How do you get the values? Basic design questions.

Sam1911

Member
2016-03-13 1:52 am
Hi!

I've got a slowly percolating first guitar amp build that I've posted about here: http://www.diyaudio.com/forums/instruments-amps/288424-critique-phonograph-conversion-amp-plans.html, but before I buy any parts I've committed to understanding a bit more about what I'm choosing and why, and that's where I'm running into trouble.

I really would like to understand where all the capacitance, resistance, impedance, and voltage numbers come from, on the basic level. Why are they chosen?

I bought Morgan Jones' Valve Amplifiers: Fourth Edition and I'm about 1/3 of the way through it. I freely admit that it's a stretch for me to understand. While he says he's not going to get into the higher math involved, he does indeed make a lot of assumptions that his audience is going to know the terms he's using, and be able to quickly apply those terms to his examples and understand why what he says is true. He also makes the (probably perfectly reasonable) assumption that a lot of basic circuit function is already firmly in the reader's mind.

In my case, that's a definite ... "sort of." :eek:

As I'm reading I'm rather constantly frustrated by the fact that I want to stop and ask, "BUT WHY?" Or "but where is that voltage coming from?" And other total newb questions, that really aren't answered clearly.

In desperation I've ordered a copy of Robert Megantz's Design and Construction of Tube Guitar Amplifiers, mostly on the merits of its reputation as a bit "too basic." :eek: I figure if I've got one that borders on astrophysics, and one that's at the "Build-It-Yourself Birdhouse" level, maybe I'll be able to connect the dots a little better. But I'm doubtful...


SO, this is the sort of stuff I can't find answers to:

1) How is it we choose the values we do for filter capacitors and the resistors that accompany them?
2) How do we choose what B+ voltage to set, and then why do we then choose the lower B+1 to send to the output tube, B+2 for a gain stage, B+3 for the pre-amp tube, and so on? Why are those values chosen? Why not just max you can get, after sufficient filtering?
3) How do we choose the values of coupling and bypass capacitors? And how do we figure out the proper values for all the little resistors that seem to appear between almost every component?
4) (I think I DO understand, basically, how to calculate the proper bias resistor values once you've got all the other numbers set.)

I've watched lots of videos like this one, https://www.youtube.com/watch?v=mrMGGHp8yLk, and read threads (and looked over quite a few schematics) and there seems to be a lot of cases where folks say (as this fellow does), "...and 1M Ohm is usually about right for this resistor to ground," or, "...and usually that's about a 1 mF capacitor, though you could make it 2mF if you want to."

Like what the heck? Are we guessing? Do we just use whatever somebody else picked and go by tradition? Is the math just above me and I'm better off finding someone willing to tell me what caps and resistors to use? Should I just crib it all off the Fender Champ and call it a day? :)

Any thoughts for the newbie? Where do I go to get what feels like a Swiss cheese worth of educational gaps filled in? Why didn't they cover this in high school?


Thanks! :)
 
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rayma

Member
2011-04-29 8:37 pm
Where do I go to get what feels like a Swiss cheese worth of educational gaps filled in?
Why didn't they cover this in high school?

I think that you need a good introductory electronics book.
http://iate.oac.uncor.edu/~manuel/l.../The Art of Electronics - Horowitz & Hill.pdf
That said, amps for musicians do tend to be rather idiosyncratic.

The fact that you are bothered by all this shows that you have the motivation
to understand the theory necessary to do proper electronics design work.
It takes mostly algebra and common sense, but knowing calculus would help too.
 
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Sam1911

Member
2016-03-13 1:52 am
Ok, thank you! I agree, certain, specific basic electronics reading is probably very much what I'm needing. That books looks very good, though that pdf copy skips the entire first chapter! :eek: I probably need that first chapter, at least for best knowledge-crack-spackling results. :eek:
 
I have Megantz' book, and refer to it all the time. It will explain most of your posted questions. A few general tips: coupling and bypass caps are picked for their filtering effect (roll off highs or lows- just calculate the cutoff freq). And the different B+ values come from tube data sheets, for preamp and power tubes-VERY useful but hard to read (look for tutorials on this). I was you about 2 years ago, and you will realize that the more you know, the more you realize how deep it goes.... BUT learn the basics, build existing circuits, then play with their values, and never stop asking "But why?". Have fun!
 

