DC bias shift due to charge buildup on coupling cap driving Asymmetrical clipper

[Vivek]

https://www.researchgate.net/publication/290534843_Triode_emulator_-_Part_2

Thanks !!!

Takeaway 1 re charge buildup on coupling cap : "For guitar amplifiers the charge time constant is about 2-10 ms, and discharge time constant is ten-twenty times larger. "


But the circuits I analysed seem to have charge time constant in the order of 80 ms !!!!


Takeaway 2 : The graphs presented are different in shape than the ROG circuit, the Peavey patent and the Levin design

All above have bigger output signal at beginning and then smaller signal

But the paper you posted shows smaller signal first and then bigger amplitude as time goes on. Maybe it is also more distorted initially and less later on.

[Steben]

I always linked "Sag" with "bloom"

[Vivek]

Does anyone know how to find waveforms, graphs and measurements of real tube amps exhibiting sag, bloom etc ?

Any PhD thesis available on the internet ?

Or only way is to buy a tube amp and measure yourself ?

[PRR]

Guitar Amplifier Overdrive First Edition by Ulrich Neumann (Author), Malcolm Irving (Author)
https://www.amazon.com/Guitar-Amplifier-Overdrive-Ulrich-Neumann/dp/132959665X/

[Vivek]

Thank you, sir Paul!

https://books.google.ae/books/about/Guitar_Amplifier_Overdrive.html?id=41gdCwAAQBAJ&printsec=frontcover&source=kp_read_button&redir_esc=y

[ThermionicScott]

Richard Kuehnel's book on the 5F6A Bassman goes into the nitty-gritty on that amp's response to a signal, from quiescent to full power.  Even if you're not totally into that particular amp, you'll learn a lot:  https://www.amazon.com/Circuit-Analysis-Legendary-Tube-Amplifier/dp/0976982250

[Quackzed]

if you swap the orientation of the 2 clipping diodes i think it'll do the sag/bloom thing, start smaller and get bigger as the cap charges?

[Vivek]

I did some internet search to try and understand how do people define Sag, Bloom. Swirl

Tube rectifiers have a delayed note response, known as "sag".  When you hit a note on the guitar forcefully, the rectifier tube is temporarily overloaded, which reduces the volume of the attack slightly.  As the tube recovers from the overload, the note will get louder.  This phenomenon is known as "bloom".   Amps with tube rectifiers are described as "smooth", "spongy", and "forgiving".

--http://www.hammersmith.rocks/pro-tips/2016/8/9/tube-amp-basics


I prefer "Swirl" which implies a shifting emphasis of harmonics as the note sustains and then decays. Is that what you mean?

-- https://www.thegearpage.net/board/index.php?threads/describe-amp-bloom.369114/#post-3844831



If all a cap can do is build up charge when the signal is on, and discharge when the signal is off,

Then it cannot do sag and bloom at the same time.

But yes, the circuits i posted of ROG THUNDERBIRD, Levin, Crate, Patent will create Swirl = different harmonic content based on time and past signal history (DC shift on integrating capacitor)

However there is a way to create swirl based on instantaneous signal amplitude by designing correct transfer function. Here the swirl depends upon current signal value and does not consider past signal history.

[Vivek]

I always linked "Sag" with "bloom"

" So the sag thingy, in reality, happens when the power amp transformer drops the voltage for a fraction of a second, it's a behavior that in this situation the signal sounds compressed.

The waveform starts with the full dB range of the initial signal and then it is choked out. The short squeeze and jump after happening for milliseconds, making the notes to "bloom" –  all of this happens after a hard picking attack and a voltage spike on the input stage produced by the guitar signal."

--  https://www.kuassa.com/caliburn-and-matchlock-guitar-amp-engine-deconstructed-sag-and-bias/

[PRR]

> how do people define Sag, Bloom. Swirl

They don't, they just run off at the mouth. Most people can't dance about sound any better than they can sing about architecture. Partly language, but that is because most people do not hear analytically, even musicians.

[Steben]

> how do people define Sag, Bloom. Swirl

They don't, they just run off at the mouth. Most people can't dance about sound any better than they can sing about architecture. Partly language, but that is because most people do not hear analytically, even musicians.

Very true.
A part of the "sag and spongy" thing is interaction between player and amp response, more than recorded sound.

Explain octave, they will listen, explain distortion vs clean, they will listen
Explain sag and bloom to non-guitarists and they will go like "yeah whatever"

[11-90-an]

lemme get this straight... you can get this "sag" and "bloom" characteristics of tube amps with a compressor with (probably) a short release that the volume would just "jump" right up?

Or am I missing something?

[teemuk]

Note that this image displays what happens to AC waveform of power supply and not what happens to signal as a consequence.

From signal's perspective you notice "sag" as dynamic decrease of clipping threshold as supply voltage drops down and reduces headroom of signal swing before clipping. Time constants of sag are largely defined by RC filter of power supply series resistance and filtering capacitance. "Attack" time is often slo100 (as filtering capacitors decharge during loading) and recovery time very fast (as filter cap charges when loading stops).

RC filters of preamps and power tube screens are such that sag is very rare occurence unless loading is extremely sustained. (Not typical with usual guitar input, which is not continuously high amplitude).

Do note that mechanisms and effects of sag are quite different to mechanisms and effects of DC bias shifting via charge of interstage coupling capacitors.

[Vivek]

lemme get this straight... you can get this "sag" and "bloom" characteristics of tube amps with a compressor with (probably) a short release that the volume would just "jump" right up?

Or am I missing something?

I would think :

Tube amp reduces volume and changes clipping characteristic due to sag. As power supply changes, the bias operating point changes and the headroom changes, leading to changes in volume and also clipping characteristics and created harmonics.

Compressor will normally only play around with envelope and volume. It should not change the harmonics. (Unless the compressor time constants are so fast that it acts as a clipper)

Hence a compressor cannot fully emulate sag.

[Steben]

And then there is cathode bias shift as well ....