...afraid to ask!
 

#2 in my series.

Bias...what is it? In vacuum tube devices is it a DC voltage (positive or negative) on the grid that doesn't vary? What's its purpose? I hear "...biasing this tube providing..." Not sure I understand the reason for biasing a tube or a transistor or a diode etc.

https://en.wikipedia.org/wiki/Biasing

much info here, but I'm sure that others can go into it in way more detail :P

I read that but I still don't understand how or why you would use it in a circuit. What does it do for you? Are you forced to use it by the nature of the circuit? I'll stick with tubes...no transistors...the gaps in my understanding with those devices is vast.

All of this, well a lot of it was gone over in the vocational electronics classes I went through in HS, most of which has been forgotten over the years of course!
But a lot of what was tough did stick, like the VERY NSFW mnemonic for the resistor color code! :D

That's another intractable hurdle for me in electronics...I'm color blind so I have to ask my wife to give me the color bands! I'll stick to B&W TVs for now!

Think of a tube as a valve (the British term for it), because that's what it does, it valves or regulates electron flow. In a triode, the grid is the control element. Negative voltage on the grid (with respect to cathode) is the bias voltage. It
controls how much current flows from cathode to anode(plate). More negative equals less current, less negative equals more current. So you want the bias point to be fixed midway between minimum and maximum plate current swings, i.e., the idling current. When a small signal is applied to the grid, it "valves" a corresponding LARGE flow of plate current. The bias point being fixed midway gives the best linearity to the amplified signal (which appears across the plate load, which can be a resistor, an output transformer, old-school headphones, etc.).

Bias voltage on the grid determines how much current flows through the tubes. Make it too negative, nothing gets through. Too positive the tube will no saturated full on or clip.

Assuming were talking about an audio amp - the AC audio signal will ride on the DC bias voltage.

In other words the audio signal will push the grid voltage a little bit more negative or more positive than the bias. Set the bias wrong and the top or bottom will be clipped out distorted.

Tying this topic subject to your last in this series: because of a phenomenon known as grid rectification tying grid to ground or some DC bias source through a high-ish resistance (enough to bleed off the miniscule grid rectification current, but not enough to load down the AC signal too much) is important. If you leave the grid with no DC path electrons emitted from the cathode will accumulate on the grid and bias it positive until the tube redplates and fails or plate load resistance limits current. Also some waveshapes can create their own bias through grid rectification...In an audio amp that will still eventually lead to replacing, but in a TV horizontal circuit where the output tube gets a constant amplitude and waveshape osc signal at it's grid engineers frequently used grid rectification to bias the tube without need for another resistor. Grid leak detectors in 20s radios and some hidden DC restorers in some TVs work off of grid rectification.

“ If you leave the grid with no DC path electrons emitted from the cathode will accumulate on the grid and bias it positive until the tube redplates and fails or plate load resistance limits current.”

I must be missing something... wouldn’t the grid change negative if elections are accumulating on it? :scratch2:

jr

So is the bias on the grid determines how many electrons from the cathode get to the plate?

As an analogy, think of the slider in a slide valve.

That's what I assumed - that a floating grid would eventually bias itself off. Maybe there's some particular I was missin'.:saywhat:

Think of a tube being like a fire hydrant, (triode) and the valve on top is the grid, the valve can be opened anywhere from 0 to 100%, allowing a constant flow of water out of it depending on how far it's open, now ad a variable, like a sine wave that will try to adjust (modulate) at valve by 20% (how fast does matter), but if it's too far open, it will hit the upper limit before you adjust it BY that 20% , and it will clip off the top, same if it's too far closed, a functional medium has to be set so you do not hit upper/lower limits of what you are trying to put through it, and thus get amplified.
I know this is a crude description, :p I'm trying to remember how my electronics teacher told it many many years ago.

OK...that's kinda starting to make some sense to me. So the 20% part of the analogy for the sine wave, that's a 20% increase of signal strength? So the sine wave will grow in amplitude by more than that if the grid "valve" is opened more than 20% by the bias voltage "wrench" but if it does, the peaks of the wave will be lost (clipping)?

For example, if it was already open at 85%, and you tried to put in a signal that would adjust it by 30% or more, it would hit the upper limit and clip, these are again crude analogies, but it's basically how it is.

Again, this is why the grid needs to be biased at the midway/ idling/ 50% point, to prevent clipping and minimize distortion. So the tube can handle the highest signal amplitude consistent with good linearity (as in an audio stage). The raucious sound of 'heavy metal' music was spawned from deliberately running tube stages into clipping.

OK...since I'm a bioscience guy who needs explanations at the amino acid level, I'm trying to reduce what's going on with the grid to a physical state. Biased at 0%, a tube grid is a grid or screen of fine wires with no voltage, and hence, no free electrons present on it. A tube biased at 100% has a screen saturated with the maximum number of electrons present it can retain. I think I understand the cathode to plate flow part that's regulated by biasing of the grid but now, I'm looking to better understand the grid itself and what's going on with it when the tube is biased.

In most cases, the control grid is biased negative with respect to the cathode. "Free electrons" is a poor decriptor of what's going on. All metals have "free" electrons. A better term is a negative charge consisting of excess electrons.
Also, you would say "the tube is biased" at a certain current, not "the grid is biased at a certain percent."

In the case of a linear "class A" amplifier, the maximum possible current (which you called 100%) is determined by the power supply voltage and load resistor when the voltage across the tube (plate with respect to cathode) is zero. Then, the entire power supply voltage is applied to the load resistor with zero volts appearing across the tube. This may be a hypothetical case, because it may be impossible for the tube to draw that much current.

In general, a class A amplifier stage will be biased at a point where the quiescent bias current is halfway between zero and the maximum possible, so there is an equal range above and below the bias point for the output signal to vary up and down.

If someone posted posted a simple tube type audio amp schematic
tht would help the lesson.

BTW if you look at tube spec sheets there is a "cut-off" voltage listed
This is the point at which the tube turns off. If you go above that point
the tube starts to conduct.

Zeno:smoke:

One can think of 'charge' this way: an excess of surface electrons is a negative (-) charge, while a dearth of surface electrons is a positive (+) charge.

A tube's cathode expresses a negative charge, while the anode(plate) has a positive charge. Thus electrons boiled off a hot cathode are attracted strongly to the anode and accelerate toward it, as though the anode were a sump or 'sink' for the electrons.

This gal gives a pretty decent mini-tutorial on the history and development of the triode. https://www.youtube.com/watch?v=0Smj2nHo9zA

OMG...I think I'm starting to understand this!

:banana::banana: If you happened to catch her follow-on vid about regeneration https://www.youtube.com/watch?v=DRDajFYCrt0 at 7:42 she incorrectly shows the PHYSICAL location of the "Wing Circuit" coil (although it's correct circuit-wise). It is PHYSICALLY located either next to, or wound directly over the antenna coil's secondary. That's where the crucial feedback coupling occurs. Hence it's traditionally been called the 'Tickler Coil'.

I did not but it was very interesting to see that a streak on the inside of one of the original Edison light bulbs was the start of it all.

If memory serves me well, I think is something about the geometric distances and metal work funcion, so is limited to some potential, insufficient to cut-off most tubes, especially power tubes (= meltdown).