Modding a Jimi Hendrix Classic Fuzz

[zombiwoof]

Let me first say that I have an older red Dunlop Fuzz Face that sounded horrible stock, and I succesfully modded it with better transistors (2N404's in place of the stock fake NKT's that Dunlop put in it, tested to the right specs), a .01uf output cap in place of the stock .1uf, and I also changed the resistors to carbon comp as an experiment.  It did sound different with the carbon comps, it took some of the high frequency fizz out and mellowed out the tone.  Also, I put in a 50k trimpot in place of the 33k biasing resistor to dial in the right voltages.  In the future I plan on putting in an external 10k pot/small resistor in place of the 8.2k bias resistor, to make bias changes easily (it really does perform differently at different temperatures, as many here know).  It now sounds great.

So, I always had my eye on that Jimi Hendrix System Classic Fuzz (the red one), because it's in an MXR sized box, has the germanium circuit, and also has an input buffer to make it work with wahs better.  I finally scored one on Ebay for about $30, I tried it once and it sounded out of bias, too sputtery and didn't clean up when I turned down the guitar volume.  So I put it away for awhile, for future modding.
Anyway, I dug it out today and opened it up, yeah it looks like the classic circuit with a bunch of other components which I assuve are the buffer circuit.  The transistors?  NTE 102A's.  I expected to find those Dunlop fake NKT's like the ones that came in my old FF.  Right off the bat, I see two trimpots, the top right one (marked TP4)  seems to be a gain pot, probably for the buffer, turning it it goes from really quiet to real loud quickly.  The other trimpot, bottom left on the board (marked TP3) seems to be a bias pot for one of the trannies, I couldn't really trace the circuit to see where it goes because I was working in my bedroom in bad light.  I played through it while I turned the pot, and it did seem to affect the voltage off the transistor.  When I first got in there, I checked the voltage, which should be about half the battery voltage, and with a battery that read 8.6 volts, I got 7.65 volts off the trannie!  No wonder it didn't sound right.  When I turned the pot that I think is the bias pot, the lowest I could get the voltage was 6.62 volts.  So I assume those NTE trannies are not the right range to work in that circuit.  I haven't been able to check the other components yet to see if they are the standard FF values, but I assume they probably are, with the added buffer stuff.

So I think I'm going to try putting a matched pair of good germaniums in there and see if I can get it to bias correctly.  I'm thinking that if this thing is the stock FF circuit, some good trannies in there might get me in the ballpark, and the buffer will be a plus when using a wah in front.  Could be pretty cool, and it has a smaller footprint than the big round FF.

If anyone else has tackled one of these before, please give some input here, let me know what your experiences were with this pedal.  I think it has potential.

[spaceace76]

would it be possible for you to trace the circuit? it's not available anywhere, and many seem to be curious about the differences. any insight you can provide would be great 

by the way, here's the datasheet on the NTE trannies
http://www.nteinc.com/specs/100to199/pdf/nte102a.pdf

[brett]

Hi

and also has an input buffer to make it work with wahs better

While the buffer is good for those who use wahs, it diminishes the "loading" of the pickups and reduces the "bloom" that FFs are famous for.  The bloom is only obtained from driving a FF directly from a pickup.
cheers

[zombiwoof]

Hi

and also has an input buffer to make it work with wahs better

While the buffer is good for those who use wahs, it diminishes the "loading" of the pickups and reduces the "bloom" that FFs are famous for.  The bloom is only obtained from driving a FF directly from a pickup.
cheers

That may be true, but it seems to work OK in the newer MXR fuzz (the MXR version of the Jimi Hendrix Fuzz Face).  Although, in that pedal you can toggle the buffer on/off.  However, I think the Classic Fuzz has potential, even before modding it I can hear the Fuzz Face character, it just sounds like the trannies aren't biased right.

I had another thought about the trim pot that I think is a bias pot for one of the trannies, since I could only get the voltage reading down into the 6.x volts range, I'm thinking there might be a fixed resistor in line with the trimpot, which is a common thing to do when implementing a bias pot in FF's.  If I can locate this resistor, I could replace it with a lower value to get the bias more in line.

I'm going to try emailing Dunlop support about this thing, maybe they can clear up what the two trimpots are for.  A schematic would sure be nice, but that might be asking too much from them, they probably don't want to give out the circuit details, although I'm pretty sure it's just a stock FF circuit with an added buffer.  Can't hurt to try.

If anyone has traced the circuit in this thing or has a schematic, please let me know.

Al