Plate Current, Plate Voltage, Ohm's Law?

[ajb7074]

I've been reading post after post on all sorts of forums for the last 10 hours and I think I have a pretty good idea of what I need to know although I need some clarification.

In different sources plate current and plate voltage are talked about as though they are two totally different things. But if ohms law states that

voltage = current (amps) * resistance (ohms)

doesn't that mean that plate current and plate voltage are the same thing just measured differently?


I've done my research. I need some things cleared up still. :neutral:

 

[jazzguitar]

In different sources plate current and plate voltage are talked about as though they are two totally different things. But if ohms law states that

voltage = current (amps) * resistance (ohms)

doesn't that mean that plate current and plate voltage are the same thing just measured differently?

Nope. They are the same thing if measured at the same device, and only if that is a resistor (more precisely, if you know two, you can calculate the third).

Plate current is current through the vacuum of a tube, and the amount of current is controlled by the grid voltage and the screen voltage (it also depends on the plate voltage, but not as much). All these voltages are measured to ground, btw.

What is usually (and often without thinking) referred to as the "plate voltage" is the plate voltage with no signal, i.e. the idle voltage. With a signal (i.e. if the amp makes a sound) the plate voltage changes by dozens of volts if not more, but with little consequence for the plate current.


The plate voltage otoh, does change according to ohms law when the idle (bias) current is changed, but the resistor in this equation is the DC resistance of output transformer and internal resistance of the power supply, both of which ideally are zero ohms and in reality a few ohms, so if you double the idle current, the plate voltage does not go to half, or double, as your logic suggests, but only a few volts down.

 

[Wally]

http://www.valvewizard.co.uk/

 

[dsutton24]

...ohms law states that

voltage = current (amps) * resistance (ohms)

doesn't that mean that plate current and plate voltage are the same thing just measured differently?

No. That's quite a leap, very much in the Wile E Coyote mode.

Voltage is the difference in electromotive potential between two points. Current is the flow of electrons in a circuit.

 

[alnicopu]

No but I understand your confusion. If voltage rises, so does current. Current also rises when resistance goes down. Changing one affects at least one of the others. It takes a resistance to develop a measurable voltage. These are moving targets with active components like tubes and transistors. You should check and do your calculations again after each change. You will reach a point when you begin tweaking values to get what you want.

 

[clintj]

The term you may seek is impedance. This is the resistance to current change in an AC circuit. It's the yin to DC current's yang (resistance). It's also difficult to directly measure, because it depends on frequency sensitive things like capacitance and inductance.

The output transformer presents an impedance of several thousand ohms to the power tubes. So, when the power tubes are asked to conduct more current when signal is applied the transformer basically says "no" and ramps up voltage to oppose that change, and vice versa. That's impedance: the amount of "no" per unit of current change.

 

[Henry Mars]

You really need to understand trans-conductance ... Roughly gm = ip / vg. Remember that the tube is an active component. So the voltage drop across the plate will change when a series resistance is added in the plate circuit. Any changes in Ip will be reflected in the voltage drop across the load resistance. gm is a way of figuring out how much a change in grid voltage will vary the plate current and change the load voltage across the plate resistor. The internal resistance of the tube varies with plate current. At any given instant the sum of the voltage drops must equal the supply voltage ... if the current Ip changes the voltage distribution in the circuit changes, but the sum of those voltages is always equal to the supply voltage. That is very basic ... there are a lot of factors to consider especially at higher frequencies but that is basically how it works.

 

[peteb]

ajb,

Yes, by first inspection, by ohms law, v=ir, voltage and current rise and fall together. If the resistance is constant. Direct Relationship:v=i x constant

But, if you Change the bias in your amp, low current leads to high voltage, and high current means low voltage. Inverse relationship:v X I = constant, which is converse to ohms law, which shows that tube resistance does NOT stay constant.

 

[Henry Mars]

In the tube, the resistance drops as plate current increases because the increase in plate current causes a larger voltage drop across the FIXED plate resistor. According to Kirkoff's Law the algebraic sum of the voltage drops around any closed loop bust equal 0V. Which is just a technical way of saying that the voltage applied (B+) must equal the voltage dropped (Vk + Vp + VRL ). If you were able to freeze a sine wave (or any other wave form) at any point during it's transit thru 360 degrees this law must apply at that instant. Just a short tech course. There are a lot of people out there farting around with tube amps that have no clue about how any of this works and it is dangerous.

 

[peteb]

Henry, I'm not getting what you mean. I agree by looking at ohms law that resistance in a power tube decreases with an increase in current.

I don't see plate (load?) resisters at the plates of the power tubes on the typical 50s60s fender amps.

All I see in the high voltage 'loop'of any fixed bias amps is the power tube and the OT primary, ground> power tube>OT primary > rectifier.

I would like to understand why the tube resistance lowers with increased current, but I could also just accept it as fact.

