...afraid to ask!

[Chris K]

#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.

[Yamamaya42]

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

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

[Chris K]

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.

[Yamamaya42]

Originally Posted by Chris K

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!

[Chris K]

Originally Posted by Yamamaya42

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!

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!

[old_coot88]

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.).

[bandersen]

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.

[Electronic M]

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.

[jr_tech]

“ 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?

jr

[Chris K]

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

[old_coot88]

Originally Posted by Chris K

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.

[old_coot88]

Originally Posted by jr_tech

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

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

[Yamamaya42]

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.

[Chris K]

Originally Posted by Yamamaya42

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)?

[Yamamaya42]

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.