Tone stack input impedance?

[hanumanlangur]

Hello, I'm confused about finding the input impedance of a tone stack in an amp. I was working a bit with the Duncan Amps Tone Stack Calculator, and I noticed that the input impedance (Zsrc) for the Marshall tone stack was 1.3k. How did they get this number? Are there simple rules of thumb for calculating these things?

I'm trying to translate this into a pedal-sized version of the tone stack, but I'm guessing that the input impedance would be quite different in a low-voltage, solid state pedal.

[Tubis]

1.3k? I donīt think that is correct. You have a 33k and 250kpot blocking the way.

[Bruce / Mission Amps]

1.3k? I donīt think that is correct. You have a 33k and 250kpot blocking the way.

Yes it is correct.
Keep in mind we are talking about "impedance" not just DC resistance.
All that stuff after the cathode is in parallel with the tube's total impedance and the cathode resistor. Remember that both ends of the tube are actually AC grounded. One end through the power supply filter caps and such, while the other is grounded through the cathode.
This makes for an abstract view of the tube but it is sort of folded over with respect to it's center.
The cathode follower is very very low impedance (the very reason it is used to drive the tone stack as the tone stack will have a much higher impedance).... the tone stack being a higher zed will have less effect on the extremely low zed triode this way.
If you had a really high power, high current tube and used a speaker with a voice coil with enough DC resistance and impedance in it's cathode instead of a resistor, you could actually drive a speaker load with the cathode.... it's that low in Zed.

[Tubis]

Yes it is correct.
Keep in mind we are talking about "impedance" not just DC resistance.
All that stuff after the cathode is in parallel with the tube's total impedance and the cathode resistor. Remember that both ends of the tube are actually AC grounded. One end through the power supply filter caps and such, while the other is grounded through the cathode.
This makes for an abstract view of the tube but it is sort of folded over with respect to it's center.
The cathode follower is very very low impedance (the very reason it is used to drive the tone stack as the tone stack will have a much higher impedance).... the tone stack being a lighter zed will have less effect on the extremely low zed triode this way.
If you had a really high power, high current tube and used a speaker with a voice coil with enough DC resistance and impedance in it's cathode instead of a resistor, you could actually drive a speaker load with the cathode.... it's that low in Zed.

Well now youīre talking about the tubes output impedance and not the tonestacks input impedance. The tonestack have a resistans at 29kohms if you count the 33k and 250k in parallell with the 25kpot turned to ground so the impedance cant be lower than that.
That the cathode follower has a low output impedance is true. But thats not the question.

[Bruce / Mission Amps]

Well now youīre talking about the tubes output impedance and not the tonestacks input impedance. The tonestack have a resistans at 29kohms if you count the 33k and 250k in parallell with the 25kpot turned to ground so the impedance cant be lower than that.
That the cathode follower has a low output impedance is true. But thats not the question.

Yes it is.... his question was "input impedance (Zsrc) for the Marshall tone stack was 1.3k....?"
the input zed is the cathode's total impedance... the Zsrc.... zed source.

[Tubis]

Yes it is.... his question was "input impedance (Zsrc) for the Marshall tone stack was 1.3k....?"
the input zed is the cathode's total impedance... the Zsrc.... zed source.

Never heard of such a thing(Zsrc).
When input impedance is mentioned with the word tonestack i think of the tonestacks input. Not the hole shit in parallel with the cathodes output impedance. When would you use such a thing?
Maybe to se how much a difference the load will make on that or what?
If the number drops to much from the output impedance the connection is not a good idea or what?

[hanumanlangur]

I'm a bit confused by this, still. Would the Zsrc be just the impedance of the cathode, then?

[Tubis]

I googeld the word Zed and not many hits at all. Must be very uncomen.
Z stands for impedance. Here in Sweden i does anyway.
I think its just the impedance of the source measured when the tonestack is connected. The Zs stands for the cathode follower and the rc for the tonestack. But i could be wrong

[Bruce / Mission Amps]

I'm a bit confused by this, still. Would the Zsrc be just the impedance of the cathode, then?

Yes Z = impedance and when you say it out loud, you say Zed, not Z.... my English Grandmother's upbringing I guess...

Don't feel bad. I'm not an engineer but I don't think this is a complicated formula using the Ra, Rp=(0) wtih the actual mu of the triode ... actually this is not my Forte' so I'm sure there are others here who love that stuff and will jump in...

it ends up that the Zed of a CF is usually around +1000 ohms or so.
Keep in mind that the 56K to 100K resistor seen in the cathode of a cathode follower is in parallel with the impedance of the cathode circuit.... so if that happens to be.... say, 1300 ohms then it hardly matters very much (within reason) what the cathode resistor is as it only is effecting the DC bias of the triode.
1300ohms||100,000 = less then 1300 ohms.
Now, if that big ass tone section has a Zed of around 38,000 ohms... it looks huge to the cathode follower and that higher impedance can't load the cathode down much at all because it only represents 1300/38000 of the total load... or less then around 2%.
Try that same thing with a common cathode with 56K to 100K plate load resistor driving a tone section (anode connect)... and you'll see a big difference in the loading effect of the tone section.

[Tubis]

Well i installed the calculator and its just the output impedance. It has nothing to do with the tonestack att all.

[Tubis]

hanumanlangur
If you try the fender tonestack you can see that it is 38k. That is the internal resistense and the plate resistor in parallell. So you see that the Zsrc is just the ouput impedance. And so is the Marshall that is driven from the cathode instead of the Plate.