21-09-2015, 04:06 PM
It's been noted that tubes conduct through free electrons which is indeed a factor of interest. This is perhaps the major big difference between transistors and tubes. As everybody here is aware, electron flow in semiconductors involves an electron jumping from atom to atom. It's not a cloud of free electrons as is the case in a tube (produced by heating a filament). This in itself poses many questions. For example:
In a tube, the alternating voltage on the grid, either speeds on or retards the electron flow. During the negative cycle, the cathode to anode flow will be retarded in velocity but speeded on positive cycles. Sometimes there is a fixed DC bias on the grid so current will always flow and sometimes the tube will be biased to cut-off. In the latter case, only positive cycles will have any effect. At any rate, electron velocity in a normal tube should be 0.001 Microsecond (so not a big problem when the cycles are at Kilohertz frequency.
I'm rambling a bit in the second paragraph but I wanted to point out that tubes need more voltage or power if you like than transistors. I noted a very simple point mentioned in a text book that tubes always lose more power coming in than they deliver coming out. Part of the power loss is thought to be due to the electrons that lag during negative grid cycles.
So, it's already been pointed out above that one way semi conductors can reduce the loss of velocity is the size of the transistor itself. So, anyone know what is the electron speed capacity of PNP AF117, for example? Do you think they are more efficient than tubes on high frequency?
I downloaded an article on Klystrons out of curiosity. It more or less said the Klystron takes advantage of electron velocity by causing the retarded or advanced electrons to bunch in cavities (hence "buncher"). The corresponding flow of electrons then amplifies the signal. I've never seen Klystron but am led to believe they're pretty big and seemingly expensive.
As an update today I put the potentiometer on the GEC radio and tested the resistance on the grid of the Double Diode Triode. This was because the weather was too poor for working on boats (and maybe also because I fell off my boat yesterday and am bruised somewhat). It now has a working switch and all is fine. I confess sometimes I have to force myself to do something on rainy days but I'm content the day's not been wasted. Ohms between neutral and live now also reads fine.
I am basically very lazy by nature and often enjoy delving into theory till the early hours of the morning. Then I watch a DVD.
In a tube, the alternating voltage on the grid, either speeds on or retards the electron flow. During the negative cycle, the cathode to anode flow will be retarded in velocity but speeded on positive cycles. Sometimes there is a fixed DC bias on the grid so current will always flow and sometimes the tube will be biased to cut-off. In the latter case, only positive cycles will have any effect. At any rate, electron velocity in a normal tube should be 0.001 Microsecond (so not a big problem when the cycles are at Kilohertz frequency.
I'm rambling a bit in the second paragraph but I wanted to point out that tubes need more voltage or power if you like than transistors. I noted a very simple point mentioned in a text book that tubes always lose more power coming in than they deliver coming out. Part of the power loss is thought to be due to the electrons that lag during negative grid cycles.
So, it's already been pointed out above that one way semi conductors can reduce the loss of velocity is the size of the transistor itself. So, anyone know what is the electron speed capacity of PNP AF117, for example? Do you think they are more efficient than tubes on high frequency?
I downloaded an article on Klystrons out of curiosity. It more or less said the Klystron takes advantage of electron velocity by causing the retarded or advanced electrons to bunch in cavities (hence "buncher"). The corresponding flow of electrons then amplifies the signal. I've never seen Klystron but am led to believe they're pretty big and seemingly expensive.
As an update today I put the potentiometer on the GEC radio and tested the resistance on the grid of the Double Diode Triode. This was because the weather was too poor for working on boats (and maybe also because I fell off my boat yesterday and am bruised somewhat). It now has a working switch and all is fine. I confess sometimes I have to force myself to do something on rainy days but I'm content the day's not been wasted. Ohms between neutral and live now also reads fine.
I am basically very lazy by nature and often enjoy delving into theory till the early hours of the morning. Then I watch a DVD.
