My Astrotone (Sam Ash Fuzz Boxxx) clone mod - built report

[yeeshkul]

Er ... where to start ... I was really eager to try out the sound of the famous Astrotone fuzz clone, so i made one and it wasn't that bad for what i expected but:

1. The input impedance was to low to work with Wah Wah (common emitter + fuzz pot in parallel to the input)
2. The common collector stage at the output side was really hard to find reason for. Maybe a low output resistance, maybe the gain stage that follows it need to work into a big input impedance but ....
3. The original tone control completely sucks. I am not a big friend of tone controls in general, but this one was waste of money and soldering iron.

so i made these changes:

1. moved the emitter follower in front of the gain stage to get a nice input resistance - and my Wah started working well
2. moved the original Fuzz pot to the middle - once i profit from the big input resistance i don't wanna destroy it by a small resistance in parallel 
3. I used Ge, Si and LED clipping diodes + switch.

so in the end i got this:



Comparing to the original design (without the tone control) the sound did not change (the first thing i have done was that i connected output right before the em. follower stage of the original schema and there was no change in sound) .... so ... i would call it improvement, hehehe

A few words about the particular pieces used (for complete beginners):

1.Transistors:
Q1,Q2 both silicon KC507 (reputedly a good substitution for the original 1N914)

Bias voltages(my battery gives 8.61V)
Vb(Q1):2.72V, Ve(Q1):7.85V                         
Vb(Q2):0.6V (just the B-E), Vc(Q2): 2.49V

2. Diodes:
As i mentioned i used Ge, Si and LEDs
Ge: the only type that actually changes the sound since they keep 200mV and thus they are able to clip the signal
Si: they have almost no audible effect because the signal swing around the collector(Q2) is about 0.7V - 1V (and they need about 0.6V to open and clip)
LEDS: no audible effect but some nice blinking - here you can literally see that the second diode works much less than the first one. It is because the first stage (emitter follower) doesn't invert the signal, but the gain stage does. The signal that comes to the Q2 base "sits" on 0.6V bias voltage which is not enough to amplify the both sides of the signal swing the same way - the negative half tends to close the transistor (Si transistor needs about 0.6V to open) while the positive half is being fully amplified, inverted and thus more clipped by the second diode. At least i guess so ...

2. Resistors:
R1 feeds a low bias current to the Q1 base and helps increase the input impedance together with R2.
R2 is necessary for a common collector stage.
R3 feeds the base (together with R4)  of Q2 AND makes so called negative current feedback that helps to keep the tranny steady (Ic(Q2) goes up -> Vc(Q2) goes down -> IR3 goes down -> Ib(Q2) goes down - > Ic(Q2) goes down))
R4 is a necessity for common emitter stage

3. Capacitors:
C1 is just an input cap
C2 is used when we connect two transistor stages with very different bias voltages (a big one on the emitter of Q1(7.85V) and a small one on the base of Q2(0.6V). To keep those voltages without interferring with each other we have to put that cap in between.
C3 cuts off the bias voltage before clipping - othervise just the second diode would work having constantly a big, positive bias voltage on
C4 is on the original schematics for reason (to put signal back on the bias). Here is for no other reason but maintain the original sound.
C5 is not necessary in this circuit. It is usually used for two common emitter stages to filter out some possible positive feedback signal from the collector of the second stage to the base of the first one (they both invert the signal which makes it "in phase" in the end). A resistors in between the collectors may be used as well to help the same way. I just put it on my breadboard just to get rid of some noises due to bad grounding.

4. Pots:
P1 - this one can be inserted to the Q2 emitter instead (and works well there), but ..... then it will be further away from the original schema ...
P2 - classic volume pot

that's it