Possibly dumb RC filter question.

[EtherealWidow]

Hey guys. I'm having a little trouble here. I'm looking in a 6G3 schematic,

http://www.thevintagesound.com/ffg/sche ... _schem.gif

and I'm trying to find where the rolloff is for the "dark cap" on the normal channel. (In parallel with V1's Ra on the "normal" channel) When I use the normal equation to find it

f=1/(2*pi*c*r)

361.9=1/(2*pi*2e-9*220000)

A lowpass filter at 362 hz? No way. What am I doing wrong?

[Merlinb]

f=1/(2*pi*c*r)

361.9=1/(2*pi*2e-9*220000)

A lowpass filter at 362 hz? No way. What am I doing wrong?

You're using the wrong resistance. The equivalent source resistance from the anode is roughly 220k || 1M || 65k (the internal anode resistance).
Call it 47k:

1/(2*pi*2e-9*47000) = 1693Hz

The tone control is also in parallel and will raise this figure a little more depending on setting, although it gets more complicated with those tone caps in there. So much easier to simulate it!

[EtherealWidow]

Thanks for the response, sir. So, there are other resistances at play like the internal anode resistance as well as the 1M pot? What about capacitance? What about the the .02uF coupling cap and the internal capacitance? In all honesty, I'd probably end up simulating it, but it's good to know why my math isn't working.

[vibratoking]

Thanks for the response, sir. So, there are other resistances at play like the internal anode resistance as well as the 1M pot? What about capacitance? What about the the .02uF coupling cap and the internal capacitance? In all honesty, I'd probably end up simulating it, but it's good to know why my math isn't working.

Tehcnically, all the Rs and Cs matter as well as the impedance looking into the output of the 7025 and looking into the input of the 12AX7. The question is how much do they matter? The rolloff frequency is also being changed by the volume and tone pots as they are rotated. It is a complex circuit with tedious math to analyze the whole thing as one. The area around the tone stack is typically the most complicated section to analyze by hand. You can analyzed it from a DC point of view by thinking of the caps as open circuits. You can also analyze the high frequency performance by thinking of the caps as shorts. Do the opposite for any inductors you might encounter in other circuits. You can also make mid frequency approximations, which I won't go into here. These methods can help you to develop an intuition regarding a circuit. Richard Kuehnel's books show this method at work in great detail. Warning: there is quite a bit of math, but you can learn a lot if you bother to understand it. It can provide a great leap forward.

BTW, the equation you used is correct for a circuit with a single time constant...i.e ONE capacitor or inductor in the general case. It doesn't apply to higher order systems with multiple Ls and Cs. In that case, the analysis gets more involved and normally uses concepts like poles and zeros and Bode analysis. The approximations I mentioned above can be very insightful and help avoid difficult details.

[EtherealWidow]

Alright then. Never thought the addition of a single capacitor would be so complicated. Simulation it is, then! Thanks gents!