Figure 12.2. This is a screen shot from the LTspice simulator showing the schematic diagram that was described in figure 12.1. It starts out the same but stick with it, it gets different after the plate of the tube. On the left is an AC generator labeled AC 1. The plus terminal connects directly to the grid of one of the triodes in a 12 A X 7. The minus terminal is grounded. The cathode goes through the parallel combination of R1, a 1.5 k ohm resistor and, C4, a 470 u f capacitor. The plate goes through R2, a 100 k ohm resistor to the plus side of V2, 250 volts DC. The minus side is grounded. The plate also goes to one end of R3, a 100 k ohm resistor. We will come back and pick up the other end of R3 later. The plate also goes through C1, a 250 p f capacitor to the top end of a 250 k ohm pot which is labeled treble. In the simulation the pot is represented by two resistors R4 and R5. The junction of the two resistors is the pot wiper. The junction of R4 and R5 goes to the top of R9, a 1 meg ohm resistor. The other end is grounded. There is a 30 p f capacitor in parallel with R9. It is C5. R9 and C5 represent the resistance and capacitance of the volume control pot. The bottom of the treble pot, R5, goes to R7 which stands in for the bass rheostat which is also a 250 k ohm. The junction of R5 and R7 goes through C2, a 0.1 u f capacitor to the hanging end of R3, the second 100 k ohm resistor. The bottom of the bass pot, R7, connects to the top of R8 which stands in for the midrange rheostat. The junction of R7 and R8 connects through C3, a 0.047 u f capacitor to the junction of R3 and C2. The bottom of R8 is grounded.
Pasted in here is the netlist created by LTspice for this simulation.
* C:\Users\Max\LTspiceXVII\Audio\Guitar tone stack.asc
V1 N007 0 AC 1
V2 N003 0 250
XU1 N001 N007 N008 12AX7
R1 N008 0 1.5k
R2 N003 N001 100k
R3 N001 N005 100k
R4 N002 N004 {PTv-PTp}
R5 N004 N006 {PTp}
R7 N006 N009 {PBp}
R8 N009 0 {PMp}
C1 N002 N001 250pf
C2 N006 N005 0.1µf
C3 N009 N005 0.047µf
C4 N008 0 470µ
R9 N004 0 1Meg
C5 N004 0 30pf
.lib 12AX7.sub
;.param PMp=5k
.param PBp=35k
;.step param PTp list 1 15k 50k 125k 250k
.param PTv=250002
.ac dec 100 10 100k
.param PTp=50k
;.step param PBp list 1 7.9k 25k 79k 250k
.step param PMp list 1 2.5k 6.5k 14k 25k
* Treble
* Bass
* Midrange
.backanno
.end
End of verbal description.
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