Can anyone please explain this power supply circuit to me. It looks like B+ (300V) is taken from the filament (cathode) of the 5Z4 diode tube. I don't get it.
Yes, that's how it's done. The heater is also the cathode (though some rectifiers have a cathode sleeve over the heater). The heater carries the AC current that heats it and the DC rectified current.
https://www.dhtrob.com/overige/idh_vs_dh_rectifiers.php#:~:text=A directly heated rectifier is operational as,needs to heat the cathode, hence indirectly heated.
Unlike an audio tube, 5VAC added to the cathode voltage, raw B+ is nothing to worry about.
Unlike an audio tube, 5VAC added to the cathode voltage, raw B+ is nothing to worry about.
Okay, thanks for the responses. But how is the power transformer output voltage going from the anode to the cathode? I'm missing something here. I only see 5VAC going to the cathode and it's confusing me.
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Notice the center tap of the transformer is grounded. Each side of the primary that is connected to a plate will swing positive depending of the side of the wave. The rectifier conducts from plate to the cathode(filament in this case). The 5 volts for the filament is a trivial voltage in this case.
A side note, this power supply may cause premature rectifier life due to excessive capacitance for the first capacitor. Maybe others can chime in on reasonable choices for the capacitors.
Having too much capacitance at first causes higher peak current at the top of the charging cycle.
A side note, this power supply may cause premature rectifier life due to excessive capacitance for the first capacitor. Maybe others can chime in on reasonable choices for the capacitors.
Having too much capacitance at first causes higher peak current at the top of the charging cycle.
Do you understand how electrons flow in an electron tube?
I am sorry if this is beneath you. We don't know what you don't know.
Forget the heater for a second, the 5Z4 is a directly heated valve, the filament IS the cathode, it's doing two jobs, it's heating the valve AND acting as a cathode. So having forgot about the heater circuit for a minute, look at the valve as two diodes, two 1N1007's, their cathodes connected, make more sense? As said in post #5, the middle of the tfmr is tied to ground, so the AC acts like a seesaw, the pivot is ground, one end of the tfmr goes positive, one diode conducts, the seesaw then goes the other way, the second diode conducts. The seesaw does this 50 or 60 times a second, like a pump, pump pump pump, pumping electrons into the first capacitor.
Both AC and DC current can flow in the same wire at the same time, when your first learning electronics this can blow your mind. I used to get very confused, a cathode is negative right, then how come it's connected to HT/B+ which is positive??? Think in terms of current flow, and it makes sense, see the symbol for a diode, see that arrow, it's showing you which way current can flow. The diode is a bit like a policeman, directing traffic, one hand waving traffic (electrons) forwards, the other hand raised, stopping traffic (electrons) from flowing the other way. His hand raised is the vertical stick the arrow is pointing at.... hope that makes sense and isn't patronising.
Going back to the schematic, the 5v AC from the heater/filament will be squashed by the big caps and the choke, the choke opposes AC current, the big caps act as big holes, any bumpy AC jumping up and down falls in the hole, all you can see if anything is the top of Mr AC's head every 100 or 120 seconds popping up above the DC level. Apologies for the simple metaphor's, but hope they aid understanding.
Andy.
Both AC and DC current can flow in the same wire at the same time, when your first learning electronics this can blow your mind. I used to get very confused, a cathode is negative right, then how come it's connected to HT/B+ which is positive??? Think in terms of current flow, and it makes sense, see the symbol for a diode, see that arrow, it's showing you which way current can flow. The diode is a bit like a policeman, directing traffic, one hand waving traffic (electrons) forwards, the other hand raised, stopping traffic (electrons) from flowing the other way. His hand raised is the vertical stick the arrow is pointing at.... hope that makes sense and isn't patronising.
Going back to the schematic, the 5v AC from the heater/filament will be squashed by the big caps and the choke, the choke opposes AC current, the big caps act as big holes, any bumpy AC jumping up and down falls in the hole, all you can see if anything is the top of Mr AC's head every 100 or 120 seconds popping up above the DC level. Apologies for the simple metaphor's, but hope they aid understanding.
Andy.
Good explanation Andy. As to pos/neg, it helps to remember that something is always pos or neg with respect to something else. There's always two terminals involved. (That's why you need two wires to measure voltage!).
A cathode is neg with respect to the anode. B+ is pos with respect to ground.
