THD is ok, then you've
implicitly increased the power rating of that tube amp in the process.

But a normal SS amp (I can't speak to class D) is good right up to its
maximum rating. Right up to the bitter end (unlike a tube amp).

NOW, if you turn that around and made the power rating to 0.002% THD,
which ss amps quite easily achieve today, SS would trounce the avg tube
(guitar) amp! SS can trounce them in quality. Tubes are noisy and the
traditional g amp tube designs are not going to provide that level of
performance that SS can.

But if you find tube distortion to be a positive feature of a tube
amp, yes, you'll get much more USABLE watts out of a tube amp with a
given power rating than out of a SS amp with the same rating.

That is all.

Discuss.

A lot of things are done in the name of tone. The two technologies are
very different to be sure. I'll also suggest that if you want an
acoustic amplifier, you want that 0.002% THD figure (not the lousy 1%).

Framing the statement "tube watts > ss watts" does not convey all of the
finer points involved.

If you're goint to agree 10% THD is ok, then your 100W tube amp would be
rated at 150W then (@ 10%THD). A competing 0.002% SS amp is perfect up to
that 100W limit and then goes ballistic in crap over 100W.

But clearly this is no mystery- we're now comparing a 150W tube amp to a
100W ss amp! It's no surprise then that the 150W amp wins. But if you
put a 151W ss amp up against it, ss would then win.

Watts is watts.

Snark.
.

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