Vacuum Tubes

[20849]

I am trying to understand vacuum tubes. I have read that they are rectifiers and amplifiers. Do some pass an AC signal thru
the tube?? I thought they were only for rectifying......is there a hobby kit that lets you set up tube circuits and gain an understanding of how they do the different things they do..??

 

[Minder]

The first detection of the vacuum tube effect was when it was discovered that when a light bulb was lit a cloud of electrons boiled off from the element, it was then discovered that if a plate was introduced and a +ve voltage WRT the element (cathode) was introduced to the plate, the stream of electrons were attracted across the vacuum creating a DC current through the lamp or what became as the first tube rectifier.
After that came the grid between the two in the current stream which then controlled the degree of current stream.
It was known as a trans-conductance amplifier, a varying voltage controlled the current
One of todays closest semiconductors to achieve a similar effect is the Mosfet.
M.
'

 

[20849]

so the higher the voltage the bigger amp effect..??

 

[Minder]

In a simple triode amplifier Cathode-Grid-Anode, the higher the grid voltage the higher the tube current.
So in this respect it is amplifying.
The vacuum tube demonstrates electron flow, negative to positive, which is in contrast to what is known as conventional flow in circuitry, of Positive to negative.
M.

 

[Ratch]

The first detection of the vacuum tube effect was when it was discovered that when a light bulb was lit a cloud of electrons boiled off from the element, it was then discovered that if a plate was introduced and a +ve voltage WRT the element (cathode) was introduced to the plate, the stream of electrons were attracted across the vacuum creating a DC current through the lamp or what became as the first tube rectifier.
After that came the grid between the two in the current stream which then controlled the degree of current stream.
It was known as a trans-conductance amplifier, a varying voltage controlled the current
One of todays closest semiconductors to achieve a similar effect is the Mosfet.
M.
'

Both FETs and BJTs function as a transconductance amplifiers.

In a simple triode amplifier Cathode-Grid-Anode, the higher the grid voltage the higher the tube current. So in this respect it is amplifying.The vacuum tube demonstrates electron flow, negative to positive, which is in contrast to what is known as conventional flow in circuitry, of Positive to negative.
M.

The charge carriers in vacuum tubes are electrons only (unipolar). The charge carriers in BJTs are both electrons and holes (bipolar). The charge carriers in FETs are either electrons or holes, but not both (unipolar). Conventional current is a mathematical protocol which assumes that all charges are positive and flow from positive to negative. The mathematical direction of flow may not agree with the real physical flow. Ammeters and semiconductor diodes are marked in accordance with the conventional mathematical protocol.

Ratch

 

[Audioguru]

A vacuum tube and a transistor do not pass AC, instead they pass DC that is modulated with an AC signal.

 

[(*steve*)]

I have the perfect answer that should answer the OP's every question.

"Valves are transconductance devices."

 

[elebish]

A vacuum tube and a transistor do not pass AC, instead they pass DC that is modulated with an AC signal.

Actually, a tube will generate a signal that looks just like the signal applied to the control grid. It does this by varying the current drop across the plate load resistance which is the output signal. All this is dependent of the plate voltage, load resistance, cathode potential and the grid current. Conduction occurs when the filament is heated. Think of a tube as a 'valve'. Remember, an open grid will allow the tube to run wide open.

 

[davenn]

Think of a tube as a 'valve'. Remember, an open grid will allow the tube to run wide open.

and there you have a rectifier

 

[Minder]

.... creating a DC current through the lamp or what became as the first tube rectifier.
....
It was known as a trans-conductance amplifier, a varying voltage controlled the current

One of todays closest semiconductors to achieve a similar effect is the Mosfet.
M.
'

ONE of today's ....
M.

 

[Ratch]

ONE of today's ....
M.

Both FETs and BJTs are transconductance devices.

Ratch

 

[BobK]

However, one of them draws a lot of current through the input, operates over a very limited range of input voltages and is far from an ideal transconductance device.

Bob

 

[Ratch]

However, one of them draws a lot of current through the input, operates over a very limited range of input voltages and is far from an ideal transconductance device.

Bob

Yes, and the filament is an energy hog.

Ratch

 

[BobK]

yep,nothing is perfect, but I think the JFET is the closest.

Bob

 

[(*steve*)]

Yes, and the filament is an energy hog.

Ratch

Had there been active development I'm sure this would have reduced. Some 1950's era valves had heaters which consumed in the low tens of mW. I wonder if there was ever research on "room temperature thermionic emitters"?

However I can't imagine an integrated tube with a billion devices in a single envelope connected to a board using BGA!

Edit: I want my vacuum tube iPhone.

 

[BobK]

I want my vacuum tube iPhone.

Me too. It would have a warmer, more expansive sound with greater depth and richness. Filaments are not a problem, my iphone is already warm enough to emit electrons when running some apps.

Bob