Elementary help in my study on valves needed

[caps]

Hi all :0)

Just a couple of things that are confusing me:

In one of the texts i have been studying, it states as follows:



" ...when a positive signal is applied, conduction through the tube will increase.The only trouble is the current through the cathode resistor will also increase. Voltage drop across the cathode resistor will increase. Remember at this time the plate is going NEGATIVE due to increased conduction through the tube. The combination of the negative going plate and the positive going cathode will decrease the electrostatic attraction across the tube and lower the conduction of the tube. This will reduce the gain of the tube."

This is obviously talking about effects of unbypassed cathode resistors.


My questions are:

If the cathode is going positive, and the plate negative, how does this decrease the conduction through the tube? Im thinking along the lines of opposite charges attract here. Im obviously missing something simple.


The other question is, Im unsure of the statement "...the plate is going positive at this time" or words to that effect. What does this actually mean? Is it simple referring to the fact that due to conduction, the plate is attraction negative charges from the cathode via the grid? Its being hit with negativer charges, hence its "going negative"?

Thank you very much.



:thumb:

 

[dabo]

The answer to your question about the plate absorbing electrons and that causing the "going negative" effect is yes.
The plate is at a much higher positive potential than the cathode will ever reach and the cathode will never go positive enough to approach the plate voltage (under normal circumstances).
Each approaching the other reduces the difference between them and thus the conduction. It's a built-in negative feedback effect of a cathode resistor biased tube stage.
A cathode bypass cap and lower power supply impedance will work to lessen this effect (if that's desirable).

 

[caps]

Thanks.

Not sure still that I grasp it though.

Is it that, even though the plate is "going negative" it is still at a positive potential due to the voltage applied to it in the first place?

Therefore, with a positive going cathode, and a plate with a positive potential, they repel electrons?

 

[PRR]

> "...negative going plate and the positive going cathode will decrease the electrostatic attraction across the tube and lower the conduction of the tube. This will reduce the gain of the tube."

This is hard to read. The author knows what he means, but has not explained it with infinite clarity (which is mighty hard to do).

> Im unsure of the statement "...the plate is going positive at this time" or words to that effect.

I don't see where it says that.

> Its being hit with negativer charges

"Negative charges"??? Dammit, get to know The Electron. Understand him. He is your friend. And he is unique to vacuum tubes, because while electrons are everywhere, it is only in vacuum tubes that they get to play unimpeded by other hunks of matter (ions, protrons).

But when you think electrons, remember that while Electrons DO flow from cathode to plate, "current" is a math abstraction and flows from plate to cathode. (The sign on current polarity was picked arbitrarily, before anybody saw an electron.)

> even though the plate is "going negative" it is still at a positive potential due to the voltage applied to it in the first place?

In any normal amplifier tube: the cathode is a few volts positive of the grid (which is usually near Ground), the Plate is always many volts positive of the grid and cathode. It is normally impossible for the plate to drop to the cathode voltage, or even close. Cathodes of small tubes live at +1V to +5V above grid, plates live at +50V to +300V. Typically the plate supply may be +250V, the plate idles at +150V, and swings to +100V and +200V to represent the audio signal. The steady +150V is blocked by a coupling cap, so the output idles at 0V and swings to -50V and +50V.

But the plate is always many volts positive of the cathode. How many volts determines how many electrons flow from cathode to plate (or how much current flows plate to cathode).

 

[caps]

Thanks PRR, I certainly need to be put in my place. :grin:

Its a real uphill battle understanding even the basics sometimes, especially when starting out.

So we have clearly established the plate, of course, is many times more positive than the cathode, as you point out thats easy to see.

So if we have the cathode going more positive, with a positive plate as stated, thats why the conduction decreases? simply like charges repelling?

Sorry to dumb it down so much, you gotta start out by asking the dumb questions sometimes . :grin:

 

[caps]

[quote author="PRR"]>

> Im unsure of the statement "...the plate is going positive at this time" or words to that effect.

