Where does plate resistance reside?

[pjd3]

Hey there,

This is one of those things i haven't fully come to understand. (I'm probably making more of it than it is).

Where does a vacuum tubes plate resistance reside? Is it simply the DCR of the plate material? Or, is it the space between the plate and other tube elements that constitutes the plate resistance?

So that's my big quandary, haven't slept in 20 years over this. But really, the inclusion of "space distance" between elements in a vacuum tube creates a veil in my ability to really get whats happening inside a tube.

Thanks for offering any insights and facts into the severe mental block o mine.

Best,
phil

[roberto]

There's a flow of electrons between the cathode and the anode due to the thermoionic effect and the potential difference.
The internal plate resistance is what limits the flow of electrons, so sets the current through the tube.

Simplifying a triode, your grid is the knob and the triode is the rheostat: change the status of the grid/knob, and more or less current will flow through the triode/rheostat.

Of course this is not something fixed, but changes with many parameters: working point, local feedback (think EG about pentodes in triode mode), etc...

[ChopSauce]

Hey there,

This is one of those things i haven't fully come to understand. (I'm probably making more of it than it is).

Where does a vacuum tubes plate resistance reside? ... is it the space between the plate and other tube elements that constitutes the plate resistance?

I'd say that. Not 100% sure what your question is, though - meaning: the distance might well be not much more significant than other factors influencing the electron flow within the tube, as discussed by Roberto.

[bepone]

it is dinamical proces, means "in action" when tube is working. when the tube is off, plate reistance doesnt exist (it is huge)
in plate curves it is slope of the curve, means Rp=delta U/ delta I..it is variable with the current, can be read from the plate curves (essential curves for the dedicated tube). how graph is not linear, slope is changing,
higher current, smaller Rp. smaller current = higher Rp

[martin manning]

Where does a vacuum tubes plate resistance reside? Is it simply the DCR of the plate material? Or, is it the space between the plate and other tube elements that constitutes the plate resistance?

It's in the space in between the plate and cathode.

Resistance is the energy (or work) required to move a unit of charge from one potential to another divided by the charge flow rate. The work here is in pulling electrons (charged particles) away from the cathode and through the electric field of the grid(s). An equivalent DC resistance would be plate voltage divided by plate current, or Va-k/Ia. For example, a 12AX7 at 250V Va-k and -2V Vg-k has plate current 1.2 mA, so its DC plate resistance would be V/A, 250/0.0012 or 208.3 kΩ. The plate resistance we are most interested in (to calculate gain for example), is AC plate resistance rp, which is the change in Va-k divided by the change in Ia at constant Vg, at a particular operating point. At the point above, rp is about 62.5 kΩ (as listed in the data sheet), and it is the inverse of the slope of the -2V Vg curve at 250V Va-k).

[roberto]

Resistance is the energy (or work) required to move a unit of charge from one potential to another divided by the charge flow rate.

Sorry Martin, I don't want to be pedant, but at the same time I prefer to specify for other people: resistance is not energy nor work.
We can consider it an energy converter (from electrical to thermal).

[martin manning]

Resistance is the energy (or work) required to move a unit of charge from one potential to another divided by the charge flow rate.

Sorry Martin, I don't want to be pedant, but at the same time I prefer to specify for other people: resistance is not energy nor work.
We can consider it an energy converter (from electrical to thermal).

Resistance is the energy (or work) required to move a unit of charge from one potential to another [Joule/Coulomb = Volt] divided by the charge flow rate [Coloumb/sec = A].

V/I = R, no?

[pjd3]

Thank you all for your feedback,

Your explanations and examples went a long way with coming to better grips with this and I've already started to get a handle on looking at the specific tube plots and learning how to generate load lines and calculating slope for the plate load resistance. This has already shed some light on the topic. There was always a question in my head regarding any material resistance in whatever formulation of metal was used for making the actual plate. Its good to be able to dispel that, finally.

I understood where Manning was coming from, I took it more as a wording thing than any blatant misunderstanding of energy transfer - that resistance is where the energy/work is necessarily manifested or carried out.

Thanks everyone, I sure appreciate you making this conversation.

Best,
Phil Donovan

[bepone]

..and I've already started to get a handle on looking at the specific tube plots and learning how to generate load lines and calculating slope for the plate load resistance.

it is very simple actually, it is tangenta on the plate curve, you can make deltaU/delta I on evert small part of the graph and you can undestand how the Rp changes

[roberto]

Resistance is the energy (or work) required to move a unit of charge from one potential to another [Joule/Coulomb = Volt] divided by the charge flow rate [Coloumb/sec = A].

V/I = R, no?

Joule is the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.

https://en.wikipedia.org/wiki/Joule#Definition

In physics, energy is the quantitative property that must be transferred to a body or physical system to perform work on the body, or to heat it. Energy is a conserved quantity;
Electric potential energy due to or stored in electric fields

https://en.wikipedia.org/wiki/Energy

Electrical energy is energy derived as a result of movement of electrons. When used loosely, electrical energy refers to energy that has been converted from electric potential energy. This energy is supplied by the combination of electric current and electric potential that is delivered by an electrical circuit (e.g., provided by an electric power utility). At the point that this electric potential energy has been converted to another type of energy, it ceases to be electric potential energy. Thus, all electrical energy is potential energy before it is delivered to the end-use. Once converted from potential energy, electrical energy can always be called another type of energy (heat, light, motion, etc.)

https://en.wikipedia.org/wiki/Electrical_energy

If resisistance was a sort of energy, you could supply everything with resistors instead of batteries (that store energy), and Ohm/s would be the measuring unit for Power.
It isn't.

[martin manning]

Roberto, I never said resistance was work, I said that it is the work required to move a change across a potential difference (J/C, which is Volts), divided by the charge flow rate (which is Amps, C/s). That's J-s/C^2, which is Ohms.

[roberto]

Hi martin, this way the discussion moves to the fact that Volt is not energy nor work. If you agree, I would stop it here.

I always read your interventions with attention, and usually these discussions lead to a boring “occupied” thread and two unhappy persons. I would like to avoid both things.

Thanks

[martin manning]

Hi martin, this way the discussion moves to the fact that Volt is not energy nor work. If you agree, I would stop it here.

Energy or work per unit of charge: https://en.wikipedia.org/wiki/Volt

[pompeiisneaks]

I think both of you have made pretty clear your thoughts on it. I am wondering if this might be a language barrier issue?

Everything I'm seeing both sides say, seems to be the same with somewhat subtle differences. I don't think we should belabor the definition of a Volt here anymore, I think one of the biggest things I might be seeing as the confusion is that a Volt is potential for work, once a voltage drops and current flows then work is done (wattage is the real work, not the voltage potential). Electrical work is most often a major heat producer, and most electric designs incorporate some means of dissipating excess heat. That is a byproduct of most circuits, (with the exception of say heaters which intentionally use it to their advantage) I think ultimately it's possible that we're all 'wrong' a bit on our understanding of this, unless we're all electrical engineers with dozens of years of practical work with electricity and electronics (I am not, I'm a software/systems engineer, and studied civil engineering for years before jumping ships, so I think I often act like i know more than I do )

At any rate, I concur with roberto's idea that the thread hijack be stalled out in favor of the initial request of where the plate resistance lives... No need for people to get angry... let's leave it at the point where there's obviously some fundamental part of the idea of voltage that is not as simple as many want it to be.

~Phil

[katopan]

It's already been said that it's in the vacuum between the plate and cathode, and is a function of the operating point of the valve. I just wanted to add - the fact that it changes with operating point is a hint that it's not just the plate material resistance (which doesn't change, and is relatively low).