Help with understanding rectifier tube

[swwhit]

Hello: I'm building a Champ 5F1 kit and trying to understand how each part operates. The rectifier tube has got me thoroughly . I know it takes in AC from the transformer and magically puts out DC but I'd like to know how. Below is a summary of what I think I understand - please feel free to tear down my assumptions! Thanks in advance.

Power Transformer: P-TF22772 (Torres)
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black - AC Primary
red - high voltage secondary (to rectifier plate)
red/yellow - secondary center tap (to ground)
yellow - 5V AC (to rectifier filament)
Green: 6.3V AC (filament heaters)

Rectifier
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5Y3GT

Pin 4 Plate/Anode (high voltage secondary AC from transformer)
Pin 6 Plate/Anode (high voltage secondary AC from transformer)
Pin 2 Cathode (5VAC from transformer)
Pin 8 Cathode (5VAC from transformer)

Pin 8 is also DC output (approx 320vdc)

Questions
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1. How many anodes are actually in the tube, 1 or 2? Are the two secondary leads from the transformer supposed to make a circuit across the anodes and back to the transformer?

2. How many cathodes, 1 or 2? It looks like the 5v wires make a circuit through a single cathode and back to the transformer.

3. (the bigee...) Describe the current flow - and how/why does it put out 300+ VDC from pin 8 which is also attached to one of the 5V leads from the transformer?

... from what I have read about tubes, the cathode heats up and sheds electrons which are attracted to the anode, sometimes through grid(s) (does not apply to rectifiers). I can't figure out how this relates to DC voltage.

[Enzo]

It relates in that DC current flows only one direction, while AC current flows back and forth. The cathode emits electrons, and they flow to the plate (anode). They do not flow from plate to cathode, because the plate does not emit electrons (and the cathoe would repel them anyway). So the tube acts like a diode - it IS a diode - and allows current to flow one direction, but not the other. That is how it turns AC into DC.

The AC flows through the diode section for half its cycle, then when the current reverses, the diode no longer conducts. At the cathode then we now have the positive half of the AC waveform, and a space at zero volts where the negative half would have been. A continuing row of bumps and spaces, but all positive.

The two ends of the transformer winding alternate positive and negative, and one will be the opposite of the other. SO if we then connect a second anode to the other end of the transformer winding, whenever the first one is not conducting due to the reverse polarity, the second one will be the correct polarity and current will flow through it. So what we now have is each of the two anodes taking turns alternatelysending a positive bump through. So instead of bumps and spaces, we now have bumps and bumps. Still rough, but better than with the spaces.

It is like the AC, the voltage smoothly goes from zero up to a peak, then back to zero, but only positive. SO it is DC, even if pulsating. We refer to it as pulsating DC, or unfiltered DC. To smooth it out, we add a capacitor. The cap charfes up to the peak value of the pulsing DC. The charge then maintains the voltage at that level between pulsing DC peaks, so we get smooth steady DC.

AN analogy to the rectifier or diode is the valve in your heart. The heart valve allows flow one way but blocks it the other. Your blood flow is DC then.

The 5Y3 has two anodes and one cathode. It is actually physically two cathodes - one inside each anode structure - but they are wired together, so electrically they are one. The schematic symbol is more clear than looking at the tube itself perhaps. Current from the cathode flows to one anode, then the other, alternating 120 times a second.

Pins 4 and 6 are the anodes - we call them plates - and while the ansformer winding is wired to them, there is not a circuit there, no current flows from pin 4 to pin 6. Current flows from the cathode, to either anode, and from there out the grounded center tap of the transformer winding.

Look carefully at the 5v winding. Nowhere is it connected to ground. The high voltage winding, though has the center tap grounded. So it is connected to ground. All the 5v does is heat the 5Y3 cathode. Ther is 320VDC on pin 8 and you will also find it on pin 2.

Ever look up at the high voltage wires on a telephone pole - 50,000 volts or more - and see birds sitting on the wire? How can they sit there without getting shocked - it is 50,000 volts???? They can do it because the birds are not also connected to ground. There is not circuit through the birds for that 50,000 volts to flow. This is analogous to the 5v winding perched on the 320VDC. The 5VAC winding is isolated, so it can float with repsect to ground, but still have 5vAC across its wires. That 5VAC heats the tube cathode, just as the little bird warmed a small part of hte wire.

[swwhit]

Enzo - thank you for your thoughtful and detailed explanation. I understand most of it !

