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

David Barnett

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Loss through the tone stack is how it works. It's a passive circuit, so it is only able to induce loss. To give the impression that bass and treble can be boosted, you must first drop the midband. Then when you turn up the bass or treble, those areas can maybe approach unity. Without loss through the tone stack, you'd lose range for the controls.

 
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elpico

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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?


Unfortunately the answer comes back to that same pesky grid current topic. Whether we like it or not, current does flow into and out of a tube's grid.

A tube's grid is actually a thin wire that's coiled closely around the cathode & heater. That's a hot place for a wire to be. The hotter the heater is run, the hotter the grid gets. The higher the dissipation of the tube, the hotter the grid gets. Get the grid hot enough and it will act like a little second cathode - electrons begin to jump off the grid and flow to the plate.

This grid current is flowing through the grid resistor so it causes a voltage to develop across the grid resistor, which makes the grid voltage more positive. More positive grid voltages make more current flow through a tube. More current through a tube makes the tube hotter. A hotter tube makes the grid emit even more electrons. More electrons shift the grid even more positive and so on... we're in a feedback loop. This is called "thermal runaway" and can destroy the tube if the grid resistance is too high.

The designers don't want to throw away gain by using a 220k grid resistor on the 6V6, they're forced to do it to make the tube run safely. Smaller grid resistor = less bias change when grid current flows = more stable and safer operation. You can find the max recommended grid resistance values for a tube in a good tube data sheet. Or if you're really keen the formulas they used to come up with those recommendations are in RDH4
 

Bendyha

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Unfortunately...........................
......Rider doesn't give us a drawing of little electrons with hot feet.
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peteb

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Thanks guys, I think we have the volume controls figured out.

now I want to look at the blackface tone stack versus the tweed tone control and look at how both affect gain thru voltage division And impedance matching.

the 5F2-A tone stack is common to the
6G2 princeton
5E3 deluxe
5C5 pro
5D4 super

we know that the blackface tone stack has an insertion loss, meaning that no matter how you set the tone controls there will be a loss of gain. We see that the impedance mismatch by lowering the output load of v1 is present in any tone control setting. This must be the insertion loss. The treble control is positioned so that it is going to cut the signal voltage like the 5F1 volume control. You can see that if the treble and bass are on 10 the tone stack is going to send the majority of the signal on to the volume pot. The blackface tonestack has a significant permenant gain loss due to impedance mismatch and tone control setting imparts additional gain loss due to voltage division of the signal.

Please look at the tweed tone control.

it is not supposed to be lossy like the bf tone stack.

what do you see in the way of impedance mismatch?

what do you see in the way of signal voltage division?


i believe there is some insertion loss, but it most likely is much less than the insertion loss of the bf tone stack.

the other big difference is that the tone control setting has no affect on the signal level. *

* this is one of the biggest questions of this thread. Can anyone explain why the blackface tone control settings can cut signal and the tweed tone control setting can not.


that is my question.

i want to be able to look at a tone stack or a tone control and be able to estimate the amount of insertion loss, and whether the tone control setting affects the signal level.

that is all


thank you



696A70A4-782A-49B1-ADCD-CAE83C1A1819.gif
 

peteb

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Finally, using the equation for the voltage gain we have:

image016



my comment:



this is the equation that shows the importance of maintaining a high load resistance.

Rl is the combined resistance of the plate resistor and the load resistor.
ra is the plate resistance internal to the tube.
 

elpico

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"Impedance matching" is only relevant to power amps and RF circuits. There's no impedance matching going on in an audio preamp or EQ.

Can anyone explain why the blackface tone control settings can cut signal and the tweed tone control setting can not.

What you're asking isn't true. They both cut signal. All passive EQ controls work by cutting signal. If they couldn't they would have no effect.
 

David Barnett

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"Impedance matching" is only relevant to power amps and RF circuits. There's no impedance matching going on in an audio preamp or EQ.



What you're asking isn't true. They both cut signal. All passive EQ controls work by cutting signal. If they couldn't they would have no effect.

"It's not a bug, it's a feature"
 

peteb

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What you're asking isn't true. They both cut signal. All passive EQ controls work by cutting signal. If they couldn't they would have no effect.

I have done a lot of signal measuring on tube amps. The signal voltage measured by a DMM is not affected by the tone control on my 6G2.

that is one of the main reasons for this thread. I want to understand why it doesn’t.

thanks
 

peteb

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Ok, some progress has been made.
I programmed the gain equation that relates anode resistance and load impedance Into excel.

