Tube Rectifier Question

[12thfret]

I'm trying to get my head around tube amp design and operation. I was making good progerss in my studies this morning until I started looking at the PS. The rectifier in particular.

I understand the concept of a diode tube, and how electrons only flow from cathode to anode. That the AC supply off either end of the center tapped seconday PT is out of phase with the other end and the rectifier only conducts on the positive side of the sign wave. Filter caps fill in the gaps but for a bit of ripple that is handled by the choke.

My big question is what exactly is going on in the tube. From this schematic, I see that 5 VAC is going to the heater and the HT is connected to each of the plates. But I aslo notice that the resulting DC output is connected to the 5 VAC line. How is it possible that these 2 are on the same "buss"? Is it because the output of the tube is only the positive side of an AC voltage and is only considered DC when in a circuit that connects it to ground?

And, if electrons only flow from neg to pos, or cathode to anode, how is a higher voltage created on the cathode? Is the cathode pos in this case?

[octal]

This is a bit of a simplification, but there is no ground reference for the AC voltage applied to the heater, so it doesn't cause ripple to be added to the DC output. The end of the heater that is tied to the cathode is AC bypassed to ground by the filter caps.

Also, remember that current flow (as taught in electronics theory) is the opposite of electron flow. i.e. the convention is that current flows from positive to negative, when in actuality electrons travel from negative to positive.

Hope that helps.

[Enzo]

What is a positive charge? It is fewer electrons than whatever it is positive with respect to. Electrons are little units of negative. (must...resist...political... reference...) So how do you make something more positive? REmove more electrons from it. When your electrons leave that cathode, that makes it more positive than it was. And conversely, adding electrons make a thing more negative.

Imagine you are a bird sitting on a power line. 100,000 volts in that line, but you can sit there safely because you complate no circuit. If a ground wire were to touch you while you sat there, BAM you are cooked. But no ground wire, so you can sit there all day and fly away whenever. Now imagine you also have a 9v battery, and you connected the negative terminal of it to that power line right next to you. No, I don't know where birds get batteries, but that isn't the point. No ground wires anywhere near. Then the remaining positive terminal of the battery is now 9v more positive than whatever thousands of volts are on that wire. But since the battery conects to nothing else, it has no effect at all on the power in that line. No one miles away will have 9 more volts in their power outlets. For your little bird electronics hobby you could consider that 100,000v power line as your common - I hesitate to use the term ground here. You could actually build a little amplifier circuit using that power wire as "ground" and running off that battery. And because there is no reference to earth ground, that 100,000v makes no difference to it, and it make no difference to the 100,00 volts.

SO think of the 5VAC as if it were that little birds battery. The current through the B+ does not flow THROUGH the 5VAC. So the 5VAC does not add to the B+ causing hum.


AC and DC have nothing to do with grounded or not. A battery makes DC whether I wire it to a copper stake in the earth or throw it into the air. Same with a transformer, transformers only make AC. But voltage only exists as a description of a relationship. VOltage at one point is always IN RELATION TO the voltage at some other point. A battery terminal does not just have 9 volts. It has 9 volts with respect to its other terminal. Set a battery down and measure the voltage between its terminals - 9 volts. Now set another one down a foot away and measure it - 9 volts. Now connect your meter red probe to one battery, and the black probe to the other battery. What do you measure now? Nothing, no volts, because the batteries are not connected together. They share no common frame of reference.

In most amplifiers we asssume a reference to ground. A winding on a transformer may have 100 volts across it, but if its wires are sticking in the air, we can measure voltage from either wire to our chassis and get not much. But connect one of those wires to chassis and now the remaining wire has 100 voltsAC - WITH RESPECT TO THE CHASSIS.

For convenience, we refer our B+ to ground (chassis), so the +480 is +480 with respect to chassis. Your tube heaters often are grounded in some way, but they donlt have to be to function. You could wire the heaters direct to the 6VAC, with no grounding, no center tap, no 100 ohm resistors, and it would work. But we add the grounds to reduce the pickup of hum by the high gain circuits. But withoout grounding, those 6VAC wires have no voltage to ground.

There are not positive batteries and negative batteries, there are just batteries. A battery terminal is always positive or negative with respect to its other terminal, but it only takes on a polarity to the real world when one end is connected to that real world

[12thfret]

Thanks for the replies and info. Much appreciated!

I think I have my head around this now. Being a visual learner, I made a quick animation that shows my understanding of the rectifier circuit. Hopefully, it's accurate but please feel free to correct me if something is wrong and I'll update the animation.



Peace,
Mark

[Enzo]

Wow. that is a lot more effort than I would have put into it.

Yes, that is the gist of how a tube rectifier circuit works. It doesn't address the 5v thing we discussed, but it does cover how the basic circuit functions.

[Cygnus X1]

I don't know 12thfret.
But that is probably the coolest thing I've seen in my 47 years.

I drank two beers while looking at it!

[loudthud]

To increase your understanding of how power supplys work, study the attached picture.

Green trace is the cathode of the rectifier tube connected to a capacitor and a 100mA load.

Yellow trace is the output of the transformer on one side only.

Red trace is the current through the rectifier on one side only (right side scale). Note how the current spikes are over 300mA peak, but the load current is only 100mA.

Blue lines i drew in to show how the current spikes line up with the yellow and green traces.

Current only flows in the rectifier when the plate is more positive than the cathode. At T1, the plate voltage (yellow) becomes higher than the cathode voltage (green). When the rectifier current exceeds the load current, the green trace starts to move positive as the capacitor charges up. As current peaks and begins to fall off, the capacitors charging rate slows down until the load current exceeds the rectifier current. Shortly thereafter the plate voltage falls below the cathode voltage and the rectifier turns off (T2). After the rectifier turns off, the capacitor discharges until T1 of the next half cycle (not shown).

Note that the difference between the yellow and green trace at the peak is the voltage drop across the rectifier.