Sam1911

Member
2016-03-13 1:52 am
I have Megantz' book, and refer to it all the time. It will explain most of your posted questions.
Oh, that's wonderful! :)

A few general tips: coupling and bypass caps are picked for their filtering effect (roll off highs or lows- just calculate the cutoff freq).
I think I get the general idea there, and understand basically how a tone stack works in its interconnected way. It just seems that every time I think I've identified the function of the caps and resistors in a certain part of a circuit I'm looking at, I notice 3-4 "left over" components that, even if I can guess what they're doing, I have no idea how their values were settled on.

And the different B+ values come from tube data sheets, for preamp and power tubes-VERY useful but hard to read (look for tutorials on this).
Ahhh, well, that does make things simpler I think. I was looking at my various copies of tube data sheets for the tubes I think I'll want to use in my amp and maybe I just aim to try and get something slightly under the max rated plate voltage to each one?

I was you about 2 years ago, and you will realize that the more you know, the more you realize how deep it goes.... BUT learn the basics, build existing circuits, then play with their values, and never stop asking "But why?". Have fun!
That's very encouraging! Thank you! I will...
 

rayma

Member
2011-04-29 8:37 pm
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There is a wealth of information on the internet.

I learned way too much about electricity and electronics in the Navy long, long ago. The NEETS modules are available in free pdf downloads. They are easy to read, and have tons and tons of solid information that all ties together. All killer, no filler, so to speak. Topics go all the way from batteries and flashlights to GPS navigation and smart bombs.
Navy Documents
If you use the NEETS series, start at module #1 and read each module in its entirety. I really cannot stress this enough. The sequence is very thorough and will give you the mentality you seek in a few hours worth of reading. Once you have enough background on what you're interested in, just stop at the end of that module. Most people not interested in computers, radio, or radar stop after module 8 or 9.

FWIW, the other modules in the index (in way too many disciplines to describe) are also extremely useful.

Another decent resource is:
Education - All About Circuits
Not as thorough, but they have videos and shorter sections with lots of pictures.
 
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Hi Guys

Guitar amps started off as hifi amps built to be portable, so followed hifi practice. The circuits are all very standard and those for clean tones have stayed that way. The power amps found in MI follow two basic patters for SE and PP with the bulk of most being 99% the identical circuits, i.e. most SEs are the same as each other; most PPs are the same as each other.

You can convert your mag pre to a guitar pre simply by removing the EQ required for phone and inserting a level control between the stages. Call this "step-1". Try it out. It will sound good inasmuch as you made a free modification and it works. It may sound a bit fat or flat for guitar and this is where the fun begins. By changing R and C values, you tweak the tone to what you want to hear. There is nothing critical in this process or the choices. You can use the guideline of the above books, or of the targeted guitar amp books like The Ultimate Tone series, or just do trial and error.

It is best to begin with a traditional circuit and values. Try it for yourself so you have a baseline tone to work from. Decide what is missing or what you would like to change tonally, then see what circuit change is needed to achieve that result. Voicing is just the selection of the Rs and Cs around the tube and you will be doing this to suit your guitar, your speaker, and your playing style. There are no wrong choices except for aesthetic ones that might inherently make the process too difficult, like deciding you can only have one preamp tube.

Most of the books cited follow tube hifi rules, which are very limiting. We follow tube safety rules which provide broader exploration. See the FAQ on my site and of course the TUT-series. TUT3 is the best book to have if you want to wire your amps for lowest noise and best note articulation.

Have fun
 
Hi!

I've got a slowly percolating first guitar amp build that I've posted about here: http://www.diyaudio.com/forums/instruments-amps/288424-critique-phonograph-conversion-amp-plans.html, but before I buy any parts I've committed to understanding a bit more about what I'm choosing and why, and that's where I'm running into trouble.

I really would like to understand where all the capacitance, resistance, impedance, and voltage numbers come from, on the basic level. Why are they chosen?

I bought Morgan Jones' Valve Amplifiers: Fourth Edition and I'm about 1/3 of the way through it. I freely admit that it's a stretch for me to understand. While he says he's not going to get into the higher math involved, he does indeed make a lot of assumptions that his audience is going to know the terms he's using, and be able to quickly apply those terms to his examples and understand why what he says is true. He also makes the (probably perfectly reasonable) assumption that a lot of basic circuit function is already firmly in the reader's mind.

