I can usually cope with the complex stuff, it's the basics that stump me 😱 I'm trying to understand simple valve pre amp design and found this example which is very useful.
I've also looked at the transfer characteristic graph for the 12AX7 to understand the bias point currents. Thing that I don't get is: what is the plate voltage here? Is it 250V or 150V for the purposes of using the transfer graph?
I think that it's 250V, as 150V would put the Q point on the curvy bit of the -1.5V transfer curve and that would cause more distortion than necessary. Am I right?
I've also looked at the transfer characteristic graph for the 12AX7 to understand the bias point currents. Thing that I don't get is: what is the plate voltage here? Is it 250V or 150V for the purposes of using the transfer graph?
I think that it's 250V, as 150V would put the Q point on the curvy bit of the -1.5V transfer curve and that would cause more distortion than necessary. Am I right?
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The plate voltage is actually the DC voltage difference between the plate and cathode,
as measured by connecting the leads of your meter to those two nodes. The power supply
has to be more than that amount, to allow the tube to generate an output voltage.
Here's a good reference: http://www.tubebooks.org/tubedata/RC30.pdf
Tubes are often biased on a more nonlinear part of their transfer curve, for practical reasons like
lowering power dissipation, improving input overload, etc. An unbypassed cathode resistor can
reduce this nonlinearity somewhat, by using feedback.
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Ah ha. So anode supply and plate voltage are at different points in the circuit? So that means that the tube is biased at the lower end of the -1.5V transfer curve in the attachment. I would have instinctively put it higher up. Why would you want to do that? I would have thought that people would sacrifice a few mA in a pre amp for better linearity any day. If you distort here, you presumably can't clean it up in the later power stages.
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There are more considerations and constraints involved. Here's another good reference for you at this point:
See p. 44 here: http://www.tubebooks.org/Books/Preisman_graph.pdf
You can plot the load resistor from the 250V, 0 current graph point slanting upward to the left,
to the 250V/220k current, 0 voltage graph point. Where this load line intersects the -1.5V line
is the approximate DC Q point of the tube. This works because the Vpk plus the Vload is constant,
and equal to the power supply voltage.
An unbypassed Rk will alter the AC operation of the graph, since its presence alters the AC Vgk voltage.
There is another process for plotting modified curves for adding in the AC effects of an unbypassed Rk.
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The Americans call the Anode the Plate, so the Anode voltage is the Plate voltage. They also call Valves Tubes. Silly really because everyone else knows that a Tube has always been what produces a picture in a Television; CRT. I don't think , in my 50+ years of servicing and reading service manuals from all makes, that any manufacturer adopted Tubes for Valves until recent times.
Anode supply usually refers to the B+. The load resistor drops some voltage so the absolute voltage at the anode will be lower than the anode supply by that amount. As Rayma said the anode or plate voltage in data sheets refers to the difference between anode and cathode voltage.
mike
mike
The phrases seem to be interchangeable and synonymous. rayma sent me a link to a RCA Receiving tube manual. It lists thermionic valves as tubes, and cathode ray tubes as tubes. Indeed, it's sub titled "Including picture tubes and industrial receiving tubes". It's copyrighted 1975. I think that they're called valves though as you have to go with what the inventors called them. There is an attachment from Fleming's obituary.
It all adds to my confusion just as B+, Eo , plate voltage and anode voltage do 😕
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We call them valves (with anodes) in the UK. Other English-speaking people call them tubes (with plates). I believe some non-English speaking use words which translate as 'pipes'?
B+ is left over from times when batteries were used for the power, called A, B, C.
E was first used for electromotive force (voltage), but due to confusion with
the E for electric field, then V for voltage became the standard terminology.
When a particular node's potential is called a voltage, it is assumed to be
referred to common, or ground. (That's another story, too.)
You are kidding, right? Did you ever read anything produced by GE? Did you ever stumble across O.H. Schade's much-referenced paper on the 6L6 entitled "Beam Power Tubes?"
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In the early days it was called the "Audion" of course, but the term "vacuum tube" has been in use at least since Hendrik Johannes van der Bijl published his book "The Thermionic Vacuum Tube and its Applications" in 1920.
This is certainly the case in Sweden where it is called "elektronrör", "radiorör" or simply "rör". The original meaning of "rör" is "pipe".
Lamp in Russia. Strictly, "Receptive - amplifying lamp", or "Generator lamp", when a big one. And Anode, of course.
No pipes, nor tubes.
In the UK we have HT (high tension) instead of B+. Also (for battery valves) LT for the filament voltage. We distinguish between filaments (for directly heated valves) and heaters (for indirectly-heated valves).
Except when it is referred to the cathode.rayma said:
I would be interested to know who the first people were to use each term and what the history of the split in terminology is. I did find an article last night written in 1930 by Lee DeForest entitled "Evolution of the Vacuum Tube." By the look of all of the articles in the same magazine, "Tube" was definitely the cemented American term by that date.
I then looked back at DeForest's patents and there is no trace of the term or the term 'valve', just "Audion" or detector made from an evacuated vessel or terms like that. I'd like to know who coined the term 'thermionic valve' if anyone knows.
I then looked back at DeForest's patents and there is no trace of the term or the term 'valve', just "Audion" or detector made from an evacuated vessel or terms like that. I'd like to know who coined the term 'thermionic valve' if anyone knows.
These early devices were in fact made from standard lab glass tubes (hence the name), closed at both ends.
https://en.wikipedia.org/wiki/Audion
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