Rectifier voltage drop.?

[trem]

Can one of you guys explain to me what is happening with/in a tube rectifier.?
I am not understanding where/how the voltage drop of different duo-diodes is impressed upon the B+ of the amp.
It seems like the B+ is taken from the Cathode. So how does the High Voltage of the plate, and the voltage drop of the tube, get onto the cathode, and then to the rest of the power supply.?
It seems a bit confusing (not real intuitive I guess). When a diode conducts, are the plate and cathode at the same potential, and would that also include the voltage drop of the tube as well.?
I hope you guys can understand what I am asking, and why I am confused.
At first glance it would seem the cathode is at 5 VAC, so how does that become the high voltage of the plate.?
Thank You So Much
if this is one of those questions that has been asked a Million times, I am sorry. very happy to search/read any link(s) that explains this.
Thanks Again

[soundguruman]

DC travels through a vacuum, it's drawn there to the cathode by molecular attraction.
There is holes in the cathode and the electrons want to fall into the holes.
As any current is drawn by any part of the circuit, it produces some amount of voltage drop
There is voltage drop across the diode. The anode and cathode are not at the same potential. There has to be some difference...
"how much" voltage drop it can take without blowing chunks, is rather a matter of how much heat it can dissipate.
As the load increases, you are bound to see more voltage drop and more heat...
across the cathode remains at 5VAC because it has no reference to the original high voltage. You see, the 5 volt winding is not connected to the high voltage winding of the power transformer.

[R.G.]

SGM is correct in concept, just a few details need beating on.

Current is carried in a vacuum tube by electrons crossing the vacuum gap between cathode and plate. They are boiled off the cathode by heat - that's where that word "thermionic" comes from; it's the combination of thermal and electronic. The cathode is heated until electrons are actually forced out of the surface by their thermal motion, making a cloud of electrons at the cathode.

This leaves the cathode positively charged by one electron-equivalent for every electron that jumps out, so with no other forces acting, the electrons loop out through the vacuum and then fall back in, attracted by the slight positive charge left over from their leaving the cathode.

If you insert a relatively faraway plate and make it positive enough, the pull of the positive voltage on the plate is enough to overwhelm the nearby pull of the slightly-positive cathode and pull electrons across the vacuum to the plate. That takes some voltage to force them to jump, generally at least 30-50V for a significant current to flow. And that's the voltage across the vacuum rectifier. It's whatever voltage is needed to make electrons leave the cathode and go to the plate.

The cathode is positive in a rectifier because electrons with a negative charge are going the other way. This is the old, old discussion of electron flow versus current flow. In a rectifier setup, the capacitors get positive because the rectifier is sucking electrons out of them to replace the ones which flow to the plate.

As noted, as the current flow increases, you need more voltage on the plate to pull more electrons. The transformer winding is providing the voltage difference for this to happen, and has several hundred volts available. The voltage drop across the rectifier is part of this; so the drop across the rectifier increases as current flow increases because it takes more voltage to force more electrons to shoot the gap.

[soundguruman]

"There is no such thing as an electron."
-Nikola Tesla
Actually, it was Tesla who invented many of the vacuum tube designs, and never patented or took credit for the achievements.

A lot of people tend to illustrate electrons as "water flowing out of a faucet," although this is incorrect. You see, AC is flowing both ways, and water from a faucet can drain anywhere, not just down a "drain."
Electrons in conventional electricity can only drain to one place, a place that is referenced to the source of the electrons, not "just" anywhere. They must return to where they came from...
And so, there can be two paths in one circuit, and each path will never be affected by the other, if there is no reference to provide the "drain."

[Jazz P Bass]

Here is a nice vacuum diode link.
There is a simple animation at the bottom of the page.
Link: Vacuum Diodes
Read the part twice about the 5 volt winding!
It is the 'difference' in voltage that supplies the '5' volts to the heater.

[tubeswell]

The voltage drop across the rectifier is part of this; so the drop across the rectifier increases as current flow increases because it takes more voltage to force more electrons to shoot the gap.

And some rectifier tubes are better than others at getting electrons to flow to the plate with less voltage drop.

[trem]

I appreciate the info so far, and I will definitely give that link a Long Look; maybe that will answer my real question.
But until then..... what I am really not understanding is how does the Cathode end up at B+ voltage.
From looking at the schematic, it looks like the B+ rail starts at the cathode, not the High Voltage plates. How does the cathode get to be 300-400 volts.?
HT winding is connected to the plates, and then the CT is connected to ground. How does the cathode get all that High Voltage.?
Thank You

[R.G.]

I appreciate the info so far, and I will definitely give that link a Long Look; maybe that will answer my real question.
But until then..... what I am really not understanding is how does the Cathode end up at B+ voltage.
From looking at the schematic, it looks like the B+ rail starts at the cathode, not the High Voltage plates. How does the cathode get to be 300-400 volts.?
HT winding is connected to the plates, and then the CT is connected to ground. How does the cathode get all that High Voltage.?

