Building a Tube Amp

[Eeyore]

Guitar distortion seems to be another fringe thing.

No it's totally mainstream. You have it exactly the wrong way round.

Graham

 

[Kevin Aylward]

It's the norm.

Having said that, its so....... much hassle though, that I am back to stomp
boxes with "amp simulators". In fact, my new tascam 2488 digital multitrack
has a special guitar processor section, and I am actually quite happy with
it. You can dial up different cabs and distortion, echo etc.


Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk

 

[Kevin Aylward]

A fallacy actually. The even order aspect is only significant when applied
to the preamp stage where the square'ish law is being used for both half
cycles of the waveform, although some odd harmonics are also produced in
practise.

SS amps usually have lots of feedback. This causes hard limiting. Valve amps
may typically only have 15db of feedback, hence softer limiting.

Not if you are using push-pull, the even harmonics cancel, leaving the
odd-harmonics as with solid-state.

Another fallacy.

Most guitar amps run in push pull *Class B* (i.e AB'ish). The cancellation
can only occur when both output devices are active simultaneously. Most of
the time one device is inactive, so you can not get the I^2 - (-I)^2 effect,
except around the x-over point.

However, I agree that both ss and valve amps have significant odd order
harmonics. If we look at the output waveform of either a ss or valve amp, it
is pretty much universal that the positive and negative halves of the
waveform are the same, by design, i.e symmetrical, i.e odd functions (x^3,
x^5 etc). hence, predominantly odd harmonic distortion. Usually, its the
output stage that dominates the distortion, even when sounding clean, hence
its irrelevant that earlier class A stages may generate more even (x^2, x^4
etc) distortion than odd.


Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk

 

[Kevin Aylward]

Not even remotely.



The best output transformers are in Europe IMHO. Lundahl and Sowter
for example but hardly cheap. SS amps don't need output transformers.

However, I am thinking of resorting to that to get my Marshall AVT to give
its full power into its own cabinet. Like, its just about impossible to get
single 4 ohm 12"s, especially at 100w+

So, I emailed marshal once and said , do you know of any sources of decent 4
ohm speakers to put in my amp. The replied, no, but why don't you just
connect up another cabinet...dah....

Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk

 

[Kevin Aylward]

You can't... the components in a tube amp react completely different
than digital algorithms. Analog solid state tends to end up sounding
very harsh(because of the way they clip as I mentioned).

Not necessarily. After being a long time valve man, its surprising just how
good some ss effects can be.

If you don't believe me go to your local music store and ask one of
the guys to show you how a real tube amp sounds. Compare it to a
solid state amp and then to a modeling amp. The tube amp wins in
sound quality.

I used to use a tube amp, but now can't be f%^&%ed with the weight. I now
use a Marshall AVT150 transister job, although it does have one ECC83 in it.
However, not for its distortion. I am happy with it, that is after I had to
cut and rewire its effect loop as the damm f%^*&sh£$ "design" techs at
Marshal have no idea that a || loop is useless in a guitar amp. Like, if I
am using its pre-amp distortion, how the f^&* do you insert a volume pedal
in the loop and expect it to control the sound down to zero. Or like, insert
a noise gate and have it actually work?

