Sandspur Fuzz biasing

[apm588]

Hi everyone,

Kind of a dumb question. I am starting a sandspur fuzz build and I was wondering how to "bias" it. I know there are two internal trim pots but was hoping someone could walk me through how to bias them with a multimeter. Specifically

1. Where do I put my multimeter poles on the circuit, and what values should I be biasing each towards?

Any help would be greatly appreciated. Thank you.

 

[bronomo]

Our own resident @jjjimi84 has a video series on the sandspur that includes biasing. Part 4 has more fiddling of the internal trimmers but this one has the multimeter walkthrough

 

[apm588]

Our own resident @jjjimi84 has a video series on the sandspur that includes biasing. Part 4 has more fiddling of the internal trimmers but this one has the multimeter walkthrough

This is exactly what I needed! Thanks for sharing. And thanks to @jjjimi84 for such an amazing video series. This is already a huge help

 

[jjjimi84]

This is exactly what I needed! Thanks for sharing. And thanks to @jjjimi84 for such an amazing video series. This is already a huge help

Anything for the community! I am glad it was helpful!

 

[Mir9]

On my build, I made the "clean" trim pot into an external control to work with different pickups. You're meant to use your guitar's volume control for that, but this helps if you use extra hot humbuckers, like I do.

That mod is offered for $25 on the real ones. This picture is from a nearly 300 page thread, but you can find the mod option on the official order page.

Analogman Sunface club

Here’s my Sunface collection. Latest addition is the 2sb175 direct from AM. It sounds awesome! Had them increase the max volume on it and it exactly what I was looking for. Added dc jacks on a couple others and can tell the difference in sound vs battery. Sounds a little brighter plugged in...

www.thegearpage.net

"Bias" is easy and that section is meant to be constantly adjusted to taste by the "Sun Dial it's in series with.
While you usually set and forget it on these type of pedals, the feature here is being able to to constantly adjust those controls to change character.

 

[giovanni]

Just finished my Sandspur and found this thread. Thank you @jjjimi84! I think this pedal sounds pretty good. But I am getting some weird artifacts if the fuzz knob is all the way up. If I pull back a smidge, the artifacts go away. These sound like a mix between a weird gating sound and space lasers. Is this normal? I biased mine so that with sundial at noon and the clean trim at zero the collector voltage is about 4.5V. The transistors are BC108 I got from Tayda. Both with hFE around 360.

 

[gtfields13]

Just finished my Sandspur and found this thread. Thank you @jjjimi84! I think this pedal sounds pretty good. But I am getting some weird artifacts if the fuzz knob is all the way up. If I pull back a smidge, the artifacts go away. These sound like a mix between a weird gating sound and space lasers. Is this normal? I biased mine so that with sundial at noon and the clean trim at zero the collector voltage is about 4.5V. The transistors are BC108 I got from Tayda. Both with hFE around 360.

Not sure I've heard this circuit sound like gating and space lasers, but I can make it squeal on high-gain NPN at max fuzz. I get feedback descriptions wrong, but when the Fuzz pot goes to zero, the feedback loop can go into runaway amplification or oscillation. I think I found this on Jack Orman's blog once, but haven't been able to find it again when I looked. Putting a smallish (10's to 100's) resistor in-line with the capacitor can cure the squeal.

 

[swelchy]

Just finished my Sandspur and found this thread. Thank you @jjjimi84! I think this pedal sounds pretty good. But I am getting some weird artifacts if the fuzz knob is all the way up. If I pull back a smidge, the artifacts go away. These sound like a mix between a weird gating sound and space lasers. Is this normal? I biased mine so that with sundial at noon and the clean trim at zero the collector voltage is about 4.5V. The transistors are BC108 I got from Tayda. Both with hFE around 360.

I've run into the same issue with high gain metal can BC108's on the same build.... Do you happen to have any BC108A's? Thats what I ended up finding with much lower HFE values and it pretty much stopped all the weird stuff

 

[giovanni]

I've run into the same issue with high gain metal can BC108's on the same build.... Do you happen to have any BC108A's? Thats what I ended up finding with much lower HFE values and it pretty much stopped all the weird stuff

Ah good to know! I didn’t socket these (the sockets I have don’t fit the PCB, for which the holes are spaced more closely to each other) so I’ll just leave them and keep the fuzz at 95%. Also I don’t have any subs at the moment.

 

[drew.spriggs]

FWIW, just slap a small NP0 cap from the base to emitter - 22-100pf is typically enough.

In the majority of situations it will remove the squeal at max gain.

 

[giovanni]

FWIW, just slap a small NP0 cap from the base to emitter - 22-100pf is typically enough.

In the majority of situations it will remove the squeal at max gain.

On Q1 or Q2? I’ll try that thank you!

 

[pfmorton]

The general rule of thumb for me is that a Ge Fuzzface style works best with the fuzz control dimed. The Si-style sounds wonky with the fuzz control dimed. I back mine off just a little bit at a time to find a sweet spot. Very cool to hear @drew.spriggs solution.

 

[drew.spriggs]

On Q1 or Q2? I’ll try that thank you!

Q1 typically - original FF has like +18dB of gain for Q1 (once you take into account the negative feedback), and around +1dB on Q2.

You can limit treble if you go too big. Just look at any of the OKF variants - most will have somewhere between 47 and 470pf across Q1, and from 0 to 47pf across Q2. You can use this as an easy way to mellow out a FF that has too harsh of a high frequency response from using high gain transistors.