The rectifier tube is a one-way valve ( Yep - that's why the UK and Euro term was invented - it's an electron valve.) for electrons. They can only go one way, cathode to plate. Plates are made so that at ordinary operating temps, before they melt down, they don't emit electrons.
(N.B. Yes, I know about secondary emission; that's a refinement, he's looking for concept.)

So whatever voltages you put across the rectifier tube, electrons only flow from cathode to plate, and then only when the voltage makes the plate more positive.

So the cathode starts at 0V, and there is 0V across the first filter cap. The transformer goes negative on the plate, positive on the CT, which is connected to the (-) of the first filter cap. The tube cannot conduct, as the plate cannot spit out electrons to get the more-positive cathode.

Next AC half cycle, the plate goes positive with respect to the cathode, which is held at 0V by the first filter cap not being charged. Now the polarity is correct, and as long as the plate is more positive than the cathode, it sucks electrons out of the cathode. These have to come out of the (+) side of the first filter cap. The transformer winding voltage pulls electrons out of the plate, and the CT stuffs them back into the (-) side of the first filter cap, charging the filter cap + and -.

When the transformer polarity inverts again, the plate goes negative compared to the cathode and the first filter cap, and no current flows.

Next half cycle, plate goes positive, and when it gets positive enough, sucks electrons out of the cathode and + side of the first filter cap. So the one-way nature of the rectifier tube lets the transformer pump electrons out of the filter cap + and shove them into the filter cap - . The circuit can then drain the filter cap, letting electrons flow out of the filter cap - and back to the filter cap +.

The one-way valve and alternating voltage let the transformer pump electrons off the + side and to the - side.

[diagrammatiks]

there are some tubes that are directly heated and some tubes that are indirectly heated.

this applies to rectifier tubes as well.

If a tube is directly heated then the heater voltage will be applied directly to the cathode element so the ac there rides on top of the rectified dc voltage coming from the plate.

in an indirectly heated tube the heater elements are separate from the plate and cathode elements.

[tubeswell]

Without the reservoir filter cap, the rectifier cathode is 'producing' +ve 'pulses' of charge. At the beginning of each pulse, the reservoir cap (which is connected directly to the rectifier cathode), quickly charges up to a +ve charge, and then gradually releases the +ve charge, but the speed with which the cap releases the charge is much slower than the speed with which it charges up. Each time the cap gets 'hit' by a fresh +ve pulse from the rectifier, it charges up again. In this way, the cap looks like it 'stores' +ve charge. As the rectifier cathode is connected directly to the reservoir cap, the cathode looks like it is continually +ve (and it more or less is, except for the slight effect of the ripple current from each pulse peak).

[big_teee]

There is also some good info on the valve wizard.
The Valve Wizard
Interesting thread I am learning a lot for an Old man.
Thanks,
T

[J M Fahey]

1)

DC travels through a vacuum, it's drawn there to the cathode by molecular attraction.
There is holes in the cathode and the electrons want to fall into the holes.

Very poetic but nonsense.
Electrons are attracted by proper positive voltage. PERIOD.
And *electrons*, not "DC" , travels through a vacuum, under proper circumstances.
To follow your idea, "AC" also travels through the vacuum if you have some reason for that and build the proper tube, so it's clear that the proper explanation mentions *electrons* and not "DC" or "AC".

2) you have always 5V across the ends of the filament which is also the cathode, because it's fed from a separate , *floating* (meaning not connected to any other winding) 5VAC winding.

3)

"There is no such thing as an electron."
-Nikola Tesla

Yea, sure.

Actually, it was Tesla who invented many of the vacuum tube designs, and never patented or took credit for the achievements.

I beg your pardon ?????
Poor Nikola Tesla's name is famous for being abused to "give credibility" to impossible things.
This is what a myth debunking site has to say about N.T.
1:27 – Nikola Tesla
Conspiracy theorists and pseudoscientists love Nikola Tesla, because he was working on a lot of weird stuff that could theoretically lead to lots of nifty science fiction machines. Consequently, if you want a machine that does X to exist, all you have to do is say that Tesla invented it and that the invention was suppressed, or that it’s an extension of something Tesla invented. After all, he died in 1943 and won’t be able to dispute you.

4)

A lot of people tend to illustrate electrons as "water flowing out of a faucet," although this is incorrect. You see, AC is flowing both ways, and water from a faucet can drain anywhere, not just down a "drain."

Fully agree with you.
I hate the "water" analogy to explain electricity because it causes *much* more confusion than what it "helps" explain.

5)

what I am really not understanding is how does the Cathode end up at B+ voltage.
From looking at the schematic, it looks like the B+ rail starts at the cathode, not the High Voltage plates. How does the cathode get to be 300-400 volts.?
HT winding is connected to the plates, and then the CT is connected to ground. How does the cathode get all that High Voltage.?

