Rob,
I think analogies to a water pipe to describe vacuum tube conduction
can be helpful in conveying very simple ideas about current and
voltage, but the parallel shouldn’t be pushed too far. Vacuum tube
conduction is inherently non-linear, which means that the plate
current is not linearly proportional to the plate voltage. In fact,
for tetrodes especially and triodes to a lesser degree, plate current
is roughly independent of plate voltage over a wide range of
conduction. The degree of independence can be seen by inspecting the
constant-current graphs of plate current vs plate voltage for any
given tube type. (For semiconductors, especially MOSFETS, the drain
current curves are nearly flat, meaning that drain current is pretty
much unaffected by drain-source voltage.)
I also respectfully suggest that it is best to avoid trying to explain
conduction dynamics in terms of electrons “boiling” off the cathode
and being drawn to the plate. It’s not that this view is wrong, but it
can muddy the waters because it suggests that current flow in a vacuum
tube is from cathode to plate, whereas in truth plate current always
flows from plate to cathode. The Brits have the right idea when they
call tubes “valves” and it’s too bad us Yanks didn’t follow their
lead.
73,
Jim W8ZR
Sent from my iPad
On Nov 1, 2019, at 3:56 AM, Rob Atkinson <ranchorobbo@gmail.com> wrote:
>> So you are not changing the bias, even though resting current has changed,
>> but
>> you are changing the conduction angle and thus the class of operation
>> (triode).
>> Question: How can you change the class of operation and conduction angle with
>> no change to bias. I apologize, but I am confused.
>
> You got a lot of answers, a lot of words, some or all correct, but not
> what I think you were looking for.
>
> A tube (triode for example) is like a ball valve in a water pipe. You
> can change the flow by cutting it almost off with the valve, but you
> can increase the flow by increasing the pressure into it or the
> suction pulling water out of it. A tube (called a "valve" in Britain
> not coincidentally) has all kinds of ways of changing its behavior
> besides bias. Increasing the plate v. alone is like increasing the
> suction on the water coming out of the valve. Higher positive v.
> exerts an even stronger attractive force on the negatively charged
> electrons boiling off the cathode, in spite of the grid v. This
> pulls more of them to the anode past the grid and the anode to cathode
> current increases.
>
> 73
> Rob
> K5UJ
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