Can someone explain (synopsize) some things for me...
I am trying to learn am radio electronics but I have a mental block in really grasping an understanding of what is actually happening in a
radio circuit. I have read and reread all the basics but still does not become clear or even half way fuzzy to me.
I understand there is AC and DC in a radio circuit. I understand how the rectifier works. I just have not got a good understanding how a signal "rides" through the circuit. Is the signal an AC waveform? Does AC and DC travel the same circuitry? I know capacitors are sometimes used to block DC and pass AC. How does the signal "move"? What makes it "GO"?
If B+ is on the plate of tubes then what kind of interaction is there between that DC B+ and the RF Signal? I try to imagine the circuit buzzing and coming to life as the tubes heat up to operating point and these electrons moving at the speed of light. Is a voltage drop like on a resistor a constant electrical charge that somehow forces the signal on through the circuit? Can someone expand on this a little (or a lot :O) I have repaired 20-25 radios and am proud of the results but would like to have a much better understanding of basics sink in somehow. I wish there were some kind of animated movie that showed this...
Anyway... any and all input will be greatly appreciated.
An illustrated hi-lighted drawing of the signal path through a circuit would be great, showing Ac and DC paths somewhat like Peter has in his "Why is the B+ distributed this way?" post
Bob E. (Not quite with it)

For one thing, AC riding on the DC is like the waves on the ocean. The DC is your water level (positive in this case). The AC is the waves riding on top. The average water level is a certain amount, and then fluctuations are added and subtracted from this amount. It isn't really alternating current, but rather fluctuating direct current. However, whenever the fluctuations pass through a capacitor or transformer, they become pure AC, since DC cannot pass through a capacitor or transformer.

A transformer is an electromagnetic device that converts electrical energy into magnetic energy, and then back into electrical energy. It works like a generator. When you move a magnet by a coil of wire, current moves in the wire. Current will only flow in the wire while the magnet is moving. If the magnet is held stationary, current will stop flowing. Current only flows when there is a change in magnetism. In the case of a transformer, alternating currents (changing currents...whether truly AC or DC) create fluctuating magnetic fields in the primary coil. As current rises, the magnetic poles face one way. As it falls, they shift the other way. Current is induced in the secondary by these changing magnetic fields. As you can see, DC could never pass through a transformer (unless it arced across the windings). A constant voltage (no fluctuations) will produce constant magnetism. Constant magnetism (of one strength and polarity) will induce no voltage in the secondary. Only when this voltage is fluctuated up and down will voltages be induced in the secondary. The voltages in the secondary will only correspond to the fluctuations in the primary. If there is a fluctuation of 1 volt in the primary, and the transformer is a 1 to 1 transformer (same turn count in both windings), there will only be a fluctuation of 1 volt in the secondary. Regardless of the steady-state DC voltage, only the fluctuation will be induced in the secondary. If the voltage in the primary is 100, and goes down to 99, the fluctuation in the secondary will be 1 volt, not 100 or 99. Same if it goes from 100 to 101 volts...the secondary will only see a 1 volt change.

A transformer's winding ratio calculates directly to voltage, and inversely to current. If the transformer is a 2 to 1 transformer (twice the windings in the primary as the secondary), voltage fluctuations will be reduced by half in the secondary. Conversely, current will be doubled in the secondary (potential current.....remember that wire gauge also limits current).

Capacitors also block DC. It's a bit harder for me to explain capacitors, though they are quite simple. Two plates are next to eachother. A charge is built up on them when electricity is imposed upon them. Electricity never actually flows through a capacitor. If one plate is charged up positively, the electrons on the other plate are going to pull towards the positive plate (opposites attract), making the other plate seem positive, too, with respect to the rest of the circuit. For me to really explain this well, I would have to draw it out. Likewise, if one plate is charged negatively, electrons will be repelled away on the other plate, and will be forced out that side of the capacitor, making it seem like a negative current is flowing. Current will only 'flow' until the capacitor is charged to its maximum capacity. Then current flow stops. Current will only flow again if something in the circuit causes it to flow in the other direction, which will discharge the capacitor, and then charge it up in the opposite direction. If you can visualize this, you can see that only AC could 'flow' through a capacitor (it doesn't actually flow, but voltage fluctuations in one part of the circuit can influence other parts of the circuit through the capacitor...much like static electricity influences your hair or a baloon, even though current doesn't actually flow...until, at least, a spark occurs). DC would charge up the capacitor, and then current flow would stop once the capacitor was full. Voltage fluctuations that ride on the DC will, however, cause the capacitor to charge up a little more and a little less. This will influence the circuit on the other side of the capacitor. If the steady DC voltage is 100, and an audio signal fluctuates the DC up to 105, and then down to 95, a 10 volt peak-to-peak wave (of whatever shape) will be seen on the other side of the capacitor.

Tubes work like this: cathode gets hot, and by its own natural phenomenon, electrons are shaken loose. A special chemical on the cathodes of most modern radio tubes helps this process along. A positively charged plate in the vecinity of the cathode collects the electrons. If a current source is connected to the tube, current will flow from the cathode to the plate, so long as the current source charges the cathode negatively, and the plate positively. Anytime a reverse polarity is present, current will not flow through the tube.

