Biasing a fuzz pedal after changing transistors.

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niblos
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Post by niblos »

I am kind of new to this sort of thing but here is my question: I recently ordered some Texas instrument cv7003 (military version of the oc44) and wanted to change just the fuzz side transistors on my 9-3700 Vox stereo fuzz wah. I wanted to try a germanium transistor for a warm, smoother fuzz tone. My question is how do I bias these two cv7003s? My friend says he can check them for leakage and that sort of thing. I have read a few things about bias control on DIY fuzz pedals but what about something minor such as exchanging transistors? Is there anything else that I need to consider?
Is this mod really worth the effort?
Your help is greatly appreciated!

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RnFR
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Post by RnFR »

you really try and post or link to a schematic with your question if it isn't a well known design. it'll make it easier for your question to be answered. i'm sure with a schem someone should be able to give you a hand and get your problem sorted. :wink:
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niblos
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Post by niblos »

Thanks. I do have the schematic for the vox stereo fuzz wah (9-3700) as well as the vox distortion booster (V8162) that was actually the very same circuit found in the fuzz/wah. However I don't have anyway to post them. I will be happy to send them to anyone who emails me at the following:
jcook27@nc.rr.com

p.s. I LOVE your Betty image!!!

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Post by RnFR »

thanks, me too!

as far as posting them, have you tried imageshack or photobucket? it's really easy. just hit upload image, and then put the [img] code for the picture into the post. give it a shot, and if you are still having trouble, i'll figure it out for you.
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R.G.
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Post by R.G. »

niblos wrote:I am kind of new to this sort of thing but here is my question: I recently ordered some Texas instrument cv7003 (military version of the oc44) and wanted to change just the fuzz side transistors on my 9-3700 Vox stereo fuzz wah. I wanted to try a germanium transistor for a warm, smoother fuzz tone. My question is how do I bias these two cv7003s? My friend says he can check them for leakage and that sort of thing. I have read a few things about bias control on DIY fuzz pedals but what about something minor such as exchanging transistors? Is there anything else that I need to consider?
Is this mod really worth the effort?
Then it's a good time for a newcomer's lesson.

1. Simply subbing germanium transistors into a circuit will not make it sound warm, smooth, vintage, etc. Transistor type is only one of a whole spectrum of things that affect tone. I suggest you go to http://www.geofex.com and read all of the "Technology of..." articles, especially the Technology of the Fuzz Face.
2. Your friend is probably checking for leakage and gain with either the circuit for testing Fuzz Face transistors from Geofex.com or one of the many imitators that have sprung up in the years since I put it up there.
3. You need to learn about polarity. The OC44 is a PNP transistor, the two schemos you posted are done with NPN transistors. That means that you will have to reversed every single polarized part, including the battery to work properly with OC44s.
4. This introduces you to the negative-ground/positive-ground issue. Reversing the polarity of the power supply means that the positive lead of the battery will be connected to signal ground, not the negative one. That means that you cannot power both the positive ground and other negative ground circuits from the same power supply because signal ground connects to and therefore shorts both sides of the power supply. A second power supply is needed. You will find people who will tell you that you can build the positive ground version then reverse the power supply to get a negative ground. In theory this works. In practice, there are some fraction of these conversions that never work. Bad idea for a newcomer.
5. Finally, as you recognize by now, exchanging transistors is not minor if you change polarity.

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Post by RnFR »

well there you go! RG to the rescue! you just got a much better lesson than i could have ever given you. i recommend reading up on everything RG had to add, he's been doing this stuff about as long as anyone out there. another thing that may make it difficult to swap in ge trannies is that the gain will probably be much lower than the silicons that were in there. you will also most likely have to rebias the transistors as well. it is going to be quite an undertaking for a beginner, and the results may not be what you expect. why not just build yourself up a rangemaster or a fuzz face? they are good beginner projects, and you know that if your transistors are good, you should end up with a decent sounding fuzz or boost. many premade boards can be ordered for rather cheap, and it is a good way to start building.
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Post by RnFR »

biffa wrote:RG is the best to go to
I find tranny types to be a bit unhelpful(for me)
I find getting a collection together of suitable types,then swapping them in & out of a terminal to be the best(again for me)
Quite often,I'll put in the correct one,to find out later,another works better!
yes, but his transistors are not the right kind for an easy swap.
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Post by niblos »

OUCH! That is WAAAY over my head. I had no idea it would be so involved just for a couple of transistors. I really appreciate everyone's help and you have really giving me some great advice where to start. I think it may be just as simple to build a few pedals to get some hands-on experience before jumping into such a project. Since it is a vintage piece I think I'll just keep it stock. The characteristics aren't that bad and kind of has it's own sound.

You have made me even more appreciative of builders like Analogman Mike and Paul Cochran. I cannot thank the members on this board enough for taking the time to answer all my questions and then some. You guys are simply amazing. It's great to see all the free information on this site.

Keep up the superb work and let me get to work!

Take care and may GOD bless,
Robbie

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Post by jrod »

Howdy!

OK, I have a n00b question, and I have actually search for this answer: When you bias a transistor in the Fuzz Face, for example, do you measure voltage accross the resistor, the resistor and transistor or transistor ground?

Thanks, fellas!

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Post by jrod »

Biffa,

Thanks for the reply. I will certainly use your pot suggetion. That sounds so simple!

Much appreciated!

jrod

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Post by niblos »

Hey Resistor Ronker,

Wow - I had no idea it was that easy, THANKS! I think I learn something every time I log on this site.

Have a great weekend,
Robbie

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