Re: [LAD] Permafrost guitar amp, was Re: RDF libraries, was Re: [ANN] IR: LV2 Convolution Reverb

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Le Sat, 5 Mar 2011 18:55:19 +0100,
Giuseppe Zompatori a écrit :

> From: Stefano D'Angelo

With triodes, the preamp will include at least 2 stages (like the 2
valves of an ECC81). With pentodes, you get higher gain and can make a
complete guitar amp with 2 tubes like in the Fender Champ :
http://www.drtube.com/schematics/fender/champ-5c1-schem.gif

Also, be aware that each manufacturer make compromise between the sound
quality and the manufacturing costs. As example, a well-know mark is
using cheap power transformers, and when at full volume, the sound will
be very bad because half of the distortion you will ear is due to
saturation into the power transformer. As a consequence, those
amplifiers are widely used by jazz musicians but almost never by rock
musicians. The laters will also often blow the power transformers.

Another well-know mark is using good but small output transformers, as
well than better power transformers than the precedent one. The
consequence is than the sound is very good at full volume, but a rock
or blues musician will often blow the output transformers.

Generally speaking, a common source of non linearities, and often
ignored, is due to a non adequate driver stage. Tubes are by design
made to work best at high impedance. To get a low output impedance,
you need a transformer. But that's expensive hardware, especially in
class A. Preamp and driver stages are class A stage, when the output
stage is generally a class AB2 push-pull where the grid can become
positive in respect to the cathode.

Common tubes for such a push-pull are 6L6. According to the datasheets,
such tube can take 0.35 watts and the driver output impedance must be
lower than 500 ohms:
http://www.mif.pg.gda.pl/homepages/frank/sheets/021/6/6L6.pdf

It is no one single guitar amp I know on the market with a driver stage
that can provide such a power to the output stage, and this is a major
source of non linearities when the output stage is driven at high
volume.

It would be simple to design such a driver : one tube like a
triode mounted EL82 in a class A stage with a high quality driver
transformer. But it will cost too much money, so no one single
manufacturer I know is using such a design. It is too bad because such
this sound just terrific, the sound is clean and fully saturated at the
same time, and its dynamic is outstanding. :)

Be also aware than the valves models used by simulation software like
spice are good for consumer audio equipments or sonorisation stages, but
than they don't take in account the saturation that a guitar amp will
provide. In other words, they are completely out of business in regard
to correctly simulate a guitar amp at high volume, or even a simple
preamp stage in saturation. For that, a much better approach would be
to use models based on the constant current family of curves:
http://www.agsrhichome.bnl.gov/AP/ap_notes/ap_note_97.pdf

My 2c

Ciao,
Dominique

>

--
"We have the heroes we deserve."
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Messages in current thread:
[LAD] Permafrost guitar amp, was Re: RDF libraries, was Re: ..., Giuseppe Zompatori, (Sat Mar 5, 5:55 pm)
Re: [LAD] Permafrost guitar amp, was Re: RDF libraries, was ..., Dominique Michel, (Sat Mar 5, 7:26 pm)
Re: [LAD] Permafrost guitar amp, was Re: RDF libraries, was ..., Giuseppe Zompatori, (Sun Mar 6, 12:38 am)
Re: [LAD] Permafrost guitar amp, was Re: RDF libraries, was ..., Giuseppe Zompatori, (Sat Mar 5, 6:14 pm)
Re: [LAD] Permafrost guitar amp, was Re: RDF libraries, was ..., Giuseppe Zompatori, (Tue Mar 8, 4:56 pm)