How does a tone stack impact gain? Impedance matching content inside.

peteb

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consider the blackface/Silverface tone stack.

the tone stack is known for allowing signal to go to ground. This certainly will decrease gain.

isn’t another reason the blackface tone stack cuts gain is, that regardless of tone Control settings, the tone stack puts resistors to ground in parallel with the load impedance. Cutting the load impedance cuts the gain of the preceding gain stage.

it must be both factors together that cause a loss of gain.
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Greggorios

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Rob Robinette's site has a lot of detailed information on it. Amp builders like Allen, Gries and Dr. Z, to name a few, offer controls to "blend out" the typical blackface/silverface tone stack.
 

andrewRneumann

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You are correct in your analysis, I think, but you leave out the effect of the source impedance driving the tone stack.

If you are driving a 100K load with an cathode follower with 1K source resistance, you won’t get much signal loss even if you drop the load resistance to 50K (-0.1dB). The cathode follower just pumps more current to keep the voltage up. (Until the cathode follower clips…which is the main problem using a 12AX7 as a cathode follower for driving heavy loads.) Considered in isolation from the source of the signal that is driving it, it is hard to say what the tone stack will do to net gain.
 

tubedude

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You are correct in your analysis, I think, but you leave out the effect of the source impedance driving the tone stack.

If you are driving a 100K load with an cathode follower with 1K source resistance, you won’t get much signal loss even if you drop the load resistance to 50K (-0.1dB). The cathode follower just pumps more current to keep the voltage up. (Until the cathode follower clips…which is the main problem using a 12AX7 as a cathode follower for driving heavy loads.) Considered in isolation from the source of the signal that is driving it, it is hard to say what the tone stack will do to net gain.
Generally true, but the OP's example is plate loaded driven tone stack.
The plate resistor and load resistor in parallel, plus the 15k thru the 47nF cap etc. drop the load impedance dramatically. A switch (pot mounted) to interrupt the tone control circuit ground will defeat the tone circuit and get back some gain.
 
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peteb

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I have two questions.
how does placing the tone stack affect the gain and why?
how do the tone controls affect gain and why?

Let’s start with the 5F1 champ. A very baseline pre-amp.

I see v1 having a 100k plate load and a 1M output load.
There is no requirement for how big the output load is supposed to be relative to the plate load.

is the output load 1M for all settings of the volume control? I see the output resistance defined as the grid resistance of the following stage. The resistance of the volume pot and the grid resistance are not the same thing. Is the load impedance one meg for all settings of the volume control?

Aiken says this:

Resistor Rl is the load resistor. It usually is also the grid resistor of the next stage. It controls the midband gain of the amplifier, because, for AC signals, the effective plate resistance is the parallel combination of Rp and Rl. If Rl is made at least ten times larger than Rp, it can usually be ignored for gain calculations.




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elpico

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Let’s start with the 5F1 champ. A very baseline pre-amp.

I see v1 having a 100k plate load and a 1M output load.
There is no requirement for how big the output load is supposed to be relative to the plate load.

is the output load 1M for all settings of the volume control?

It's the same for all settings if the signal is small enough to stay clean. If the signal grows large enough to overdrive the second stage the control setting could have an effect on the load the 1st stage sees. You could think of "9" on the dial as the second stage having a grid stopper and "10" as no grid stopper.
 

peteb

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Thank you Elpico,

I think we have agreement that the load impedance is similar to but not quite the same thing as the grid to ground resistance. In contrast, the input impedance is the entire resistance the signal overcomes to reach ground as it gets to the tube. The exact Level of the signal fed to the grid and the resistance to ground at that point is not important. The load impedance of v2 on v1 on the 5F1 is always One Meg ohms regardless of volume pot setting.

Next example and next question
 

peteb

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On the 5E3, consider the volume control on the ‘lower channel’ and disregard the interaction with the upper channel.

The important difference seen on the 5E3 volume control is the signal enters the pot on the wiper as opposed to exiting the pot on the wiper as on the 5F1.

can we agree that the 5E3 volume control is not a voltage divider like the 5F1 volume control but rather a reducer of the load impedance for v1, reducing the gain of the first gain stage, reducing the amount of signal coming out of v1 rather than cutting the signal going to v2? Are we all on the same page here before going forward?

any and all input is appreciated.

thanks


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andrewRneumann

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The 5E3 is a perfect example demonstrating why we need to know the output impedance of the source doing the driving. The only reason lowering the impedance seen by the valve actually drops signal is because the valve and it’s load resistor have their own resistance too. If this type of volume control was driven by a cathode follower or other low impedance drive, it would limit voltage drop even as the knob was turned substantially.

Here’s a sketch showing that this is still a regular voltage divider, but just not as obvious as the typical volume pot.

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elpico

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The exact Level of the signal fed to the grid and the resistance to ground at that point is not important.