Supposedly there are many factors in tube amps that make them sound
better

~~~~~

Different.

You know the one about meat and poison?

Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk

 

[Eeyore]

And plenty of bands that are all transistor.

Not a single one I know of, and I am in the business.

Graham

 

[Eeyore]

Of course. We are talking about the subjective opinions of people who
are probably drugged-out most of the time and hearing-impaired all of
the time. None of which involves electronic design.

And you're being a complete arse through total ignorance of actual practice. Drugs
aren't 'cool' any more anyway.

Graham

 

[Eeyore]

No, I want none of that.

You'd better only listen to jazz or orchestral then.

Never liked say Led Zeppelin, The Beatles, Black Sabbath, The Kinks or any other
'rock or pop' band ?

Graham

 

[Kevin Aylward]

Not in a Vox AC30 it isn't, despite the name.

Its that "probability to failure" and how much can you afford bit again. I
remeber when ECC83s were less than a pound.

Kevin Aylward
[email protected]
www.kevinaylward.co.uk

 

[Kevin Aylward]

No it's totally mainstream. You have it exactly the wrong way round.

Graham

Yeah, that is a really interesting comment from John. I do have some clean
guitar work, but by and large, I generally play solos with distortion. If we
went to see a band live, its essentially, a certainty that the band will be
playing with distortion. So, its ceratinly main steam...

However, statistically, how often is the guitar played with distortion
against without? I am not so sure on that one. The majority of music is not
Rock. apparently, its err "Bass and Drums". Oh for the days of Joe Tex,
Bogie nights, and Gloria Gaynor...music you could actually dance to with
some interesting chord sequences and bass lines...yes boys and girls I
uncelebrated my 50th on the 2nd, so its nostalgia time, all over again...

Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk

 

[Eeyore]

Actually I was thinking about you as I was dating my information, and
puffing off theory with little recent practical knowledge.

What _is_ current usage? (or do they let you into the recording
studios?)

How do you think I met Knoppler's wanky guitar tech. I do more live sound though.

I would assume that the Euro-pop bands are all over the DSP
stuff,

What DSP stuff ?

but I also assume that there's going to be more inertia among
'traditional' rock musicians.

Why change what's not broken ?

I knew a guy (only 10-year old information!) who was in a Seattle
grunge-rock style band who used Great Big Tube Amplifiers along with a
volume control made out of the heating element from an oven. Basically
they treated the gain control on the amp as a "tone" control, then used
power attenuators before the speakers to fit the sound down to the size
of the hall.

There are idiots to be found everywhere.

But what gets used "normally", if you can pick just a few examples of
"normal"?

For guitar amps, valves. You only use transistor if you can't aford a real valve amp and
even then they 'try' to make it sound a bit 'valvey' now. For electric bass it's not
such a big issue. maybe 50/50 tube vs SS depending on your preference. Keyboards
probably mostly SS now.

Graham

 

[Eeyore]

For 'amplification' one would like as little distortion as possible.
For wave shaping, it is better done at lower power levels.

If you can specify the waveform (distortion type) you want,
then you can can make that either digitally or analog at a low
level, and then use a HiFi power amp.
At least you would have a decent amp

There were people saying this in the 70s.

They were wrong then too ! A few people have tried hybrids but they all sank
without trace.

Graham

 

[Eeyore]

Wrong type of distortion. Compare the transfer characteristics. Tubes produce more
even-order harmonic distortion, transistors odd. And odd order sounds horrible.

That just gives the 'fuzz box' sound.

You should read 'semiconductors' in the widest sense.
As I pointed out AD - EPROM[s}- DA can make any curve,
and those are semiconductors too.
I used that to make gamma curves for a lightshow, it works for
audio too.
You could perhaps do that in FPGA and use blockram, and have different curves
selectable on the fly.
Or just use SRAM and some multiplexers in good old 74HCXXX logic, done
that too, have the PC or a micro write the curves to the SRAM.
Not so many points for 16 bit audio, although for guitar 8 bits would do likely ;-)
mm maybe they even only use 20 dB dynamic range....[/QUOTE]

DSP attempts to model the tube sound have been made with varying degrees of success but
modelling the loudspeaker is a real killer. Especially if you want the 'edge sound'
rather than the 'centre sound'. The mixing engineer does this simply with mic
placement.

It's easier just to use the real thing.

Graham

 

[Eeyore]

Guitars can be played with any kind of amp and don't necessarily
need to be driven into distortion. Thats determined by the artist.

That's for jazz guitarists.

Graham

 

[Eeyore]

The type of speaker distortion is also part of the power output
stage.

No it's part of the speaker. Does 'cone break up' mean anything to you for example ?

Graham

 

[Eeyore]

There is a group of people with special skills who can
make it so that the low level stage *does* have the
same "irregularities" as the power output stage.

They call us "Engineers"...

Yes, I've considered that. I wonder if it's patentable ?

Graham

 

[Eeyore]

I disagree, the speaker distortion is dominated by the speaker
characteristics.

You are, as is so often the case, quite correct. That's why they make special
loudspeakers for guitars that don't even remotely resemble hi-fi speakers.

Graham

 

[TheM]

There have been some acoustic-based isolated power couplers, usually
solid-rod piezo things. Probably more efficient than a
tube-amp/speaker/microphone combo.

John

Sure, that combo is insane. I'll add "<irony on>" tag next time

M

 

[Eeyore]

That argument could easily be applied to building violins out of
plastic, or just using sampled violin sounds in a synthesizer instead of
a string orchestra.

Yet somehow, when I go to the symphony it's all flesh-and-blood people
playing wooden instruments.

Strange that ! ;~)

Circa 1985 you couldn't build a solid state 'distorter' that would
satisfactorily replicate the sound of a tube amp.

Not very well at all actually but nm.

You may be able to now,

It'll be DSP now.

but it takes a tremendous amount of work to
design. There's a lot of dynamic nonlinearities at work, in the tubes,
in the transformers and in the speakers. And it's not limited to the
audio path -- even the power supply sag modifies the sound ("adds
crunch", per the folks who study this).

Yes, yes and yes.

Even today, if you want the genuine vacuum tube sound the easiest way by
far is to build a genuine vacuum tube amp.

Remarkable isn't it ? They work and sound like musicians want them to sound.

There may be value in
learning all of the little details of an amp and replicating it (and,
AFAIK, it's done and marketed for high-end guitar amps) -- but there's
still a lot of value in making them the old way, as witnessed by all the
new vacuum tube guitar amps out there.
Tons.


So yea, for reproduction the "vacuum tube sound" probably makes no
sense.

Indeed not, because it's adding more distortion to what the producer and mix
engineer wanted you to hear in the first place.

But given that the distortion of the amplifier is an integral
part of rock & roll guitar, I think it'll be a while before vacuum tube
amps can really be replaced.

I can't see it happening in my lifetime. The cost of the DSP to recreate every
subtlety of the control settings, input and output stage distortion and speaker
choices would make it financially prohibitive.

Graham

 

[Eeyore]

Tim Wescott wrote


Maybe you have not noticed, but whole orchestras come out of the paper cone of your loudspeaker,
and the plastic membranes of your headphone.

Do you never attend live performances ?

Graham