Plate resistors on preamps

[bluefoxicy]

What does a plate resistor on B+ on the preamp do? On a 12AX7 for example, power comes from the power supply to the anode of the 12AX7. I have heard raising the resistor increases gain; this makes no sense to me, wouldn't raising the resistor make the anode more negative and thus closer to the cathode charge, lowering gain?

[jrfrond]

In the typical grounded-cathode stage with a plate and cathode resistor, the plate resistor is the plate load. If it wasn't there, the tube would not amplify and would become a cathode-follower (grounded-plate amp). The signal needs a load to drop voltage across to vary the plate current. The higher the value of the resistor, the higher the voltage gain.

[hasserl]

The tube amplifies current, not voltage. In order to get an amplification of voltage you need some resistance; per ohms law, I * R = E. Increase the R and you also increase the E, or vice versa.

[6267]

The way I see it, which probably isn't the EE way, is that the tube draws current through the resistor which produces a voltage. A bigger resistor will allow for a bigger voltage up to a point. You are correct in that the B+ voltage at the top of the resistor will be higher than at the anode. Generally, you might have 250-350 B+ at the top of the anode resistor and something more like 100-200 between anode and cathode. It's function is not to limit voltage, but to produce a variable voltage drop depending on the current draw of the tube, which in an amp is related to the voltage of the signal at the grid. This variable voltage drop is 'communicated' through the capacitor (which blocks the B+, but passes the AC voltage drop pattern of the resistor). You just want to have a high enough B+ that the resistor doesn't limit the plate voltage too much.

[hasserl]

I don't think that is very accurate. The "function" (purpose?) of the resistor is NOT to produce a variable voltage drop depending on the current draw of the tube, It is to produce a voltage gain of the signal. Without the resistor the tube would still produce an increased current as a result of the signal at the grid, but there would be no gain. In order to have gain there has to be resistance, that is the purpose, or function, of the plate load resistor.

[Bruce / Mission Amps]

I don't think that is very accurate. The "function" (purpose?) of the resistor is NOT to produce a variable voltage drop depending on the current draw of the tube, It is to produce a voltage gain of the signal. Without the resistor the tube would still produce an increased current as a result of the signal at the grid, but there would be no gain. In order to have gain there has to be resistance, that is the purpose, or function, of the plate load resistor.

Kind of nebulous... here's my take on it
The variable DC voltage drop across the plate load resistor and tube is called "AC"... AC by the pure definition of what AC is ... the AC in this case is audio.
There really is no such thing as "voltage gain" as the variable voltage across the tube is merely a higher voltage duplication in AC of what is happening on the grid, I don't think there is anything gained as that implies an additive component and we all know that any AC appearing on the grid, really goes nowhere but to ground through the grid load resistor or equiv.
However, the extremely low impedance of the coupling cap connected to the plate (with respect to the plate load resistor and tube impedance) allows that AC appearing at the plate to be transfered somewhere else.

[Enzo]

Which points out the difference between current and signal.

There is no flow of current from the grid to the other elements, but the signal voltage at the grid controls the current through the tube, and that current through the plate load resistor causes a voltage drop across it that varies with the signal. The gain is to the signal. That signal can be sampled off through the coupling cap.

[kg]

being the "R," it provides the load in an RC coupled amp, plain and simple.

[hasserl]

Try this, remove the plate load resistor and what do you get? The tube will still work. A signal applied to the grid will still cause an increase in current through the tube, but what good will that do for voltage amplification? That is not amplification, that is conversion (per the NEETS manual).

There is a variable DC voltage drop across the load resistor, it is a result of the load resistor, but it is not the purpose of the resistor. There is also a variable AC voltage drop across the resistor. A big voltage drop (down to zero). Apply a sine wave to the grid of the tube and measure the AC voltage on either side of the load resistor. You'll find AC on the anode side of the resistor that is an amplified version of what was applied to the grid (that is voltage gain), on the power rail side the only AC you'll find is unfiltered ripple from the power supply. That's the voltage drop that is important. Because without it you've got nothing. Take the resistor out and that's what you get, nothing.