 

[jazzguitar]

Of course there are no plate resistors at power tube plates, as the purpose of power tubes is to put out power, and a resistor just dissipates power (it will get warm).

So at the power tube plates there is the speaker instead of a resistor, and only because speakers need high currents at low voltages and tubes carry low currents at high voltages, the output transformer is connected in between.

Now, resistance is defined as the ratio of current change vs. voltage change, and for an Ohm's type resistor this is a constant at any voltage (before the resistor fries), and the ratio is given a figure in Ohms.

For a tube this does not make much sense, as the tube's current changes/determines the voltage (to a degree determined by the plate resistor or the power supply's internal resistance), and the current is determined mostly by the grid voltages as I stated in my above post.

However, in a mathematical sense, with a constant voltage source (such as the rectifier-filter cap node) at the plate, a change in tube current will result in a different "resistance value" of the tube.

Now, as the output transformer or the plate resistor is between the voltage source and the plate, the changed current will result in a changed plate voltage, the latter being the result, not the cause of the changed current.

In Ohm's law, the change in voltage is the cause of the change in current.

Hope this helped ....

 

[peteb]

Thanks Jazz, that makes sense at least I get that a tube is not a resistor and so it does not act like a resistor.

 

[tele_player]

Thanks Jazz, that makes sense at least I get that a tube is not a resistor and so it does not act like a resistor.

It acts like a resistor whose resistance value can be varied by varying the voltage between the grid and cathode.

 

[peteb]

The tube acts like a resistor in that if you change the bias, lowering the resistance, you get more current.

If it was a resistor, and you lowered it, the current would increase, same, but the voltage would stay the same or close.

but a tube, if you lower the resistance by changing the bias, the current increases, same, but the voltage across the tube will drop, unlike a resistor.

in an equation like V=IR, if nothing is held constant, the values are all interdependent on the other values and it gets kind of confusing, like what came first, the chicken or the egg.

 

[Henry Mars]

Of course there are no plate resistors at power tube plates, as the purpose of power tubes is to put out power, and a resistor just dissipates power (it will get warm).

So at the power tube plates there is the speaker instead of a resistor, and only because speakers need high currents at low voltages and tubes carry low currents at high voltages, the output transformer is connected in between.

Now, resistance is defined as the ratio of current change vs. voltage change, and for an Ohm's type resistor this is a constant at any voltage (before the resistor fries), and the ratio is given a figure in Ohms.

For a tube this does not make much sense, as the tube's current changes/determines the voltage (to a degree determined by the plate resistor or the power supply's internal resistance), and the current is determined mostly by the grid voltages as I stated in my above post.

However, in a mathematical sense, with a constant voltage source (such as the rectifier-filter cap node) at the plate, a change in tube current will result in a different "resistance value" of the tube.

Now, as the output transformer or the plate resistor is between the voltage source and the plate, the changed current will result in a changed plate voltage, the latter being the result, not the cause of the changed current.

In Ohm's law, the change in voltage is the cause of the change in current.

Hope this helped ....

FYI ... the Plate resistance is the DC resistance and Zt of the OT. ... I under stand the difference I was just trying to simplify an answer. The key word here is "active" component as opposed to passive.

 

[elpico]

The tube acts like a resistor in that if you change the bias, lowering the resistance, you get more current.

If it was a resistor, and you lowered it, the current would increase, same, but the voltage would stay the same or close.

but a tube, if you lower the resistance by changing the bias, the current increases, same, but the voltage across the tube will drop, unlike a resistor.

in an equation like V=IR, if nothing is held constant, the values are all interdependent on the other values and it gets kind of confusing, like what came first, the chicken or the egg.

You're a little mixed up there. A tube acts exactly like an adjustable resistor. You change it's resistance with a control signal on it's grid. It doesn't "change the voltage across it" or anything special, you seem to be thinking of a tube with a resistor between it and the power supply. If you look at just a tube by itself and connect the plate straight to the power supply you'd see that the current through it increases and decreases as you adjust the control signal on it's grid, but the voltage across it does not change. Exactly what you'd expect from an adjustable resistor.

When you add a resistor between the tube and the power supply you've formed a voltage divider. Voltage dividers share the voltage across them according to the resistance of each section. When the tube (aka adjustable resistor) is adjusted to have the same resistance as the load resistor they will share the voltage equally. So if we have a 300v supply it will be divided up equally into 150v across the resistor and 150v across the tube. If we adjust the resistance of the tube to a lower value then the voltage sharing will change, more of the 300v will be across the load resistor and less will be across the tube, but it's the voltage divider formed by that load resistor that's causing the voltage across the tube to change - not a property of the tube itself.

 

[peteb]

that makes sense, that the OT is the plate resistor, and that it's voltage goes up with increased current like expected with ohms law, and also that the reduced resistance in the tube gets less voltage per Kirchoff's law.

thank you all three of you


it turns out it was simpler than I thought