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For reading/setting the bias for tube amp I put one ohm, one %, half watt resistors from the cathode pin to ground for each tube socket. Measure the millivolts across the resistor for each tube using a auto-ranging digital VM. Volt meter will read -20 or whatever. The higher the number the more draw. The plate has a positive charge and opposites attract. I know how to adjust and to what range but don't understand the term 'negative voltage'. Is it actual negative voltage (how can this be)? Tad confused. Thanks, PMLibonateIII "Trip"

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Thanks Al, Thought there was more to it than that. Negative charge introduced into the tube to increase/decrease draw. So a negative charge would have more electrons right? Postive charge more protons? What's a balanced charge called? Is this true only with DC? What the deal with all that? Reading everything I can get my hands on but seem to missing a few major points. Thanks again for your time, Trip

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Thanks Al, Thought there was more to it than that. Negative charge introduced into the tube to increase/decrease draw. So a negative charge would have more electrons right? Postive charge more protons? What's a balanced charge called? Is this true only with DC? What the deal with all that? Reading everything I can get my hands on but seem to missing a few major points. Thanks again for your time, Trip

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Cyberman,

No.. protons don't do anything! Only electrons move. The + and - is like the direction water is miving through a hoze. If it's moving left to right and you walk around to the other side of the hoze, now is moving right to left! That's DC. AC is alternating + then - then + then - at high speed.

In the tube circuit you mentioned, current is flowing up from ground and from cathode to anode (plate). it is +on the cathode with respect to ground. Since the grid is connected to ground it sees - with respect to the cathode... So.. it sees - bias on the grid just becasue the cathode is +.

Neat huh?

Al K.

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A voltage is positive or negative with respect to a reference point, like positive and negative temperatures. It your reference point is 100V (black lead) and the test point is 80V, you'll read -20V. It's a relative thing and can be used to show the direction of moving electrons

John

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John and Al are right on point as always. However, let me expand a bit. (Ask an engineer what time it is, and he'll tell you how a watch works.)

"Voltage" is also called, and is a unit of, Electro Motive Force because it forces the electrons to flow. It is also called Electo Motive Potential.

Please recall problems from high school physics. If a bowling ball is 6000 feet above mean sea level above Denver, its potential energy is measured by the fact that it can fall 1000 feet to Denver at 5000 feet above mean sea level. If the same bowling ball is at 6000 feet MSL above Death Valley, it has more potential energy because Death Valley is at -200 MSL. (Excuse me if altitude numbers are a bit off.)

So, by analogy, EMP or voltage reading on a meter depends on what the probes are connected to. Following John's point, the meter probes could be connected to 1000 volts and 1001 volts. You'd see a reading of 1 volt or -1 volt depending on how it is connected.

In all this talk of relative voltage, it important to understand that there IS a universal reference voltage for us. That is the electrical voltage of the planet Earth. We take that as zero. It is not theoretical. The "ground" connection of your electrical outlet is actually connected to a metal stake or equivalent where the service comes into your house. The "neutral" connection is too.

In design of electrical components, the metal chassis is considered "ground". Even in these days of plastic housings, the circuit board and ground buses are connected to the ground prong on the power plug. Often, the speaker minus or black is connect to it too.

We have to be careful about what is called "conventional current flow". This convention is that current flows from the plus voltage to the (relative) minus voltage. In truth, the electon has a minus charge. So, electrons always flow from the lower voltage to the higher. Our "sign" convention is a bit off.

Al is describing the function of the triode (three connections) vacuum tube. This was the great invention of Lee DeForest. He created what the British call a "valve". For all the virtues of the transistor and ICs, this was monumental and made electonics what it is today. Or at least what it was in first part of last century.

As you can anticipate, the vacuum tube "valve" (or transistor in later days) does not truly "amplify" in the sense of making something small into something big. It just allows a small voltage change to control large flows of electrons which orignate from a power supply.

You start with two leads in a vacuum. One is connected to the plus (anode or plate) the other to the minus (cathode). Not much current flows because the electons are attached to the metal. However, if you heat the source of electrons, the minus side, the molecules start to jitter and the electons are not securely attached. Now, they are free to move across the vacuum to the plate.