RG's explanation is perfect. Just to repeat it using other words, plates *are* connected to the cathode whenever they are , say, 30V more positive than it, or in another words, *positive* voltages at the plates will appear at the cathode, but "losing" 30V in the way.
So if a , say , 300V AC winding is connected to the plate, its peak voltage will be 300x1.4142=424V, and *up to* (424V-30V)=394V will appear at the cathode.
Since it's usually connected to a capacitor, it will charge up to this voltage.
Why not less?: because if it is more negative (hasn't yet reached) than that, it will still attract current.
Why not more?: because once it's reached that peak value, the plates can't go any higher (more positive) and the tube stops conducting.
What happens if that supply is connected to a load? (An amplifier for example)
The amp will "use" electrons stored in that capacitor, its voltage will fall, and said voltage difference will start moving electrons again.
That's why normal Power supplies have a certain "no load" voltage, which starts falling when consumption starts to climb.

Let's make something clear: there's a "Conventional current flow", which says *current* flows from positive to negative, and "electron* flow, which being negative run the opposite way, I know it can be confusing to some.
Why is that convention maintained? Because it helps explain many complex electric things in a *much* simpler way. So far as you remember "Current is positive, electrons are negative and flow the other way" there's no confusion. Sorry but it's the Physics consensus.

Tubeswell explained this charging/discharging the filter caps.
Let me add that this causes a certain up/down rhythmic variation in available voltage, at the line frequency (or twice it, depending on the rectifier configuration).
This up/down voltage is called "ripple".
As you can imagine, lower current consumption and/or bigger filter capacitance will lower ripple.
Poorly filtered supplies fail to suppress it well, and said ripple appears as annoying "hum" mixed with the audio signal.

[trem]

DC travels through a vacuum, it's drawn there to the cathode by molecular attraction.
There is holes in the cathode and the electrons want to fall into the holes.

I thought this was something peculiar to Semi Conductors. Did not realize it also happens with tube diodes. But every time I ask a question, I realize I know even less than I thought I do. I feel less "smart" anytime I open my mouth on this forum. I guess that is the price you pay for learning.
Anyway..... I have A LOT of info/links to digest here. But if anybody wants too add/say more please do so.
I REALLY need a classroom setting with a teacher. I am not an idiot (you will just have to take my word on this) but I really benefit with that situation. That is not going to happen however. Teaching a guy to trouble shoot to the component level seems to be a thing of the past..... so I come to you guys for help.
I probably seem like a loss, but you guys would be shocked at the number of amps I have been able to fix/build for players in the last few years. For the record, I NEVER tell anybody I am an amp tech. I tell them the truth. I have a general understanding of electricity/electronics, and I can probably fix-repair-bias-mod-add a kit to their amp. Just wish I had the formal education some of you guys do, but I do not.
Somebody mentioned Electron/Conventional current flow. Right or wrong, I always think/look in terms of current flowing from Negative to Positive. The first real book I read on this topic was by Grob. He wrote in terms of electron flow, so I just stick with that.
Anyway..... I will say THANK YOU once again
I appreciate It

[R.G.]

... I guess that is the price you pay for learning.
Anyway..... I have A LOT of info/links to digest here. But if anybody wants too add/say more please do so.
I REALLY need a classroom setting with a teacher. I am not an idiot (you will just have to take my word on this) but I really benefit with that situation. ...Just wish I had the formal education some of you guys do, but I do not.

You're very welcome to any help I can add.

I did sit through the classroom settings for electronics. It helps. But you also get stuffed full of someone else's idea of what you need, too. I spent a whole lot of time in classes for things I've never really needed. It is entirely possible to have a successful career as an EE with no more than a very good understanding of Ohm's law and power calculations.

One guy I know (actually, I work for him!) does not have any formal EE training. He's self taught, and very well. His understanding of electronics is not the same as mine, and he has gaps, but he's very capable of doing pro-level EE work in most cases.

The bottom line is that the classroom/formal approach has pluses and minuses, like everything else.

One really, really pertinent item that has always stuck with me is an offhand comment from my systems and controls professor. There is an idea in Control Theory called the Theory of Optimism. Simply stated, it says that no matter what situation you find yourself in, the best result is obtained if you look around, pick the most likely path to get what you want done, and go that way. Figuring out the best way to start from somwhere you're not is generally no help at all. I can still see the guy's face telling me that, and I can't remember any of his lectures.

[Steve Conner]

Yes, "holes" are mostly used in the context of semiconductor physics. There are no holes in vacuum tubes. Electrons are the charge carriers. (SGM's explanation was incorrect and confusing. I would venture that every time he opens his mouth, the whole forum actually gets a little less smart.)

I believe that some beginner courses in electronics ignored the existence of electrons and taught that "holes" were the carriers of conventional current in regular circuits. This might be consistent, but it has nothing to do with physics. It is impossible to understand the operation of tubes this way.

Viewing circuits in terms of electron current can be confusing. Most circuits run off a positive voltage, so the electrons travel in the opposite direction to the power. In other words, power flows from the power supply into the amplifier, where it is dissipated as heat or converted to sound. But this is supported by a flow of electrons going the other way.

Of course the same electrons are getting fed back from the power supply to the amp through the ground wires, so they really go both ways, but we tend to ignore what happens on the ground side.

There was a colossal thread debating electron vs. conventional current. It went on so long, even Enzo got sick of it.