In my case, that's a definite ... "sort of." :eek:

As I'm reading I'm rather constantly frustrated by the fact that I want to stop and ask, "BUT WHY?" Or "but where is that voltage coming from?" And other total newb questions, that really aren't answered clearly.

In desperation I've ordered a copy of Robert Megantz's Design and Construction of Tube Guitar Amplifiers, mostly on the merits of its reputation as a bit "too basic." :eek: I figure if I've got one that borders on astrophysics, and one that's at the "Build-It-Yourself Birdhouse" level, maybe I'll be able to connect the dots a little better. But I'm doubtful...


SO, this is the sort of stuff I can't find answers to:

1) How is it we choose the values we do for filter capacitors and the resistors that accompany them?
2) How do we choose what B+ voltage to set, and then why do we then choose the lower B+1 to send to the output tube, B+2 for a gain stage, B+3 for the pre-amp tube, and so on? Why are those values chosen? Why not just max you can get, after sufficient filtering?
3) How do we choose the values of coupling and bypass capacitors? And how do we figure out the proper values for all the little resistors that seem to appear between almost every component?
4) (I think I DO understand, basically, how to calculate the proper bias resistor values once you've got all the other numbers set.)

I've watched lots of videos like this one, https://www.youtube.com/watch?v=mrMGGHp8yLk, and read threads (and looked over quite a few schematics) and there seems to be a lot of cases where folks say (as this fellow does), "...and 1M Ohm is usually about right for this resistor to ground," or, "...and usually that's about a 1 mF capacitor, though you could make it 2mF if you want to."

Like what the heck? Are we guessing? Do we just use whatever somebody else picked and go by tradition? Is the math just above me and I'm better off finding someone willing to tell me what caps and resistors to use? Should I just crib it all off the Fender Champ and call it a day? :)

Any thoughts for the newbie? Where do I go to get what feels like a Swiss cheese worth of educational gaps filled in? Why didn't they cover this in high school?


Thanks! :)
You are starting at a "too high" level, even the "simplest guitar amp book" or Hi Fi or whatever assume you already have the basic knowledge.
Mind you, you'll see people in Forums who seem to know, "speak the language" ... but then say things which show they don't really grasp the concepts ... oh well.

What I always suggest to people who want to "learn it right" is to start with a Physics book :eek:

Try to get, generally at a high school library, a Physics book dealing with "Electricity and Magnetism".

The older the better, 1900's to 1960 or so is fine, what we want is a book which does not get knee deep in Math, specially not Calculus, but which solves everything with simpler algebraic Math: 4 basic operations, up to some squares , nothing further, and concentrate instead on showing and explaining on the "phenomenon" itself, just what happens and why ...... which is what you want and need :)

But ..... Physics? :confused: .... I want to learn Electronics.

Sure enough, see what those old Physics books will teach you (in no particular order): electron - current - conductor - insulator - resistance - battery - voltage - watt - meter (old needle type multimeters) - magnet - wire - coil - electromagnet - attraction/repulsion - dielectric - generator - resistor - capacitor - potentiometer - switch - etc. etc. etc.

Knowing that, you will learn much better stuff which is basically a combination of the above basic knowledge, such as, for example:
Tube Diode:
it's made out of 2 electrodes, separated and in vacuum (insulator) , where you apply positive voltage from a battery to one of them, no current flows while cold, but if one (cathode) is red hot because it dissipates some watts it will emit electrons which will be attracted by the other one (anode , a.k.a. "plate") , both filament and plate voltage can be measured by a voltmeter, cathode and plate current can be measured by an ampere meter, you can calculate the watts dissipated by the plate.

Notice how *all* Electronics parameters are actually Physics parameters and concepts, if you know these you will know Electronics very well ;)
 
What JMFahey said. Physics encomasses and ties together many subjects.

For a "noob" or someone not familiar with the fundamental concepts, the NEETS manuals cited before start off with wooden ships, static electricity, and magnetic "needle" compass examples (in module 1) to form a historical and solid foundation to build upon. All the relevant terms are introduced from a perspective that is easy to read, understand, and remember for later use.

With any of the refernce materials of this nature, a student will wonder what this ancient material has to do with modern technology. The answer is simple: you have to know about how this stuff was originally thought up and then refined and improved to modern standards to understand how it works.