Good general electronics forum?

[mushy]

In an amp, how about the signal riding the bias voltage on its way to the output tube grids.

There was an MXR multiFX unit back in the 70s', that "floated" control signals on the 12V DC supply to the pedal, and connected pedal to amp with any quarter-inch cable, nothing special needed. Another type of case of AC superimposed on DC.

Does the signal become modulated dc between stages (after each tube output, before each coupling cap)?

[Tom Phillips]

Does the signal become modulated dc between stages (after each tube output, before each coupling cap)?

I'd say that you have the correct idea but I wouldn't use the term "modulated" because that means something else in electronics.

[Enzo]

In a circuit, you can take both readings and describe it all in whatever terms make the best sense at the time. But you don;t have AC and DC voltages flying around separately in the same wire. You only have voltage. At any instant you can measure that voltage. It is only after we spend some time with it that we can call it AC or DC.


Yes, they affect one another. You already seem to get that one rides the other. But there will usually be some pedantic discussion over AC. I myself use the term AC component of a voltage, though in speaking I might merely say AC. But if you have +160v on the plate of a preamp tube, and the tube wiggles it 20v either way from there, the voltage was ALWAYS DC. It might be wiggling from +140 to +180, but it remains positive, and that is DC. Now with a cap we can separate out just the motion, and so capture a 40v peak to peak AC voltage.

[mushy]

Ok, I think I have that straight for now. Spending some time playing with a scope would probably be helpful...hmmm, I forget that I have some simulation software (multisim). I guess I'll install it and see what I can make of it. We never used this software in school. I picked it up to play around with at home, but I never had time while attending school.

[SpareRibs]

Two semesters of electronics classes, and we never did this stuff... For anyone considering taking classes, check out your school thoroughly before signing up. Had I done a little prying, I could have found out that every single student in the program preceding mine failed their certification exams, that the school's overall graduation rate is lower than 20% (only really graduating business and nursing students), and that the school was having a very hard time finding competent instructors for the electronics tech classes. Believe me, it became obvious that there were major problems after just a few days in classes. I should have dropped right then, spending the money on books and tools instead. /End whatever this is.[/QUOTE]

Hello,
That is the same problem in this state, leading to the most highly educated McDonald, and Wal mart employees. I am sure they have heated discussions in the lunch rooms about nuclear physics, liberal arts, political science, and the lot.

[eschertron]

Does the signal become modulated dc between stages (after each tube output, before each coupling cap)?

To steal a quote: "an amp is a modulated power supply", which allows me to think of a gain stage this way, using only Ohm's Law --

The anode resistor and the preamp tube form a voltage divider. The tube is treated as a variable resistor in this model, even though it's actually a 'transconductance' device. when the resistance of the tube is near infinity (tube in cutoff) the voltage divider = INF/(Ra+INF) - or almost unity - so the signal peaks near B+ at that node. when the resistance of the tube is low, the divider = LOW/(Ra+LOW) - or almost zero - so the signal troughs near ground level (cathode level, actually, but this is a simple model and ignores that). We've taken the power from the power supply and made a high-voltage [inverted] copy of the signal appear at the anode. Did we really modulate the power supply? Not at the preamp stage, we hardly affected the power supply at all. But we used SOME power, maybe a fraction of a Watt, to create our amplified signal.

For a practical example, the voltage divider is in a quiescent state about half of the B+, more or less. The designer has picked values for the resistors (cathode and anode) to meet their requirements. So the signal at the anode is seen as +/- around THAT voltage, up to the limits described above. Looking at preamp load line charts can make it visually apparent.

On the other side of the coupling cap, the voltage can be seen INDEPENDENT of any DC reference, so for a typical preamp, a grid leak resistor gives it a new reference to ground (so the AC that passes through the cap is now +/- around ground). At a fixed-bias output stage, the signal is referenced to the negative bias voltage. It works for whatever the new reference is.

[silverfox]

Too much good stuff

Here are a couple sites I use. On the first one you can download all the lessons as .pdf files. Videos and everything: All About Circuits : Free Electric Circuits Textbooks

Next up, check the forums. Same as here you can post questions: Electronic Circuits - Free Electronic Circuits

Video lessons on lots of stuff: EEVblog - YouTube

Physics- Basic stuff useable at times: The Physics Hypertextbook

Try a search for electronics forums and learning tools etc..

Good luck have fun,

That's what the Fox Says

[Joe L]

If you have a program to simulate circuits, that would be a tremendous help in learning electronics! And if you don't, Google and download LTspice. Set it up then search on YouTube for some tutorials on using it. All these questions you are asking can be simulated along with every meter you can imagine. Want to know a voltage at a point in the circuit? Click the dc meter probe on that spot and get an instant reading. Same for current, according volts, etc. Man, I wish I had access to software like that when I took my training in the 70s.

[mushy]

If you have a program to simulate circuits, that would be a tremendous help in learning electronics! And if you don't, Google and download LTspice. Set it up then search on YouTube for some tutorials on using it. All these questions you are asking can be simulated along with every meter you can imagine. Want to know a voltage at a point in the circuit? Click the dc meter probe on that spot and get an instant reading. Same for current, according volts, etc. Man, I wish I had access to software like that when I took my training in the 70s.

