Fuzz Face mod in LTspice (Cap c-b)

[yeeshkul]

Hello, i just started playing with LTspice ... so here you can see how the capacitor between collector and base of the Q2 influences frequency output.

C range: 10pP - 100pF
Step: 10pF.

[gaussmarkov]

cool.  please post your schem with the .op commands showing so that we can
see how you did it. 

[yeeshkul]

Here is a content of .asc file. Make the file and open it in LTspice.
Please either replace all pots with resistors or go to yahoo group for LTspice and find out more bout pot model in files / tutorial.
----------------------------------------------------
Version 4
SHEET 1 880 680
WIRE -224 -288 -880 -288
WIRE -80 -288 -224 -288
WIRE 64 -288 0 -288
WIRE 64 -192 64 -208
WIRE 112 -192 112 -240
WIRE 112 -192 64 -192
WIRE 64 -160 64 -192
WIRE 288 -160 64 -160
WIRE 432 -160 352 -160
WIRE -224 -144 -224 -288
WIRE 64 -48 64 -80
WIRE 64 -48 -48 -48
WIRE 64 -32 64 -48
WIRE -880 -16 -880 -288
WIRE 432 0 432 -160
WIRE -224 16 -224 -64
WIRE -112 16 -112 -48
WIRE -112 16 -224 16
WIRE 0 16 -112 16
WIRE 704 48 480 48
WIRE -544 64 -608 64
WIRE -384 64 -464 64
WIRE -304 64 -320 64
WIRE -288 64 -304 64
WIRE -608 128 -608 64
WIRE 64 144 64 64
WIRE 112 144 64 144
WIRE 704 176 704 48
WIRE -304 208 -304 64
WIRE -112 208 -304 208
WIRE 64 208 64 144
WIRE 64 208 -32 208
WIRE 208 256 112 256
WIRE 208 304 208 256
WIRE -880 368 -880 64
WIRE -608 368 -608 208
WIRE -608 368 -880 368
WIRE -416 368 -608 368
WIRE -224 368 -224 112
WIRE -224 368 -416 368
WIRE 64 368 64 288
WIRE 64 368 -224 368
WIRE 208 368 64 368
WIRE 432 368 432 80
WIRE 432 368 208 368
WIRE 704 368 704 256
WIRE 704 368 432 368
WIRE -416 432 -416 368
FLAG -416 432 0
SYMBOL pnp -288 16 R0
SYMATTR InstName Q1
SYMATTR Value AC128lg-hfe75
SYMBOL res -240 -160 R0
SYMATTR InstName R2
SYMATTR Value 33k
SYMBOL voltage -880 -32 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName baterie
SYMATTR Value -9
SYMBOL res 16 -304 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 470
SYMBOL res 48 -176 R0
SYMATTR InstName R5
SYMATTR Value 3k1
SYMBOL pnp 0 -32 R0
SYMATTR InstName Q2
SYMATTR Value AC128lg-hfe75
SYMBOL cap 352 -176 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 10n
SYMBOL res 688 160 R0
SYMATTR InstName kombo
SYMATTR Value 1000000k
SYMBOL cap -320 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 63 32 VBottom 0
SYMATTR InstName C3
SYMATTR Value 2.2µ
SYMBOL res -16 192 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 100k
SYMBOL potentiometer_standard 416 -16 R0
SYMATTR InstName volume
SYMATTR Value Rtot=500K wiper=1
SYMATTR SpiceLine ""
SYMBOL voltage -608 112 R0
WINDOW 123 24 132 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC 1 1
SYMATTR InstName kytara
SYMATTR Value SINE(0 100m 800)
SYMBOL potentiometer_standard 48 -304 R0
WINDOW 3 46 -21 Left 0
WINDOW 38 48 -55 Left 0
SYMATTR Value Rtot=1K wiper=1
SYMATTR InstName bias
SYMATTR SpiceLine ""
SYMBOL polcap 224 368 R180
WINDOW 0 -49 48 Left 0
WINDOW 3 -45 84 Left 0
SYMATTR InstName C1
SYMATTR Value 20µ
SYMBOL potentiometer_standard 48 192 R0
WINDOW 0 48 99 Left 0
WINDOW 3 -88 206 Left 0
WINDOW 38 -87 243 Left 0
SYMATTR InstName fuzz
SYMATTR Value Rtot=1K wiper=1
SYMATTR SpiceLine ""
SYMBOL res -448 48 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 1
SYMBOL cap -48 -64 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C4
SYMATTR Value {CX}
TEXT -640 504 Left 0 !;tran 0.1
TEXT -648 528 Left 0 !.ac oct 20 20 20k
TEXT -456 -456 Left 0 !.include potentiometer_standard.lib
TEXT -456 -408 Left 0 !.STEP param CX 10p 100p  10p
--------------------------------------------------------------------------
here is A128 model (copy it and paste to standard.bjt)

.model AC128lg-hfe75 PNP(Bf=75 Vje=0.2 Is=1.41f Xti=3 Eg=1.11
+ Ne=1.5 Ise=0 Ikf=80m Xtb=1.5 Br=4.977 Nc=2
+ Isc=0 Ikr=0 Rc=2.5 Cjc=9.728p Mjc=0.5776
+ Vjc=0.2 Fc=0.5 Cje=8.063p Mje=0.3677 Tr=33.42n
+ Tf=179.3p Itf=0.4 Vtf=4 Xtf=6 Rb=10

In case you decide to draw your own scheme just use directive .STEP param CX ************** (see help file) for a component value sweeping.
Here, you will have to  set the VALUE of the observed component  with the same symbol you used in the directive .step - here CX,  so {CX}.