This is analagous to doping in semiconductors which make electons "free" or able to move about.

This heating and vacuum is the effect which Tom Edison discovered. We can have flow of electons across the vacuum. Nothing gets in their way. They "see" the positive charge on the plate. Called "The Edison Effect." An early rectifier. It is a type of switch because it allows the electons to flow in only one direction. It is a magnificent invention in its own right.

DeForest put a grid in the path and arranged the power supply to give the grid a negative voltage. Now the electrons boiling off the heater don't quite "see" the positive charge of the plate and they don't flow as much.

But, suppose we can make the grid a little less and more negative. Then, the flow of electrons going to the plate is controlled. With a bit less negative voltage on the the grid, more electons flow to the plate. This is the "valve" action.

So, you can see the overall situation. The input to the tube at the grid is a change in voltage, always biased to the negative, but modulated by the input signal. As a result, there is a change of current between the heater (cathode) and the plate (anode).

By some other circutry, the much greater flow of current in the plate circuit goes to our loud speakers.

What DeForest created was now a days called a voltage controled current source.

What time is it.

Gil

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Gil, Have you considered peening a "tube amps for idiots" ? or perhaps "high-end tweaking for idiots" or even "audio for idiots" ? at least one copy can be considered pre-sold! thanks for explaning to me, in words I can understand (no mean feat!), how a tube functions! warm regards, tony

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It always seemed to me that the whole issue of electron flow in a electrical circuit really doesn't confuse things much unless your looking at a circuit that includes vacuum tubes. I guess it's because the operation of a vacuum tube forces you realize how the elecron are actually flowing. Just the fact that your applying a positive plate voltage in order to attract electrons, just seems counter-intuitive.

There was an English guy that came in between Edison and DeForest who actually invented the first vacuum tube. It was called the Fleming valve and had the operation of a simple diode. DeForest took this idea and inserted the control grid which resulted in the first amplifying vacuum tube, or triode. Unfortunately, DeForest didn't understand how his invention worked. Later on, while DeForest was trying to keep himself out of jail, a guy by the name of Edwin Armstrong (who clearly understood how the triode worked) invented the first circuitry (employing feedback) that made the transmitting and receiving of radio signals truly possible. Armstrong later developed the Superhetrodyne receiver which is the basis for all radio receivers, even today.

Interestingly enough, DeForest became known as the "Father of Radio", whereas Armstrong, later in life, leaped out of a third floor window.

Kerry

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Hey, Gil,

Liked your lucid explination of tube amp output stage. Want to take a crack at explaining how the variations in current caused by the grid are converted to an alternating current flow going to the speakers?

Where you left off, we have a current flowing from cathode to plate, which goes from more current to less current, depending upon the signal being applied to the grid, but it's always flowing in the same direction. How is this "variable DC" signal converted to the AC signal the speaker sees? confused.gif

If you could mention OTL's in there as well, that'd be cool... biggrin.gif

Ray

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I started work on the transformer story the other night. But the computer ate my homework. Back to the word processor.

A work project is going to prevent me from doing anything "educational" for a couple of weeks. Also, I should buy a scanner so we can have some diagrams to look at.

Thanks for kudos re writing. It is part of my profession.

May I point out that many of the posts on the bbs are a bit unclear. That is not because of lack of content. Rather, people have not been breaking up the text into paragraphs.

If you want a "works every time" tip: Do try to put three sentences into each paragraph, plus or minus one. Do try to put one thought in each paragraph. If you have more than one, hit the return key to break it up.

You don't have to put all the connected thoughts into one sentence so that we start with the beginning and go the middle and then to the end and go back to explain how the middle step relates to the beginning step and to the end step which they are both connected to but I'm not sure where to put a period because then they're not connected but I wanted to let you know they're all connected in my humble opinion but then most things are opinion. =8^o

It is a rote matter, but it (use of paragraphs) works every single gosh darn time. Spelling and punctuation count. Errors there, however, do not grossly effect readability or clarity of thought.

It is always a pleasure to write about what you love, to a receptive audience. I love this stuff. Most of you do too.

Thanks again.

Gil

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