Input stages, biasing

[peacocksuit]

I've been doing some reading up on tube biasing, particularly on input stages. I have a question regarding gain and distortion and how it controlled. Say I have a PP power stage (6v6's or EL34's) and 3 12ax7's. 2 of these for amplification and 1 is a cathodyne PI. That's 2 x 12ax7's = 4 gain stages (2 per bottle) excluding the Cathodyne which make unity gain, right?

The signal will flow through one stage into the next. Say I bias the first stage to be clean and neutral. At some point I want some controllable distortion so presumably I'd bias a stage cold so the signal clips. Firstly, at would stage would this be introduced, and secondly, given that the signal for the whole chain flows through this stage, how is it controlled? I understand how volume can be controlled by reducing the signal at the begging or end of the chain, but how do you implement control over the distorted signal, being able to add it when needed and be able to dial it in across the net volume range?

Sorry if this is a newb question but I think it's a missing piece of the puzzle for me here.

Thanks

[Carlsoti]

This bit of info might make you hate me, but if you learn how to read, draw, and interpret loadlines, the answer should present itself fairly readily. I say that because I'm not adept enough to give a completely proper written answer, and I don't feel I understand loadlines well enough to be able to teach it to someone else.

[acheld]

I can't answer your question directly; I'm not smart enough, and I can't type that fast . . . but there is hope.

Rich Kuehnel's books are very good (if dense reading) at explaining what you are asking.  See  https://www.ampbooks.com/mobile/books/  and take a look at his design series.  His early book on the Bassman is terrific.

I've also been a big fan of Merlin Blencowe's "Designing Tube Preamps for Guitar and Bass."  Very well written.

Last, not least, is study schematics.   How did Dumble approach overdrive, as opposed to Jim Marshal, or Mike Soldano? What did they do?   The schematics tell the tale.

It's a journey!  Enjoy it.

[PRR]

> presumably I'd bias a stage cold so the signal clips

Why? Cascade enough stages, it always clips!!

> I understand how volume can be controlled by reducing the signal at the begging or end of the chain

-OR- in the middle. You can put an attenuating pot between every stage, if you wish. If you read ALL the schematics you can find, you will run into a few of these. They tend to be more than a handful; the elegant designer figures how to make many choices with few knobs.

At guitar level you usually do not want a pot in front of the whole chain, dumping your signal into universal hiss.

And with common power tubes you *may* not want a pot too close to the power tubes because you sacrifice some of the precious Power you paid for. (Or you may like it, especially at home.)

Agreed, Kuehnel is not an easy read but this is not an easy subject.

[peacocksuit]

Thanks for all these replies, very helpful stuff. I've already asked father Christmas for Merlin's book on guitar and bass amp preamps for Christmas 😀. So that should be a useful.

The thing is I'm not a big reader. Which is probably why I didn't pursue any of this sort of stuff academically when I was you younger. Despite having the potential to get my head around this stuff I'm not easily able to take information onboard by reading. I find watching YouTube videos on these subjects helps me understand it a lot better. I started off with uncle Doug (legend in my opinion), I've been watching some of headfirst amplification's stuff lately, most recently a video on preamp gain stages. He shows how to use load lines and it does make sense, I'm starting to grasp it. I understand how the bias of a stage can create or decrease headroom, or make a signal clip and I understand the relationship of the cathode bias resistor to the relative negative voltage of the grid etc. That is all starting to make sense to me. I'm just struggling to understand how the distortion can be controlled Vs a clean tone through a range of volume if each stage might be biased differently given that the signal is cascaded.
I think I'm nearly there I'm just waiting for the penny to drop.
I take the point about volume control and the possibility to put this at any stage in the chain. I get that. But say we had four stages, three of them for biased balanced and relatively hot giving hesdroom, and I have one stage which is biased cold to allow for clipping to  occur, will this clipped signal just pass on to the following stage(s)? How do we control this?
I will obviously do more reading up and watch more videos but sometimes all it takes is the right person to explain it in the right way for that Eureka moment 😁.

Thanks again for taking the time to reply l, it's always appreciated.

[PRR]

Fundamentals of Guitar Amplifier System Design
Richard Kuehnel

("the red one")
https://www.ampbooks.com/mobile/books/system/
https://www.amplifiedparts.com/products/fundamentals-guitar-amplifier-system-design
may be at Amazon (they had problems when it came out)

[tubeswell]

I've already asked father Christmas for Merlin's book on guitar and bass amp preamps for Christmas 😀.

Merlin has the 1st chapter on line, which covers the basics of tube characteristics (amplification factor, transconductance, and plate resistance) and DC load lines and AC load lines.
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf


The book has more on signal coupling and input and output impedance between stages etc, which will help answer some other things about your questions.

[dwinstonwood]

Also, take a look at Rob R's annotated Simplified SLO 100 schematic:

https://robrobinette.com/How_the_Saldano_SLO-100_Works.htm#Simplified_SLO-100

He shows the percentages of attenuation that Soldano's voltage dividers dump between gain triodes. As PRR said, "the elegant designer figures how to make many choices with few knobs."

Merlin's preamp book also goes into great detail about using voltage dividers to adjust the gain of each triode. He also shows how cathode bypass caps - or, lack of - are used to shape a stage's gain.

[HotBluePlates]

... At some point I want some controllable distortion so presumably I'd bias a stage cold so the signal clips. Firstly, at would stage would this be introduced, and secondly, ... how is it controlled? ... how do you implement control over the distorted signal, being able to add it when needed and be able to dial it in across the net volume range? ...

... The thing is I'm not a big reader. Which is probably why I didn't pursue any of this sort of stuff academically when I was you younger. Despite having the potential to get my head around this stuff I'm not easily able to take information onboard by reading. ...

It's true that many folks don't learn easily from the written word, but absorb information better by "seeing" or "doing."

The problem here is that there are many book-chapters devoted to the exact question you're asking.  If converted to a lecture, I can see filling 2 years of a college course-load going through all the ins & outs, and foundational knowledge required to make sense of it.  Convert that to video format, and you're into hundreds of hours.

Why?  There are many ways to get to the end-result of "introducing & controlling distortion, balancing with clean-volume" and no one has found the ideal method yet.  That has resulted in our having:

   - Amps with master-volume circuits and multiple channels
   - Non-master volume amps run cleanly & used with pedals for dirt
   - Non-master volume amps run almost-dirty & used with pedals for more-shades of dirt
   - Attenautors and re-ampers for making the dirty amp quieter
   - Digital modeling of a dirty amp to deliver any desired loudness (or zero loudness of recording or silent-stage)

And there are many variations present within each sub-class noted above.  All these different approaches exist because no one has found an approach that is "always best, for all users, for every application."