[gaussmarkov]

sweet.  thanks yeeshkul!

[Gus]

Look

at a transistor circuit output at the collector with a cap from C to B

on a scope

Then look at the output of a lowpass like  3tran/ rocket  on a scope.

[yeeshkul]

my friend i have no idea what you mean, i am sorry

[Gus]

It is good you are modeling it but

If you build a circuit with the C to B to roll off the highs Use a scope on it and look at the output waveform

Then build a circuit with a passive RC lowpass at about the same freq and look at the wave form.

If you look at my schematics I don't use a C to B cap.   I use a simple RC

http://www.diystompboxes.com/pedals/gusOverdrive.gif

http://www.diystompboxes.com/pedals/3tran.JPG

http://www.diystompboxes.com/pedals/gusFuzzFace.gif

There are reasons

[yeeshkul]

Thanks Gus, now i see what you mean. I will try it. 

[yeeshkul]

Strange, i had the transient analyse made while changing the cap (0.01n-100n) but all the shapes seem to be the same ... is the "scope" the same thing? I put the sond at the very output and before the output cap.

... or did you mean to use a real machine on the real circuit? I am, sorry for being slow

[Gus]

Build a real one and check the waveforms with a real scope

[gaussmarkov]

on another matter, yeeshkul, you have put the value of your R5 at 3K1.  this resistor is usually
specified to be 8K2 as in the basic fuzz face schemo in R.G.'s The Technology of the Fuzz Face.
i found that i had to do the same thing to get a sensible bias for the second transistor when
i first entered the fuzz face into LTSPICE.  any thoughts?

thanks again for sharing your work!  it's helpful to compare notes.  and you taught me about
the .STEP command. 

cheers, gm

[flopoeko]

I´m an absolute LTspice novelist. How exactly do I create a schematic and simulation from that net list?

Here is a content of .asc file. Make the file and open it in LTspice.
Please either replace all pots with resistors or go to yahoo group for LTspice and find out more bout pot model in files / tutorial.
----------------------------------------------------
Version 4
SHEET 1 880 680
WIRE -224 -288 -880 -288
WIRE -80 -288 -224 -288
WIRE 64 -288 0 -288
WIRE 64 -192 64 -208
WIRE 112 -192 112 -240
WIRE 112 -192 64 -192
WIRE 64 -160 64 -192
WIRE 288 -160 64 -160
WIRE 432 -160 352 -160
WIRE -224 -144 -224 -288
WIRE 64 -48 64 -80
WIRE 64 -48 -48 -48
WIRE 64 -32 64 -48
WIRE -880 -16 -880 -288
WIRE 432 0 432 -160
WIRE -224 16 -224 -64
WIRE -112 16 -112 -48
WIRE -112 16 -224 16
WIRE 0 16 -112 16
WIRE 704 48 480 48
WIRE -544 64 -608 64
WIRE -384 64 -464 64
WIRE -304 64 -320 64
WIRE -288 64 -304 64
WIRE -608 128 -608 64
WIRE 64 144 64 64
WIRE 112 144 64 144
WIRE 704 176 704 48
WIRE -304 208 -304 64
WIRE -112 208 -304 208
WIRE 64 208 64 144
WIRE 64 208 -32 208
WIRE 208 256 112 256
WIRE 208 304 208 256
WIRE -880 368 -880 64
WIRE -608 368 -608 208
WIRE -608 368 -880 368
WIRE -416 368 -608 368
WIRE -224 368 -224 112
WIRE -224 368 -416 368
WIRE 64 368 64 288
WIRE 64 368 -224 368
WIRE 208 368 64 368
WIRE 432 368 432 80
WIRE 432 368 208 368
WIRE 704 368 704 256
WIRE 704 368 432 368
WIRE -416 432 -416 368
FLAG -416 432 0
SYMBOL pnp -288 16 R0
SYMATTR InstName Q1
SYMATTR Value AC128lg-hfe75
SYMBOL res -240 -160 R0
SYMATTR InstName R2
SYMATTR Value 33k
SYMBOL voltage -880 -32 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName baterie
SYMATTR Value -9
SYMBOL res 16 -304 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 470
SYMBOL res 48 -176 R0
SYMATTR InstName R5
SYMATTR Value 3k1
SYMBOL pnp 0 -32 R0
SYMATTR InstName Q2
SYMATTR Value AC128lg-hfe75
SYMBOL cap 352 -176 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 10n
SYMBOL res 688 160 R0
SYMATTR InstName kombo
SYMATTR Value 1000000k
SYMBOL cap -320 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 63 32 VBottom 0
SYMATTR InstName C3
SYMATTR Value 2.2µ
SYMBOL res -16 192 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 100k
SYMBOL potentiometer_standard 416 -16 R0
SYMATTR InstName volume
SYMATTR Value Rtot=500K wiper=1
SYMATTR SpiceLine ""
SYMBOL voltage -608 112 R0
WINDOW 123 24 132 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC 1 1
SYMATTR InstName kytara
SYMATTR Value SINE(0 100m 800)
SYMBOL potentiometer_standard 48 -304 R0
WINDOW 3 46 -21 Left 0
WINDOW 38 48 -55 Left 0
SYMATTR Value Rtot=1K wiper=1
SYMATTR InstName bias
SYMATTR SpiceLine ""
SYMBOL polcap 224 368 R180
WINDOW 0 -49 48 Left 0
WINDOW 3 -45 84 Left 0
SYMATTR InstName C1
SYMATTR Value 20µ
SYMBOL potentiometer_standard 48 192 R0
WINDOW 0 48 99 Left 0
WINDOW 3 -88 206 Left 0
WINDOW 38 -87 243 Left 0
SYMATTR InstName fuzz
SYMATTR Value Rtot=1K wiper=1
SYMATTR SpiceLine ""
SYMBOL res -448 48 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 1
SYMBOL cap -48 -64 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C4
SYMATTR Value {CX}
TEXT -640 504 Left 0 !;tran 0.1
TEXT -648 528 Left 0 !.ac oct 20 20 20k
TEXT -456 -456 Left 0 !.include potentiometer_standard.lib
TEXT -456 -408 Left 0 !.STEP param CX 10p 100p  10p
--------------------------------------------------------------------------
here is A128 model (copy it and paste to standard.bjt)

