GGG Boutique 70's Fuzz Question

Started by afrogoose, January 15, 2008, 12:29:51 PM

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afrogoose

This was my first stompbox build.  It works fine and sounds good, but I'm a bit confused by the schematic.  The schematic shows three diodes in the circuit, the build diagram shows only two, and the parts list shows only one.  I'm not sure which one to go by.  Has anyone built one of these, and do you have any insight on this?  (FWIW, I built it with no diodes and it works)

Thanks!



petemoore

  Yes, I have just read there are differeces showing between the various drawings, if the schematic shows three it would also show how they are used.
  Generally three diodes can be for reverse PS polarity [to prevent damage during transient reverse applications], and clipping.
  That's what I vaguely remember as in the 70.
  Other uses include reverse polarity to a transistor B/E junction to stabilize bias, or inline with SP to produce a compression effect, or in the feedback loop of a 1 transistor gain stage for clipping.
Convention creates following, following creates convention.

afrogoose

Pete

Thanks for the help!  I can identify the diode that protects against reverse polarity okay.  There is another one coming off of the output cap(??) (the lone film cap in the circuit) which goes to ground.  This one is a germ diode.  I'm not sure what this one achieves.  Also, the other diode I am clueless about is used in a way which I don't understand yet because I'm still an idiot.  Here's the schem.

http://www.generalguitargadgets.com/pdf/ggg_ff5_sc_b70.pdf

If anyone has any insight into what these two diodes do, I would really appreciate it.

Thanks! 

yeeshkul

D2 looks like a clipping diode to me, it's just one because the signal swing may be asymetrical, so the other would be no use. It also kills the signal a lot because Ge diodes keep lower voltage than Si or LEDs (about 0.2V). That is indeed very strange.
I don't dare say anything about the D1.

yeeshkul

actually the output signal seem to be sym., so the diode makes it asym.

frankclarke

#5
R9 and R8 are shorted in that schematic. My gutshot photo in a blue case has a 2.2uF input cap and no diodes at all, so there may be variation.

petemoore

 D3 is a reverse protection diode, doesn't do much until battery is reverse touched.
D1 is some kind of diode...biases or holds the base of the transistor somehow [not too sure..probably would work without this diode, not sure, never seen it, looks like it'd keep AC voltage on the base one diode drop from Gnd. or something...
D2 Is a Ge clipping diode, it clips on one of the half cycles [top or bottom half of wave, negative portion I guess.]
  D4 is the indicator LED, is separate from the signal circuit, it just shows what position the BP switch is in by lighting when the two relevant switch lugs provide power supply to it.
Convention creates following, following creates convention.

afrogoose

Thanks again for the insight!  I think I might put a socket in D2 if it is indeed a clipping diode to see what it sounds like with Ge and Si diodes as well as a LED.  One complaint I have is that the pedal is too quiet, so I'm reluctant to squash the signal anymore. 

petemoore

  Ge clips lower level than Si, IIRC the circuit output doesn't break threshold of Si clipping diode, and can't tell it's there.
Convention creates following, following creates convention.

yeeshkul

Using the Ge clipping diode definitely kills the loudness. This circuit is a Fuzz Face circuit and they are quiet. Try to change R4 to something like 1k and you will see. Also, the D1 doesn't seem to have much influence. Just drop it.

afrogoose

#10
Thanks for the heads up on the Ge diode.  I'll just leave that one off.  Also, since D1 isn't part of the classic fuzz face circuit, I can do without that one too.  I have a 500k pot in the mail for the volume.  I originally bought it it to get more bass out of the pedal, but would a higher value get me any more volume?
Also, I was reading the other thread about Film vs Electro caps and I was wondering if it made any sense to try two parallel 1uf film caps in place of the 2.2uf electros?  Is that just nonsense?



yeeshkul

This was discussed many times round and around with no particular result. Indeed, different people like different things and more over - different brands of the same cap type can sound different, so the only point of using film caps is their durability. If you wanna play the same fuzzface in 20 years the YES :)). But check out the both and you will see what to choose in the end.