[Steve Conner]

I don't think there is anything gained as that implies an additive component

At the risk of sounding like a real dweeb, gain doesn't imply adding things, as it's defined as a multiplying factor. It's signal out divided by signal in. If you put 7 volts of signal in and got 42 volts out, your stage had a gain of 6.

[CitizenCain]

What does a plate resistor on B+ on the preamp do? On a 12AX7 for example, power comes from the power supply to the anode of the 12AX7. I have heard raising the resistor increases gain; this makes no sense to me, wouldn't raising the resistor make the anode more negative and thus closer to the cathode charge, lowering gain?

Here's a neat link I ran across describing what each part of the preamp circuit contributes and how to change it for more/less of something.

http://www.regiscoyne.com/tech/preamp_mods

[carlg]

Maybe this has been answered to death already, but nobody mentioned a tube is a transconductance device. That's the nerd-speak way of saying that it converts a voltage on the control grid into a current through the tube. It's all a tube does, converts a control voltage to a plate current according to some ratio set by the design of the tube. This control voltage can stay the same or be dancing around, so the current the tube spits out can be either a constant current or a varying one. Now some varying current might be useful for some other job, but in the preamp all we care about is getting a higher voltage signal to send to the power amp. To turn a varying current into a varying voltage you need a load. The bigger the load the bigger the voltage produced across it by the same amount of current. It doesn't have to be a resistor, it could be a coil of some kind, a transformer etc.

In your example if you made the load resistor large without adjusting the bias resistor or increasing the supply voltage the no signal plate voltage would indeed drop, but less plate voltage doesn't mean less gain. You still have higher gain because the same varying current is now running through a higher resistance. Reducing that plate voltage mainly reduces your headroom, assuming the stage was biased right in the first place.

[hasserl]

nobody mentioned a tube is a transconductance device. That's the nerd-speak way of saying that it converts a voltage on the control grid into a current through the tube. It's all a tube does, converts a control voltage to a plate current according to some ratio set by the design of the tube. This control voltage can stay the same or be dancing around, so the current the tube spits out can be either a constant current or a varying one. Now some varying current might be useful for some other job, but in the preamp all we care about is getting a higher voltage signal to send to the power amp. To turn a varying current into a varying voltage you need a load. The bigger the load the bigger the voltage produced across it by the same amount of current. It doesn't have to be a resistor, it could be a coil of some kind, a transformer etc.

Seems like that's what I said:

The tube amplifies current, not voltage. In order to get an amplification of voltage you need some resistance;

Without the resistor the tube would still produce an increased current as a result of the signal at the grid, but there would be no gain. In order to have gain there has to be resistance,

[carlg]

I worried my reply might annoy people because the question had been approximately answered in the first reply by jrfrond, sorry about that. Unfortunately some of the explanations were inaccurate and both of the quotes you selected there are good examples like: "The tube amplifies current, not voltage". A tube has an extremely high input impedance so you're not putting a current into it for amplification like a BJT. Only the voltage on the control grid matters, but this doesn't control another voltage it controls a current. I thought maybe this point was being overlooked, a relationship where a voltage controls a separate current is called transconductance and this is all a tube does. It doesn't amplify a current or a voltage. It can be used as part of a circuit to do those things, but that's determined by the circuit not the tube.

And the second quote about no gain without a plate load, well I've been through that argument here years ago and don't want to start it again. People who work mostly with these high input impedance devices in voltage amplification circuits tend to think the word "gain" can only mean voltage gain. I'm nobody to tell you it aint so, build in peace.

[Steve Conner]

Well, triode tubes aren't quite transconductance devices, either. The electric field of the plate causes a kind of negative feedback: the plate voltage affects the space charge the same way that the control grid voltage does.

So a triode looks like a current source (the transconductance part) shunted by an internal resistance which we call the plate resistance, rp. For gain calculations, you have to consider rp as being in parallel with whatever plate load resistor you use. This puts a fundamental limit on the gain of a triode, no matter how large a plate load resistor you use. This is why mu is a popular figure of merit for triodes: it's gm * rp.

http://www.guitarstudio.tv/Splawn/ex..._must_read.htm

With pentodes, the plate is isolated from the control grid by the screen grid. So rp is almost infinite, hence pentodes have higher gain.