So in this case the cathode being at 300V only means that the anode has to be even higher to satisfy cathode is neg wrt anode. And it pans out: you always loose some voltage across the tube because the anode has to be pos wrt the cathode to conduct, so anode needs to be above 300V.
Jan
Okay, thanks for taking the time to write that up Andy and Jan further clarified the point. I was confused by the fact that a vacuum tube diode CAN conduct fron anode to cathode. I'm still struggling with the notion of the filament withstanding 300V, but that's because I was thinking of a directly heated cathode more like a heater than a cathode. I'm coming around now.🙄 Now I'm wondering how the anode develops an electron cloud.
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The heater does not have to withstand 300V as such, the whole heater including the secondary winding 'floats' on 300V. It's the xformer that has to withstand 300V because parts of it (core) is at ground potential.
And conduction in a vacuum diode is exactly the same as in a triode, sans grid.
If you understand a triode, you understand them all.
Jan
And conduction in a vacuum diode is exactly the same as in a triode, sans grid.
If you understand a triode, you understand them all.
Jan
It doesn't. Usually. Any electrons near a positive anode are sucked right up. You are thinking of triode grid-cathode space.
Looking at the schematic, it appears the transformer secondary is wired to the anodes while the cathode is supplying B+. It stands to reason the cathode is energized by the anodes, but how does an anode become an emitter without a heater? I'm still having a hard time grasping this.
The heater is the cathode.
The heater windings (5V) float in respect to ground. The current supplied to cathode/heater may have 5V across it, but the entirety of it is at B+ potential.
The anodes are connected to the HV windings. The electrons flow between the anode and the cathode. Where they ‘emit’ from is more of a simplification than an actual thing, as it’s the extra electrons banging around the envelope and moving that actually matters.
The heater windings (5V) float in respect to ground. The current supplied to cathode/heater may have 5V across it, but the entirety of it is at B+ potential.
The anodes are connected to the HV windings. The electrons flow between the anode and the cathode. Where they ‘emit’ from is more of a simplification than an actual thing, as it’s the extra electrons banging around the envelope and moving that actually matters.
Apologies to sound negative. If you have a problem understanding something as basic as this I suggest you start looking for another hobby where you do not risk electrocuting yourself. It is not going to end well for you with your level of understanding.
This is a deep misunderstanding.
Which way do the ELECTRONS flow in the B+ line? Toward the rectifier.
Which way does the CURRENT flow in the B+ line? Away from the rectifier.
Why? Electrons are negative things. They show on the current meter as a negative number.
For more fun: for a hundred years textbooks taught Current Flow (and didn't say so). 30+ years back there was a fad to teach Electron Flow. Followed by release of textbooks in TWO editions.
So if you have a DIY education you may be reading opposing points of view and not know it.
Follow the blue arrows.
When you get a shock from the B+, it is not because electrons flow out of the rectifier. We see that any electrons to your hand "really" come out of the 6V6 or 6P1 power tube anode, in a hurry to get to the rectifier.
The nature of a thermionic cathode does not encourage any other explanation.
But this is compelling only in a vacuum tube. In a transistor (or a gas tube), BOTH + and - charges exist, and they usually go opposite ways at the same time. Now you may say the positive charges are rushing from the B+ wire inTO your finger.
When you get a shock from the B+, it is not because electrons flow out of the rectifier. We see that any electrons to your hand "really" come out of the 6V6 or 6P1 power tube anode, in a hurry to get to the rectifier.
The nature of a thermionic cathode does not encourage any other explanation.
But this is compelling only in a vacuum tube. In a transistor (or a gas tube), BOTH + and - charges exist, and they usually go opposite ways at the same time. Now you may say the positive charges are rushing from the B+ wire inTO your finger.
Thanks PRR - you've been most helpful. It's the contradiction of electron theory versus current flow that had my mind twisted with this.
This confusion is sometimes blamed on Ben Franklin, but really nobody knew that electrons existed until the very early 20th century. Current moved too fast to see, so could have been in either direction, for early experimenters. They picked one; it wasn't right for vacuum diodes, but could have been right for later semiconductors; can't change things now.
All good fortune,
Chris
All good fortune,
Chris
I recently saw this book being promoted here, and I can vouch that it is excellent in helping one get to grips with some of the concepts of thermionic emission and tubes in general. My problem has always been understanding plate resistance. I'm not all the way through the book, but I have a much better understanding of how chemistry, and the structure of the tube (spacings/alignments), dictate its behaviour.
Inside the vacuum tube
Inside the vacuum tube