Like I said, I picked up multisim when I was in school (before I dropped). I saw exercises for it in all of my textbooks, and I only wished that I had time to learn it and use it, or that it was being used by the school (it wasn't). Oh well. I'm using it now. It seems nice, albeit a little slow in performance. Compared to a modern daw or video game, it is really slow. What I have is an educational version (no student verification required) from amazon, for something like $50-60, and it came with a book for learning basic circuit analysis, which I'm working through at the moment, along with a textbook.

I'm weak in math, and I never properly learned how to use a calculator (working on that today). I may need to pick up a math book for electronics, too. Any suggestions? I'm eyeballing schaum's basic math for electronics.

[mushy]

To steal a quote: "an amp is a modulated power supply", which allows me to think of a gain stage this way, using only Ohm's Law --

The anode resistor and the preamp tube form a voltage divider. The tube is treated as a variable resistor in this model, even though it's actually a 'transconductance' device. when the resistance of the tube is near infinity (tube in cutoff) the voltage divider = INF/(Ra+INF) - or almost unity - so the signal peaks near B+ at that node. when the resistance of the tube is low, the divider = LOW/(Ra+LOW) - or almost zero - so the signal troughs near ground level (cathode level, actually, but this is a simple model and ignores that). We've taken the power from the power supply and made a high-voltage [inverted] copy of the signal appear at the anode. Did we really modulate the power supply? Not at the preamp stage, we hardly affected the power supply at all. But we used SOME power, maybe a fraction of a Watt, to create our amplified signal.

For a practical example, the voltage divider is in a quiescent state about half of the B+, more or less. The designer has picked values for the resistors (cathode and anode) to meet their requirements. So the signal at the anode is seen as +/- around THAT voltage, up to the limits described above. Looking at preamp load line charts can make it visually apparent.

On the other side of the coupling cap, the voltage can be seen INDEPENDENT of any DC reference, so for a typical preamp, a grid leak resistor gives it a new reference to ground (so the AC that passes through the cap is now +/- around ground). At a fixed-bias output stage, the signal is referenced to the negative bias voltage. It works for whatever the new reference is.

Thanks for the reply. I understood most of that, and I'll give it another look when I make it out of noob-land. I'm a visual thinker, so imagining waveforms and such is very helpful. A little off topic here: I wish that math were taught in a much more visual way. For example, the concepts of exponents, squareroots, and logarithms can be demonstrated with squares and cubes very easily to small children. A sine wave can be demonstrated with a piece of paper passing under a pencil attached to a wheel that is perpendicular to the paper. I think that most people who have issues in math (and technical subjects explained using math) get caught up in the notation, jargon, and forumulas, because the root concepts are never demonstrated clearly enough.

[silverfox]

Amp is Modulated Power Supply

To steal a quote: "an amp is a modulated power supply"

I was about told how untrue that statement is on another Forum. But those are real Gurus over there. Anyway, that is the conclusion I've reached also.

Coincidentally today I also reached the same conclusion regarding a triode and the internal resistor resembling a potentiometer with the control, (grid), being the shaft. That analogy works much better than a moving grate that varies water flow. But then, flow is pressure over opposition.

I may actually have figured out how a triode stage works. The output voltage varies between levels that do not necessarily have 0 volts as the crossover point. Only a variation in output level.

Silverfox.

[diydidi]

OK, I'm learning too, but I'll take a stab at it.

First a disclaimer: the AC in a guitar amp (for example) can be amplified and sent to the output terminals BECAUSE of the DC, so the assertion that the AC and DC don't change another might be a bit misleading.

In terms of measurement instruments, some are designed to sense whether there's more AC than DC present (or vice versa) and display a reading based on the stronger of the two elements. Other meters require the user to pick. Some may be better at rejecting DC when measuring AC than others, some may be better at averaging AC when measuring DC. A less-than-professional grade meter may be poor at differentiating the two sources and display garbage when both are present.

The distinction between AC and DC for the engineer or technician is an arbitrary one. DC is nothing more than an extremely low frequency AC wave component (zero Hertz, actually!) and can be treated that way mathematically. DC on your AC signal can be called an 'offset', and is common in an amp (remember the disclaimer). Between gain stages it is removed by using AC coupling capacitors. The DC component can be ignored for much amp circuit analysis, but is very important to keep in mind for calculating headroom, power, etc. We talk about the AC and DC like they're two different things, because their origins are usually different (in an amp, at least). But the AC wave gets more powerful from input to speakers as it takes energy provided by the DC.

Some multimeters can measure the combination of DC and AC signal in RMS.
For instance when a AC waveform is riding on a DC offset.
Have a look at BRYMEN TBM869. It has a DC+AC True RMS range. It uses the formula: Square root (DC squared + AC Squared)

[mushy]

Random stuff:

Multisim has some obstacles (bugs, organization, etc.), but it is pretty neat after getting past the stuff that is in the way. I think it is going to be very useful as a learning tool. Creating shortcut keys for commonly used components and instruments saves alot of clicking and digging through menus.

I think that I will not forget again resistor color coding (or meter resistance settings) after labeling a crap load of resistor bundles in a big variety pack, double-checking with a meter, organizing them by 1's, 10's, K', M's. How do you guys organize component variety packs, or do you deal with them at all?

[tedmich]

Mathematics for Basic Electronics
by Bernard Grob

and of course the free treasure trove from Pete Millet
Technical books online

not a bad math review here:
http://www.mfoww.org/down%20meba%20math.pdf