However I'm still unclear as to why there is 320vdc on pins 2 & 8. We have high alternating voltage causing electrons to flow in one direction away from the cathode to either anode and out through the center tap to ground.

We have a wire attached to pins 2 or 8, which is eventually connected to ground.

Here's my guess at this point (to continue your wildlife analogy ... )
We have a herd of electrons stampeding from cathode to anode inside the tube - do some of them 'break out of the corral' at the cathode pins and gallop across the wire because it is grounded?

thanks again,
SW

[Enzo]

Pin 8 is not grounded. Whatever load you place there might be connected to ground on its OTHER END, but no ground connection to pin 8.

The most important concept in current flow is the CIRCUIT. There must be a complete path for current from its source, through a load, and returning to it source. The typical example is a battery and a light bulb. Current from - end of battery flows through bulb to + end of battery - complete circuit, current flows. Run a wire from the - end of the battery to the bulb but omit the wire from the bulb to the + end of the battery, and ther is no complete circuit, no current flows.

Circuit as a word comes from the same root as circle. For current to vlow there mist be essentially a circle path.

What am I blathering about? Your rectifier current has to have a circuit. It is the same buffaloes stampeding around and around in it. Pick a spot, any spot, in the circuit, and we have to find a path through the circuit back to the start. Those electrons start at the cathode pin 8 - there, I picked the starting point - they fly over to the anode, then through the transformer winding (half of the winding really) to ground. But that is only half the circuit. They continue through ground over to the bottom end of the load, then through the load and voila, back to the cathode pin 8. That is the complete circuit. The "load" is whatever the power supply powers. For us let's just pretend it is a resistor from pin 8 to ground. In reality it is all the accumulated current paths through the tubes in the signal path.

Let us step back one. Transformers put out AC current - electrons moving one way, then turning around and moving the other. Endlessly moving and turning, back and forth over and over and over, same old back and forth day after day after day, and does Mrs electron appreciate it? Not on your life buddy, she gets a charge out of... Oh... ahem...

The secondary winding produces an AC voltage for us to use. Let us just connect a resistor across it. Now current flows through that resistor, first one way then the other back and forth - AC. One circular loop of wire. Think of the resistor as just another wire but one that is harder for current to flow through.

Now add that diode - tube or semiconductor, doesn't matter, either kind is a one way valve - in between the transformer and the resistor. Doesn't matter which end for this example. Current can flow through the rectifier one way but not the reverse. Remember the cathode emits electrons which flow to the plate, but the plate cannot emit electrons and even if it could, the cathode would repel them. That means now that the transformer voltage can send current through that diode one way, through the load and back to the tansformer, but when it reverses and tries to flow the other way, it is blocked by the diode's one way aspect.

So we have current flowing through the load one way, then it stops when current TRIES BUT CANNOT flow the other way, then flows again when the current flows the correct direction. WHat this is is current that flows one way only - and that is the very definition of DC current.

Electrons are negative, they have a - sign. When you take them away from something it is less negative. That is the same as saying more positive. We discussed this here not long ago briefly. WHat is happening is that the ground has more electrons than the cathode, because the cathode is emitting all it can towards the plate. SO electrons are rushing from ground through the load and rushing TOWARDS the cathode. They have to to replenish the ones the cathode is emitting all the time. Don't worry, they'll just run through the circuit and be back in a flash. Well maybe a jiffy is preferred to a flash.

Is that part of the confusion? That 320VDC is not a collection of excess electrons, it is a 320v SHORTAGE of electrons.

[swwhit]

Enzo - thanks for the info. You've provided a lot of good information, which I'll print out and keep as as a reference.

One of my fundamental misunderstandings was that Ground could carry voltage as part of a circuit - since the center tap is connected to a ground screw on the chassis, the entire chassis must be part of the circuit carrying 320v(right?)

[Enzo]

It carries current, but yes, it is very much a part of the circuit. In theory, we could attach any one part of the circuit to ground. For example we could run a circuit where the B+ was ground and the tubes ran -320v on their cathodes. It is the same 320 volts across the tubes either way. Or you can connect nothing to ground. There is no connection to ground on your walkman or iPod, though they do have internal commons.

In most of our amps, common and ground are the same, and they are usually the negative end of the power supply. Not always of course. The negative end of B+ goes to ground, but in the bias supply the positive end goes to ground so we get a negative supply.