Greetings everyone. I'm new to this forum, so please cut me some husk if I screw up. I'm currently learning about antique tube radios and ran across this book titled: Old Time Radios!Restoration and Repair-by Joseph J. Carr. On the workings of vacuum tubes, on the top of page 48,he makes referance to:quasistatic dc and ac process. He is explaininglate resistence and plate impedance. The paragraph reads as followslate resistance and plate impedance are related terms, but are differant in that one is a quasistatic dc process and the other is an ac process. Plate resistance is defined as: Rp=Vb/Ip. Rp=plate resistance. Vb=plate voltage-dc. Ip=plate current-dc. The second paragragh reads: Plate impedance,on the other hand, is defined in terms of alternating current, so it is a dynamic measure of the same thing. rp=Δvb/Δip(Vc=constant). I forgot. He also gives a formula for grid voltage necessary to effect cutoff: --Vco=Vb/μ. My question is:What is a quasistatic dc and ac process?The only thing I can find on the internet just deals with physics explanations such as refering to entropy and equilibreum of things like cylinder pressure. Any help with this inquiry will be greatly appreciated.

 

By quasistatic ac and dc process (I've never heard the term before) I think he means that the tube has a static DC bias voltage and current, and a small AC signal term riding on top of that.
But that's just my guess.

 

Ohm's Law: the voltage across a resistor equals the current through the resistor times the value of the resistor in ohms. E = I x R

If you turn that around, R = E / I. So anywhere you see a DC voltage across an electronic component, such as the forward voltage of a conducting diode, or the voltage between the collector and emitter of a transistor, if you know the current through the device then you can use Ohm's Law to calculate it's "equivalent" resistance. Of course, it isn't a real resistor, but under those specific circumstances it is emulating the characteristics of one. This is actually used in RF and video circuit design as diode switching circuits.

Good book. Carr is basically the Carl Sagan of RF circuit design.

ak

 

By quasistatic ac and dc process (I've never heard the term before) I think he means that the tube has a static DC bias voltage and current, and a small AC signal term riding on top of that.
Crut

By quasistatic ac and dc process (I've never heard the term before) I think he means that the tube has a static DC bias voltage and current, and a small AC signal term riding on top of that.
But that's just my guess.

Ohm's Law: the voltage across a resistor equals the current through the resistor times the value of the resistor in ohms. E = I x R

If you turn that around, R = E / I. So anywhere you see a DC voltage across an electronic component, such as the forward voltage of a conducting diode, or the voltage between the collector and emitter of a transistor, if you know the current through the device then you can use Ohm's Law to calculate it's "equivalent" resistance. Of course, it isn't a real resistor, but under those specific circumstances it is emulating the characteristics of one. This is actually used in RF and video circuit design as diode switching circuits.

Good book. Carr is basically the Carl Sagan of RF circuit design.

ak

Thank you Crutschow and Analogkid for the quick responces. Both of your answers answered my question perfectly. Questions like this is why I try to buy books by differant authors of the same topic. I might not understand author number one, but author number two makes sence. Analogkid. Thanks for the info on Joseph J. Carr. I'll try to buy more books by him now I know of his stature of tube radios. Again. Thank you guys very much for your answers.

 

Greetings everyone. I'm new to this forum, so please cut me some husk if I screw up. I'm currently learning about antique tube radios and ran across this book titled: Old Time Radios!Restoration and Repair-by Joseph J. Carr. On the workings of vacuum tubes, on the top of page 48,he makes referance to:quasistatic dc and ac process. He is explaininglate resistence and plate impedance. The paragraph reads as followslate resistance and plate impedance are related terms, but are differant in that one is a quasistatic dc process and the other is an ac process. Plate resistance is defined as: Rp=Vb/Ip. Rp=plate resistance. Vb=plate voltage-dc. Ip=plate current-dc. The second paragragh reads: Plate impedance,on the other hand, is defined in terms of alternating current, so it is a dynamic measure of the same thing. rp=Δvb/Δip(Vc=constant). I forgot. He also gives a formula for grid voltage necessary to effect cutoff: --Vco=Vb/μ. My question is:What is a quasistatic dc and ac process?The only thing I can find on the internet just deals with physics explanations such as refering to entropy and equilibreum of things like cylinder pressure. Any help with this inquiry will be greatly appreciated.

Quasistatic simply means that things are changing slowly enough that you can use equilibrium (static) measurements to work out what happens.

For example, you can measure a curve of plate current vs grid voltage by changing the grid voltage and recording the plate current, say you get I(Vg). This is a static measurement.

If you apply a changing voltage to the grid, say v sin(wt), then the plate current will be I(v sin(wt)) as long as the quasistatic assumption holds. For audio work with valves the quasistatic assumption is fine, you start to get problems when the grid/anode voltages change significantly whilst the electrons are moving between them. I know little about these limits, it's been a long time since I had to know anything about this!