BD5C0D90-EB0A-4206-844A-45D8F998EB39.jpeg



at first I noticed the lowering of the load impedance had not too much affect on gain, until I got all of the way to the bottom where output impedance was cut all the way down to 100K, and the combined impedance of the reduced output impedance and the plate load resistance was cut in half to 50K ohms. I noticed the gain is 44. I knew I was getting somewhere then. I have done a lot of signal measuring including on two blackface champs and the gain of V1, regardless of tone control settings, regularly ends up at 44-45. This is the signal measured at the plate of v1 divided by the signal at the grid of V1.


these are the facts. The gain equation is given in post 25. The anode resistance of a 6v6 is 62.5K ohms. The gain of V1 in a blackface tonestack is 44.4, backed up by modern fender schematics showing signal.

PRRI: gain = Vp/Vg = 1.3 / 0.03 = 43
DRRI: 1.6 / 0.037 = 43
SRRI: 0.8/0.037 = 40.5
TRRI: 1.6 / 0.037 = 43.


if the anode resistance is 62.5k ohms, and the gain is 44.4, then the load impedance has been cut by a factor of ten from 1M to 100k and the combined load of the output impedance and the plate load is 50K ohms.


Now onto the tweed tone control. I looked at my signal measurements of my 6G2. The gain at V1 is now 52. Looking at my excel chart, the gain of 52 occurs when the output load impedance is cut by a factor of 5 to 200k and the output impedance combined with the plate load is 66.6k.
 

peteb

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reference:





Finally, using the equation for the voltage gain we have:

image016
.



image017
 

elpico

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I have done a lot of signal measuring on tube amps. The signal voltage measured by a DMM is not affected by the tone control on my 6G2.

that is one of the main reasons for this thread. I want to understand why it doesn’t.

thanks

A few possibilities that come to mind:

- you're not using an appropriate test signal
- you have the volume set to 10

Try a 10kHz signal with the volume control set to 5.
 

peteb

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A few possibilities that come to mind:

- you're not using an appropriate test signal
- you have the volume set to 10

Try a 10kHz signal with the volume control set to 5
thanks Elpico,

Interesting point about the volume on 10 making taking the tone control out of the circuit. I kind of see that and I kind of don’t, and I don’t think that was the case.

I will check my notes about the frequency and control settings. This was with a signal generator and a dummy load. I know that if I tested it on volume 10 it would only have been the briefest test. The dummy load freaks me out because you can’t hear what is going on.

why 10k Hz? That is way high for guitar.

when I did my signal testing this was probably the thing that stuck with me the most. It really surprised me when I first saw it.


along with your observation about 10k making more difference, the only way I can explain it is that the high frequencies are bled to ground, leaving the lower frequencies which carry the most energy. Not sure how this would work when testing with a single frequency.

I was playing the 6G2 last night and tested the tone control. My findings agree with what both of us feel about low versus high frequencies.

I play my Tele with the tone rolled just about all of the way off. Hitting the low e string with this tone setting and quickly moving the tone control back and forth to try to get a wah affect made me think the tone control was inactive as I heard no wah at All. Then I set the tele tone all the way up and switched to the high e string and then the tone control had a huge effect.

the tweed era circuit was capable of a piercing high tone, and the tone control was able to bring it back into a reasonable listening range.


I will check my frequencies.
 

peteb

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you're not using an appropriate test signal
I checked what it was, 125 Hz and mostly 250 Hz.

I know Those are low for testing purposes.


I am going to try an easy test tonight. Repeat my wah test with a DMM on the speaker. open low e at 82 Hz, open high e at 330 hz, and then up towards 1K Hz.
 

peteb

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My previous test with a signal generator showed no affect on the signal level from the tone setting at 250 Hz.

yesterday using a guitar I could see the tone control affecting the signal level at 330 Hz, the open high e string.


I don’t think I am the first or last person who looks into the operation of the simple tone control that ends up shaking their head and walking away.
 

peteb

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This has the more common tone control section modeled and you can change the values to see what effect a component change has and what different drive and load Zs behave
thanks

i may have to do that.


does it have a lone tone control?

I have played around with the DTC before.

my memory is that it had common tone stack arrangements, but not the lone tone control as seen on the more basic tweed circuits.
 




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