.model AC128lg-hfe75 PNP(Bf=75 Vje=0.2 Is=1.41f Xti=3 Eg=1.11
+ Ne=1.5 Ise=0 Ikf=80m Xtb=1.5 Br=4.977 Nc=2
+ Isc=0 Ikr=0 Rc=2.5 Cjc=9.728p Mjc=0.5776
+ Vjc=0.2 Fc=0.5 Cje=8.063p Mje=0.3677 Tr=33.42n
+ Tf=179.3p Itf=0.4 Vtf=4 Xtf=6 Rb=10

In case you decide to draw your own scheme just use directive .STEP param CX ************** (see help file) for a component value sweeping.
Here, you will have to  set the VALUE of the observed component  with the same symbol you used in the directive .step - here CX,  so {CX}.

[cctsim]

The signal traces in the original post seem to be very low at -126dB. When I run your simulation model I get a different plot which makes more sense.





Make sure the model for the transistor is linked properly. The easiest way to paste it as spice directive (.model).

[Rob Strand]

There are capacitances in the transistor model.  To me they look too small.  If you ferret through many old datasheets you might find a number. Off hand CJC for old non-RF germaniums should be maybe 100pf or 150pF.
This value is already comparable to the values you are adding.

The way spice works is the capacitance value in the model is at 0V, it gets smaller for reverse bias and larger for forward bias.  The value you  use is therefore not the value in the datasheet but a modifier version of it.

Cjc=9.728p

This appears in parallel with BC.

Cje=8.063p

This one appears in parallel with BE.

[Rob Strand]

Here's model I was playing with at some point.
Be warned I know for a fact I didn't verify this 100% so I will accept complaints.
    **** See bottom of post this is incomplete.
I'm a bit reluctant to to release it except for the fact there's some pretty bad AC128 models out there.
Tweak the gain to suit what you want.

* Hfe = 100
* ML medium leakage Iceo ~ 160uA
.model QAC128L150_RS pnp(Is=1500n Bf=100 Nf=1.0 Vaf=300
+                 RB=200 RBM=0 RC=1 EG=0.68  XTI=3.0
+                 ISE=0 ISC=0
+        VJC=0.7 MJC=0.333 VJE=0.7 MJE=0.333
+                 Cjc=175p Cje=350p Tf=106n)

* Hfe = 100
* LL low leakage Iceo ~ 10uA
.model QAC128L10_RS pnp(Is=100n Bf=100 Nf=1.0 Vaf=300
+                 RB=200 RBM=0 RC=1 EG=0.68  XTI=3.0
+                 ISE=0 ISC=0
+        VJC=0.7 MJC=0.333 VJE=0.7 MJE=0.333
+                 Cjc=175p Cje=350p Tf=106n)

[Edit:   I can see from some of the parameters I was playing around with these models doing something specific.
  EG has been "fixed" to be correct for germanium. 
  VAF=300 is just thrown in.
  For now,  maybe just steal the last two lines plop them in another model.
]