A grid placed between the cathode and the plate can influence electron flow. If it is placed very close to the cathode, very small voltages on the grid can greatly influence electron flow from the cathode to the plate. Electrons at the cathode are moving a bit slower than they are once they reach the plate (they are influenced more and more by the plate as they get closer to it, which makes them move faster and faster). A grid placed really close to the cathode has great electron 'leverage.' Tubes don't actually 'amplify' voltages. All they are is a valve with good leverage. A big voltage flows from the cathode to the plate, and the grid is placed so that a very small voltage on it can greatly manipulate the voltage flowing from cathode to plate. Small voltage fluctuations at the grid cause big fluctuations in the current flowing from cathode to plate. Like charges repel. The grid is usually charged negatively, and signals imposed upon the grid (like waves on the ocean) make the grid more or less negative, which, likewise, causes cathode-to-plate electron flow to decrease or increase.

Other grids are often placed in amplifier tubes. A screen grid is placed a bit farther from the cathode. It is charged positively. It accelerates electron flow, but is far enough away from the cathode that electrons are often already moving too rapidly to collect in large amounts on the screen grid.....so most pass by and on to the plate. Another grid, called the supressor grid, is placed even further from the cathode. Sometimes electrons strike the plate with such velocity that they are knocked off. They can bounce back to other elements, or cause varying space charges, which will create unwanted feedback and oscillation. The supressor is charged usually as negative as the cathode, is so far away, and the electrons are moving so rapidly, that it doesn't really slow them down much, but prevents those that bounce off of the plate from going to unwanted places....and usually the supressor, since it is negative, repels the electrons back towards the plate.

Hope this clarifies things somewhat. There are quite a few good books worth reading.....regarding radio theory.

T.

As for a signal...whether it be audio or RF, it is an alternating current. The signal coming into your radio is an electromagnetic/electrostatic impulse....mostly electromagnetic. A coil of wire that is in parallel with a capacitor resonates to a certain frequency (when a current alternates back and forth at a certain rate, that rate is called a frequency). Capacitors tend to pass higher frequencies better than lower ones. The opposite is true with coils. A neat thing worth studying is the resonance that occurs when the coil and capacitor pass a certain frequency equally (higher frequencies will be passed more by the capacitor and less by the coil, and the opposite will be true with lower frequencies....so they will tend to not resonate in the circuit, since one of the two components is always opposing the flow of those frequencies). The current flowing back and forth at the resonant frequency bounces back and forth in the tank circuit (coil and capacitor) quite freely. All other frequencies are absorbed and lost by the tank circuit. The resonant frequency, since it is not absorbed, can influence the grid of the 1st RF tube that it is connected to. Various things then occur, depending on the radio. The signal is strengthened by the process I explained before. It is then sent on to each successive stage through capacitors and coils and tubes, as I described before. It is then detected (something that requires a bit more explanation...I'm going to bed now)....and then is sent on to further audio amplification stages using the methods I described.

T.

Bob, have you tried going to www.funwithtubes.net? This site has a lot of good basic stuff along with explaining the all American five tube sets. Just type in fun with tubes in your search engine and have a look.

Also, there is a book that you can download called Elements of Radio Servicing by Marcus and Levy. Just type in Elements of Radio Servicing, and your search engine should find a place where this can be downloaded.

I printed the whole 450 plus pages, and it helped more than any other literature I have come across.

Good Luck, Doug

Thanks Doug,
I have printed out a few articles at funwithtubes and will be printing more later. I have A 4th printing copyright 1947 copy of Elements of Radio Servicing and
need to delve back into that too. For me though it is difficult reading and comprehending when there is no one to ask a question when you come to a stumbling block and for me that could be every other paragraph.
I know that somewhere there probably exists an old Navy training film that covers some of this basic stuff in an animated fashion. I think I remember that from my electronic school I took in the Navy in 1968. Trouble is/was that I never put any of that training to work until a year ago and now I remember why.:O) Thomas had some good explanations for me and I will respond to his post too. In the meantime thanks and if you come across
anything else let me know. Bob

:Bob, have you tried going to www.funwithtubes.net? This site has a lot of good basic stuff along with explaining the all American five tube sets. Just type in fun with tubes in your search engine and have a look.
:
:Also, there is a book that you can download called Elements of Radio Servicing by Marcus and Levy. Just type in Elements of Radio Servicing, and your search engine should find a place where this can be downloaded.
:
:I printed the whole 450 plus pages, and it helped more than any other literature I have come across.
:
:Good Luck, Doug

Thanks Thomas,
I liked your wave analogy. Thats the kind of thing that helps me understand a little more. I will be reading your post a few times and then I may have a question
or two. Please feel free to expound on other basic theory as well. Every little bit will help. I pick up quite a few tips on this site. It has helped me quite a bit. Bob E

:As for a signal...whether it be audio or RF, it is an alternating current. The signal coming into your radio is an electromagnetic/electrostatic impulse....mostly electromagnetic. A coil of wire that is in parallel with a capacitor resonates to a certain frequency (when a current alternates back and forth at a certain rate, that rate is called a frequency). Capacitors tend to pass higher frequencies better than lower ones. The opposite is true with coils. A neat thing worth studying is the resonance that occurs when the coil and capacitor pass a certain frequency equally (higher frequencies will be passed more by the capacitor and less by the coil, and the opposite will be true with lower frequencies....so they will tend to not resonate in the circuit, since one of the two components is always opposing the flow of those frequencies). The current flowing back and forth at the resonant frequency bounces back and forth in the tank circuit (coil and capacitor) quite freely. All other frequencies are absorbed and lost by the tank circuit. The resonant frequency, since it is not absorbed, can influence the grid of the 1st RF tube that it is connected to. Various things then occur, depending on the radio. The signal is strengthened by the process I explained before. It is then sent on to each successive stage through capacitors and coils and tubes, as I described before. It is then detected (something that requires a bit more explanation...I'm going to bed now)....and then is sent on to further audio amplification stages using the methods I described.
:
:T.