Jordan Bosstone problems.

[Branimir]

I've built two bosstones, and both of them osciallate when the attack is maxed and guitar volume down, and if it doesn't oscillate (lower freq, like 200 or 300hz sound), it pickups up AM radio.

I've used 2N2222 for the npn and the BC556 for the pnp, 100nF in and out caps.

What's gone wrong here?


Here's the schem:


Also, I heard about Voodoo Lab Superfuzz is just bosstone with a few more controls.

Any ideas how to incomplement them here? Rat style tonecontrol before the clipping diodes?

Thanks!

[R.G.]

Try inserting a 1K resistor in series between R5 and C3.

[Branimir]

While this topic is still active...

Okay I'll try the 1k resistor, I guess it goes on the line that comes from the 18k, that's where I should insert the 1k right?

What does that C3 actually do? Since it's such a small value I guess it should shunt radio frequencies, but it's not towards ground, it's towards the 9V, so I'm a bit confused...

I tried switching the C2 for 100nF, went back to 22nF and I didn't notice much of a change, I guess this capacitor is here just to separate the 9V line from the changes on the base of the first transistor?

1k like this?

[aron]

It looks to me like a high frequency roll off filter. Very similar to the caps used on the Fuzz Face on the 2nd transistor.

[George Giblet]

Try a 50pF to 220pF shorted across the base and emitter of Q1.

If that doesn't work, in addition to above,  try leaving in the cap and adding a 1k resistor in series with C1, at the circuit end of the wire.

If that doesn't work, in addition to above, try adding a 100nF cap across the supply.

If that doesn't work,  need to play with the ckt to work out what is the source of the problem.

[Branimir]

I've tried 50pF from the base of the Q1 towards ground already before, and it did not help (at least not the oscillating), maybe picking the AM was not as present.

If I forgot to mention, The circuit works normally, but it starts to oscillate only when the attack is maxed out and input shorted to ground (guitar volume turned down). There's no oscillating and no picking up AM when the attack is maxed and the guitar's volume too, while playing or not, doesn't matter:)

[George Giblet]

>maybe picking the AM was not as present.

If something changes sometimes that can mean you are on the right track.  I'd play around with increasing the cap and adding the resistor.

Once you find a stable configuration you need to check/estimate  that added parts won't affect the tone.  If they do, you need to do some series investigations.

Problems like this can be exagerated if you have used unshield cables to the footswitch and or pots.

[R.G.]

1k like this?

No. On the other side of R5/C3, in series with the base of Q2.

If that doesn't work change to 470R between R5/C3 and 470R series between C3 and base of Q1.

The Bosstone is a beast to stabilize under some conditions. What you have to do is to introduce large enough losses at RF that it kills the oscillation. Generally this is thorugh shunt capacitance to ground, or effectively to ground. Because of the peculiarity of the Q1/Q2 cascade and the low impedance of the Q2 output, it's hard to do here. Where shunt capacitance doesn't work, you have to introduce series impedance. Just shunt capacitance may not damp the RF resonances enough.

This is another circuit where there ought to be some components that don't show up on the schemo. The +9V should be bypassed to ground before R6 with something like 10-47uF and a monolithic ceramic cap of 0.01uF or so. The electro gives a low impedance at audio, and the ceramic does the same job at RF where the electro can't.

It's interesting that input shorted to ground makes it oscillate. That almost yells that the feedback that causes oscillation is coming through the emitter of Q1; AC-grounding the base of Q1 makes for higher RF gain in common-base on Q1. That presents an alternate way of killing the oscillation - insert a low resistance in series with the base of Q1 as well. Try the resistance at R3/C5, and if that doesn't help try resistance at Q1's base.

In some cases shunt caps from base to emitter doesn't help with killing RF at all, it merely provides the same signal from a lower impedance, which makes it oscilate harder. Series R can introduce enough losses to kill the RF gain in those cases. But feedback through power supplies is always lurking.

[R.G.]

It's only fair to mention that although I've built a number of Bosstones with great success, there was one I could never cure with any of the tricks mentioned in this thread, and many others. I wound up beating it to power with hammer and building another, which worked first time.

Strange world, RF.

[Sir H C]

It's only fair to mention that although I've built a number of Bosstones with great success, there was one I could never cure with any of the tricks mentioned in this thread, and many others. I wound up beating it to power with hammer and building another, which worked first time.

Strange world, RF.

You Widlarized it? 

(from Bob Pease):

We still have a sign around our lab, "This is not a black-smith shop." But there were times when Bob would discover he had wasted a day or two, just because one bad part had screwed up his circuit. He would bring this bad part -- a capacitor, a pot, a transistor, an IC, or whatever -- over to the vise and lay it on the anvil part. Then he would calmly, methodically beat it with a hammer until the smallest remaining part was indistinguishable from the dust on the floor. Then he would go back to work and get the right answer. He explained that it makes you feel much better if you do this, and, you know that bad part will never come around again and goof you up. He was right. And I recommend that you join me in doing this "Widlarizing" when a bad component fools you. You will feel a lot better.

http://www.national.com/rap/Story/widlar.html

[R.G.]

I did. And I did so specifically in honor of Bob Widlar.

Bob Widlar was a true genius, one of the ones we lost too soon. You gotta love a man who works for a semiconductor company and who copes with the bean counters' cutting back on grounds keeping costs by sending out a purchase order for rolls of rope, stakes and goats. I find it particularly telling that National filled that purchase order.

[Sir H C]

My boss comes from the days that Widlar was designing in, cutting ruby to make masks and all that. Impressive stuff.  Have you read either of Jim Williams books on analog circuit design?  He is another biggie that rules and writes clearly and has brilliant ideas.