Quote from: QReuCk on July 24, 2012, 05:21:46 AM
Thanks for this explanation. Not so sure I understood all of this, but it's an interesting point of view.
I'm just wondering if these non linearities that are enabled either by a tube/output transformer or by a SS amp with CF are in certain cases what causes the dynamics of the playing to be exagerated.
Wouldn't say for sure I have a comprehensive understanding of what "linear response" to dynamic playing should hear like, but I'm usually pretty good at producing the sound (both tone and volume) I want with accoustic guitars and at controling the distortion level of a crunching preamp with just how hard I attack the strings. Fact is, on some tube combos pushed in there usually prefered volume range, I often struggle to control these dynamics, generating barely audible sound when plaing soft and ice-pick-through-the-ears far too loud notes when picking just a bit harder. Just in the interest of better understanding, could you tell me if the dampening factor might play a part in this?

CF will not present you with a solution to your endeavours sadly.  It is so easy to get wrapped up in technical 'stuff' that might bring hopes of an instant 'playing' problem cure.  Guitar sounds, in any genre, are helped mostly by the ability of the player and many years of experience of playing.  Particularly in a live situation where 'feeling' the sound is so important and balancing pick pressure against drive and volume is a real skill to be learned. 

Trying to abtain the characteristics you seek in a domestic environment might not bring much fruit.  Attempting to recreate recorded sounds are too, a waste of time.  They are mic'd with microphone(s) (there may be two or three used in various placements to create a sound) that add their own harmonics to the tone, then EQ'd by an EQ that works at different frequencies to guitar amp EQs, compressed... need I go on?  A complete non starter!  Not even the original artist can copy his recorded sounds exactly!

I know some very clever electronics designers who cannot design a decent guitar amp.  Designers should have 'experienced guitarist' on their CVs to qualify.  Many of the 'nonlinearity' and other attributes that pure technicians bestow upon guitar amps, although measurable, provide very little audible difference to a guitar amp tone.  By the same token, I know many excellent players who just cannot find a great sound, even with boutique equipment.  They either have it or they don't, or they need to practise more playing live.  I guess... to be brutally honest.  Most really experienced players can get a reasonable sound out of almost any old amp... on demand.  CF is not the panacea.  But it does help a tranny amp to sound much closer to an amp with an output transformer.

Ice-picky tone at volume is normal.  The human hearing responds better to high and low frequencies at higher volumes (see Fletcher & Munson's work).  You should EQ the amp for the volume you're playing at!  Amps with a cheap output transformer, like guitar amps, also have narrowing bandwidth ability as they become heavily driven/magnetised.  Hence Peavey's 'T-Dynamics' circuitry.  But you can simply EQ the amps for loud playing... always refer to what your ears are telling you!

Hope this is helpful.

Quote from: teemuk on May 26, 2012, 09:26:44 AM
I think it really falls down to how each individual perceives certain characteristics in tone.

Personally, I hear the effect only as slight boost at low and high frequencies. I usually have to struggle to hear it as well.

Then again, I've encountered people who say it makes an astounding difference. Last one saying so wasn't even talking about current feedback per se but about differences of running a 100% tube amp to either purely resistive dummy load, or to a dummy load that mimicked speaker impedance. The effect / difference is essentially the same.

For him the resistive dummy load was too sterile, the reactive was "squishy" and responding to playing dynamics making a "Night/Day" difference. Personally, I had to struggle to hear the slight difference. ...as usual.

I once angried someone by stating that only thing the "Reactance" control in his Rocktron Velocity did was basically equivalent to diming bass and treble controls of a generic HiFi -style EQ - nothing else. He got mad insisting the control turned his amp to touch-responsive dynamic setup that sagged like a real tube amp.  ...all that from a generic boost of low and high frequencies. The control didn't even try to mimic the unique response of a poorly damped amp driving a loudspeaker. It just introduced a basic treble and bass boost.

It's all in how you perceive things, and perceiving can be based 99% on imagination and 1% on "real" auditory information. It's always more or less subjective. Therefore I wouldn't jump to definite conclusions too quickly. Yes, objectively viewed the damping barely has a slight effect on frequency response... but so far I never encountered anyone who would sense things 100% "objectively".

CF is more about the control of the speaker than more treble or bass... although that is also perceived.  It's allowing the speaker to add 'mechanically generated' harmonics.  At frequencies above 1000hz approx, the HF response of a typical guitar speaker is largely synthesised... not natural HF sound coming from the amp.  This is produced by 'cone break-up modes,'  which are characterised by the somewhat jagged speaker response curves above 1000Hz seen in speaker manufacturers publications.

Most players think of a speaker as reproducing the sound made by any amplifier.  Which is incorrect to a high degree for tube amplifiers and modern SS amps with CF.  In fact, the amplifier does produce a controlling signal, but the guitar speaker simply uses it as a motive force and, basically, interprets that signal as a 'new sound' containing those harmonics which did not come from the amplifier at all!

The art of good guitar tone is understanding these 'beneficial defects' in the components you use.  When you understand what is going on in the complete article, then it is possible to exagerate them or subdue them, to whatever your sonic aims are.  This is exactly why there are so many speaker models available on the market.

A great guitar sound wil be determined mainly by the speaker and how it interacts with the OPTX and cabinet loading.  So CF is only the 'enabler', whereas the speaker really creates the sounds you actually hear... not so much the trannies or valves, or any other component to any significant degree for that matter.  Without CF in a tranny amp, the speaker is effectively just a resistor that makes a noise in line with what it's fed; who's loudness is proportional to its changing impedance with frequency, unlike a compensating transformer output stage.

The designer plays a big part in this to.  If a designer is into metal... then he will not be able to design a good country, blues or any other type of amplifier.  As with any 'niche' market product, the designer wares the true assets in the form of his knowledge on how to bring all the convoluted defects together and achieve a desired outcome.

It ain't just about the technology or circuitry used!  The understanding of how the sound is made is more important than being a superb electronics technician!  It's a black art... and so is designing guitar speakers!

Quote from: J M Fahey on November 06, 2011, 08:08:19 AM
I noticed that effect on my own a ***long*** time ago.
I started making guitar amps in 1969 , all tube of course.
In 1972 Argentina defaulted on its external Debt (what Greece is about to do now) and, not having U$$ available, imports were impossible.
Tubes dissappeared from the shops, just like that .... or were worth their weight in gold.
Started using Transistors, which were still affordable, but noticed that the sound was not the same.
Part of it was that SS amps had too high damping (approx. 100 ) which caused "dry" bass; I measured my Twin Reverb type tube amps, and damping was around 1.
I added current feedback (straight from SS design books) until I got the same value.
It helped a lot.
I guess I invented "Valvestate" on my own, about 15 years earlier than Marshall, go figure.
Necessity is the Mother of Invention.

Hello Gents,

Just joined to make a few comments.  You might guess who I am from my username.  :-)

It was almost impossible for Jim Marshall to have known about CF, as he didn't even understand valves.  Until he started to employ decent designers post 1988... nothing very revolutionary came from this stable.  So it was his new design team that introduced CF to the Marshall brand!

CF circuits were used as the basis for 'reverb'  drive circuits designed by Accutronics back in the early days of tranny amps.