I don't see where it says that.

[/quote]


Sorry, it does actually say exactly that, I just missed it when typing out the quote.

 

[dabo]

The conduction (current) decreases because there is less voltage difference between the cathode and anode.
It's Ohm's law.

 

[bcarso]

caps, what is the reference? There might be a better one :grin:

 

[PRR]

The more I re-read the passage you cited, the more I think you should look at some other text. What that says is not wrong, but when I try to clarify it even I get confused.

 

[caps]

[quote author="dabo"]The conduction (current) decreases because there is less voltage difference between the cathode and anode.
It's Ohm's law.[/quote]

Thanx dabo, Ill have to revisit ohms law.

PRR/bcarso,

The text comes from the NEETS navy Electrical engineering modules. So far its been well explained and clear for me.

I have other texts ill be moving onto soon, so I should come across another explanation I would think.

Thanks for the answers. :thumb:

 

[caps]

[quote author="dabo"]The conduction (current) decreases because there is less voltage difference between the cathode and anode.
It's Ohm's law.[/quote]

Thanx dabo, Ill have to revisit ohms law.

PRR/bcarso,

The text comes from the NEETS navy Electrical engineering modules. So far its been well explained and clear for me.

I have other texts ill be moving onto soon, so I should come across another explanation I would think.

Thanks for the answers. :thumb:

 

[bcarso]

Have they discussed the concept of incremental resistance/impedance, rather than just ohm's law?

If you move the plate voltage on a triode by a small amount, you will see a small change in plate current (all other things held equal). Take the ratio of the plate current change to the plate voltage change---that is the plate conductance at that operating point. It is emphatically NOT necessarily the same as the plate current divided by the plate voltage (or more properly the plate-to-cathode voltage).

I have to run right now but I'll be back.

 

[caps]

[quote author="bcarso"]Have they discussed the concept of incremental resistance/impedance, rather than just ohm's law?

If you move the plate voltage on a triode by a small amount, you will see a small change in plate current (all other things held equal). Take the ratio of the plate current change to the plate voltage change---that is the plate conductance at that operating point. It is emphatically NOT necessarily the same as the plate current divided by the plate voltage (or more properly the plate-to-cathode voltage).

I have to run right now but I'll be back.[/quote]


Yes, well they have in a rather simplistic way. However, I guess not understanding something initially makes me sit back and think it over hard, which I hope would be a good thing .

I dont think thy have touched on exactly what your getting at though bcarso. Maybe the only related thing to what your talking about is when they talk about calculating plate resistance and how that changes for a given plate voltage.

But do go on, lookin to learn....

:grin:

 

[PRR]

> Ill have to revisit ohms law.

You MUST know Ohm's Law, like you know how to fall off a log. Voltage goes down, what happens? You need to "feel" the answer.

http://www.john-a-harper.com/tubes201/ is a very good essay on tubes, but at a much higher level than the Navy manuals.

Pete Millett www.pmillett.com has something like "Our Friend the Electron" but his server isn't taking connections right now.

 

[PRR]

http://www.pmillett.com/tecnical_books_online.htm

http://www.tpub.com/content/neets/14178/

 

[caps]

[quote author="PRR"]> Ill have to revisit ohms law.

You MUST know Ohm's Law, like you know how to fall off a log. Voltage goes down, what happens? You need to "feel" the answer.

http://www.john-a-harper.com/tubes201/ is a very good essay on tubes, but at a much higher level than the Navy manuals.

Pete Millett www.pmillett.com has something like "Our Friend the Electron" but his server isn't taking connections right now.[/quote]


Tubes201 is a fantastic resource of information I havent seen in any other texts I have been studying. Just the ticket. Good one PRR, fills in alot of detaile the Navy stuff misses out. The two in combination cover a hell of alot I think. Thanks! :thumb: :thumb: :thumb: