Rectifier explanation needed

[gui_tarzan]

Ok, I am totally stumped as to how a rectifier tube takes the high voltage (360v+) from the transformer to pins 4 & 6, which I understand to be the plates, and the second (5v) tap to the heater/cathode on pins 2 & 8 and pumps 360v+ out of one lead of what appears to be the low voltage section of the tube on pin 8? With diodes you're running both B+ through a diode which then goes to the common B+ feed for the caps and other parts of the circuit. With a tube rectifier the B+ doesn't touch the common B+ circuit physically.

This makes no sense to me. How can you get B+ of over 3xx volts from a section of the tube that runs on a 5v supply? Does that make sense or did I not word my question correctly?

I read an explanation on another site but it still doesn't make sense.

[R.G.]

The "5v supply" is a floating 5Vac winding. It's not referenced to ground at all. So the B+ voltage through the diodes pulls the 5V supply up with it.

[Enzo]

Ever see birds perched on a high voltage wire along power poles? How can those birds sit on 13,000 volts without electrocuting themselves? Because the birds never make a circuit with ground. Your 5v heater circuit is not 5v to ground, it is 5v across that heater. The whole heater is sitting at B+ voltage now.

[gui_tarzan]

Obviously it doesn't but it seems like that B+ coming back across the 5v supply would fry it.

[Enzo]

But that is just it, the B+ is not ACROSS the 5v. The B+ just connects to one side of it.

Look at it in terms of circuit. You have a 5v transformer winding. And it has insulation think enough that 500v won;t arc it to ground. You have a 5v tube heater across that winding. Tube heater current flows through the winding. Now connect one end of the 5v to B+. Where is there a path to ground - the other end of the B+ supply? There isn't one. If we touch any part of the 5v circuit with a grounded wire, then yes, instantly, it burns up. But we deny it a ground path.

[eschertron]

Because the rectifier is directly heated, the heater and the cathode are referenced to each other - but nothing else. So the heaters "just happen" to float at whatever voltage the the cathode is at.
Think about the 6.3v heater winding. Same thing. It can be referenced to ground, or to some other voltage; even B+ Of course, preamp tube heaters at B+ could cause some strange behavior, up until the time they cease to function! For the rectifier tube, the heater is designed to do what we're asking of it. so it's OK.

[R.G.]

Danger!! 50,000,000 ohms!!

[gui_tarzan]

Ok, that sort of makes sense. It's just hard to fathom how that much voltage can travel only across one lead and not affect the 5v section of the tube. I get that there is no path to ground. It's still amazing to me how inductance can "create" voltage and current, you know? I think these tube designers were geniuses, more so than people today.

[Jazz P Bass]

Try to keep in mind that the rectifiers heater (the 5volts) does not care where the 5 volts comes from.
If one end is 385 volts & the other is 380 volts, that is a 5 volt difference & that is all that matters.

Also, remember that voltage is 'relative'.

Relative to what you may ask.

Relative to where the common probe is attached.

[R.G.]

My relatives were all low-voltage.

[eschertron]

Ok, that sort of makes sense. It's just hard to fathom how that much voltage can travel only across one lead and not affect the 5v section of the tube. I get that there is no path to ground. It's still amazing to me how inductance can "create" voltage and current, you know? I think these tube designers were geniuses, more so than people today.

Voltage doesn't "travel". Voltage just "is", and it's all through the 5v section of that tube. Again, think of the 6.3v heaters. In an old design, one side of that 6.3v may be grounded, so the other side is 6.3v from ground. Move the ground connection to the cathode of the cathode-biased power tube (heater elevation) and now one side of the 6.3v heater is at 22vdc (for example) and the other at 6.3v away from 22vdc. And this is all along those heater wires, even inside the PT.

Same thing with the 5v heater, only we're elevating it by hundreds of volts instead of a few. The PT secondary coil doesn't care that the wiring is that far from ground because the theory says it doesn't know anything about it. It simply has 5v AC across the windings.

Yes, the physical wiring must be voltage rated appropriately, but that's a different thing entirely.

[Enzo]

Maybe this will help. Imagine the rectifier tubes came with a 5v battery strapped to the tube, so it can heat itself. That battery would be under no stress from B+ because it has no connection to the rest of the world. One side of it will be on B+ but nowhere else.


Maybe another analogy might be air travel. If you are on an airplane 7 miles up, you can walk down the aisle, and not worry about falling 7 miles if you stumble. You are totally within the airplane system. That 5v heater circuit is totally part of the rectifier tube. The 5v winding may be on the transformer, but it is isolated from all other windings, just as the 6v and B+ windings are separate.

[gui_tarzan]

I get what you guys are saying but it's still one of those things that makes me marvel at the science of it.

[NateS]

We like to dismiss magical thinking, but as the parent of a toddler... simple representative models can often take you quite far, and are vastly easier to learn. Particularly when attempts to harmonize bits of science require much much deeper understanding that 1) you currently posses or 2) need to accomplish your goal. We even do it inside science and engineering. Friction = Normal Force * coefficient of friction. Any time you see "coefficient of ..." it's likely a sign that "We're representing something vastly more complex with a mystical stand in right here". Most engineers and even physicists never ever learn how friction truly works on a molecular level. It's more like a Masters level material science concept. I'd even argue, that without the simple representative models, we likely wouldn't have gotten to the point where we can develop more complete models.

So if little greenies and their desire to party helps you understand... party on!