One tone knob tone stack wiring...Is it different?

[psychepool]

It is a tone stack of 6G3 Brownface Deluxe.
Is there a difference between the two circuits below?

tonestack.jpg

I try to put several tone stacks into one amplifier by changing the component values with a rotary switch.
5E3 / 6G3 / 18W / Spitfire and so on.

It would be fortunate that if there is no difference between the two wiring schemes above.
However, even if there is a difference, may be I will make it unified to one side in the end.
If there are differences, I want to know what the difference is.

[pdf64]

There's a difference, in that the 'cut' cap is on the input to the vol control on the LH circuit, but on the output of the vol control on the RH circuit.
The impedance on the former is around 40k and is pretty much independent of the vol control setting; that equates to a low pass filter corner freq of ~181Hz.
The latter varies according to the vol control setting, eg 40k or less at extremes of the vol control, and rises to a max of ~250k at the electrical mid point of the vol control setting, so the corner frequency varies from 181Hz down to 0.034Hz!
If the tone control isn't used to 'cut, eg turned below noon, then the difference is probably negligible.

BTW 'tone stack' is usually used to describe the regular Fender / Fender derived T&B (&M as applicable) arrangement, variable filters 'stacked' on top of each other; your circuits may be better described as 'single knob tone controls'.

[psychepool]

Thank you. It is very difficult for me, a non electronic major, until I hear this simple circuit explanation.

If there is no difference between the two circuits, the wiring I plan on would have been much simpler, but it seems a bit more complicated now.

[dorrisant]

Pete, how are you doing your impedance calculation? Not calling you out for error or anything... Just asking.

[pdf64]

Pete, how are you doing your impedance calculation? Not calling you out for error or anything... Just asking.

Have you got your head around Thevanin (voltage source) and Norton (current source) equivalent circuits?
http://hyperphysics.phy-astr.gsu.edu/hb ... venin.html
http://hyperphysics.phy-astr.gsu.edu/hb ... orton.html
And how you can easily swap between them, as for any equivalent circuit, the source impedance term is the same for both? I still remember the class where that was explained to me, 35 years ago; a real light bulb moment for me

Sometimes when analysing a circuit, although either can be used, the circuit lends itself better to one or the other.

When looking at the (following) grid circuit in the RH circuit, I find that it's easier with Norton; at low frequencies, I look for all the parallel paths to 0V. The max value will occur when the vol pot is about mid way, so one 500k resistor from grid to 0V, then another 500k from grid to the preceding plate.
We know that the output impedance of a regular CC stage is around 40k, so that second 500k has a total impedance to 0V of 540k.
There's also the impedance of the tone control to consider, but at 'cut' CCW it's the 22nF cap // the 1M track of the tone control in series with the 'boost' cap, which in turn is in series with the preceding plate impedance before we get down to 0V.
But for lower frequencies, we can assume that the reactive impedance of the boost cap is very high, so we can just ignore that path.
So, from the perspective of the following grid, the paths to 0V are 500k // 540k (I just rounded that to 250k), the total of which is // with the reactive impedance of the 22nF cut cap.
The (signal) voltage that appears at the grid will be due to the equivalent circuit current source driving that impedance. And we know that current source and source impedance can be swapped with a voltage source and the same impedance.
We can plug those numbers into the simple low pass filter calculation and get the corner frequency.
Hope that makes sense?

[Mark]

Glancing a quick eye over the two circuits I thought they'd sound pretty much the same.

[dorrisant]

Thanks Pete... Good stuff!!