It is important. In the champ the load on V1 is the combined (parallel) impedance of it's 100k plate resistor, the 1meg volume pot, and the input impedance of the next tube. At low signal levels the input impedance of the second stage is very high, many mega ohms, but as soon as you overdrive it the input impedance plummets from mega ohms to only a couple kilo ohms. That's a much more significant load than either the volume pot or the plate resistor so can not be ignored.

In the real world a champ turned to 10 is almost certainly going to be overdriving the second stage so the load on the V1 is NOT 100k||1meg regardless of signal level.

The load impedance of v2 on v1 on the 5F1 is always One Meg ohms regardless of volume pot setting.

Like I said at high volume settings the second stage is going to be loading the first and in that situation the exact volume pot setting does matter because settings less than 10 insert some series resistance between V1 and V2. That isolates V1 somewhat from the low input impedance of V2. In other words, the load on V1 is lighter at volume setting 9 than volume setting 10.

You can choose to ignore overdriven playing I guess and discuss only how the circuit works when playing clean. You could do this for example by setting the champ volume to 10 and the guitar volume to 1.
 

andrewRneumann

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Yes, overdrive is a special case. And it complicated because grid current only occurs at the tops of the signal swings, so the impedance is changing “mid swing”. The load line of the driving stage is bent.

IMHO, we should stick with non-overdrive conditions to simplify the discussion and focus on first principles.
 

Bendyha

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Yes, overdrive is a special case. And it complicated because grid current only occurs at the tops of the signal swings, so the impedance is changing “mid swing”. The load line of the driving stage is bent.

IMHO, we should stick with non-overdrive conditions to simplify the discussion and focus on first principles.
Yes, but, with a circuit as fundamental as the Champ, and its function so iconic, from the cleaner lower settings, up to the idiosyncratic dimed hoot, not talking about the interesting bit is ignoring the best bit.
"How does a tone stack impact gain? Impedance matching content inside." was the all embracing theme, so it would be a shame to limit the discussion to linear Hi-Fi conditions here on a guitar amp oriented sub-forum.

"How does a tone stack impact gain? Impedance matching content inside."(assuming we are relating to a passive tone circuit) It introduces a negative gain due to component reactance in specified frequency bands with partially selectable parameters, through variable resistance interacting with set capacitance to effect circuit impedance.
For a slightly deeper delve into the subject, maybe start with the following paper.
 

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peteb

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It is important. In the champ the load on V1 is the combined (parallel) impedance of it's 100k plate resistor, the 1meg volume pot, and the input impedance of the next tube
I am saying that the contribution of the 1 Meg resistance of the volume pot stays the same regardless of volume setting. I thought you agreed with this in post 9. I was restating in post 10 what you had agreed to in post 9, that the volume control setting does not affect the load impedance.

I am confused what you mean by the input impedance of the tube. I just learned about a week ago that the input impedance to a tube is the grid resistance in the simple case of a grid resistor from grid to ground. It gets a little more complicated when a volume pot is attached to the grid. I thought we were in agreement that the volume pot on the 5F1 has 1 Meg of input impedance at all volume settings.

the input impedance is the impedance the signal has to overcome to reach ground. The signal does not flow thru the tube. I have just been learning that the input impedance of a tube has to do with how the tube is connected to the circuit, not a property of the tube itself. The 5 Meg grid resistor is 5 megs of input impedance whether it is attached to the grid of a 12ax7 or a 6V6.


It's the same for all settings if the signal is small enough to stay clean.

it is true that I am primarily concerned with cleanish operating conditions.


next question, why does the output stage of the 5F1 and Blackface champ have a input impedance to plate impedance of 220k/100k where the pre amp uses the recommended 10/1 ratio, 1M/ 100k?

are they just freely giving up gain? Why? They don’t need it?

is the 10/1 rule for 12 AX7? And not for a 6V6?
 
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peteb

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Yes, but, with a circuit as fundamental as the Champ, and its function so iconic, from the cleaner lower settings, up to the idiosyncratic dimed hoot, not talking about the interesting bit is ignoring the best bit.
That is a good point.

in my opinion one has to first understand clean operating conditions first. The power output of an amp is measured from a clean signal.

grid Current? Grid current only happens at several gradients beyond the full power of the amp measured from a clean signal.

I refuse to go there.
 

peteb

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The 5E3 is a perfect example demonstrating why we need to know the output impedance of the source doing the driving
What is the output impedance of v1 of a 5E3? 100k?

I have not done anything with output impedance yet.

the signal forms across the plate resistor.

I see what you mean by the old style volume control being a voltage divider. The coupling cap would have reactance, I calculate 3k ohms at 500 Hz. you are right, the pot setting would Determine how much signal drops across the coupling cap and how much signal drops across the bottom of the pot.

thanks


you are right Andrew the older style volume control acts like a voltage divider, similar to the newer style volume control.


the difference is the old style volume control affects the load impedance and the newer style volume control does not.


what I am trying to understand now is how much does lowering the load impedance, say by half, affect the gain of the precious stage?

that is what we see in the blackface tone stack. We can see that for DC the load resistance is cut lower than half. I suspect for AC the load impedance is cut by even more.
 
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