Silicon Tone Bender MKI

[mac]

This is my silicon version of the Tone Bender MKI.
I tried it to be as original as possible, except for some minor and necessary changes.

The input buffer has the same Z-In, since 4m7//1m2=1m
The second stage has a smaller cap because the fuzz pot does not need to go to 1k8 as in the germ version because of the bigger emiter-base voltage drop. Since there is a minimum 22k I reduced the 22u cap to 2u2.
The third stage has a trimmer at the base to set the voltage of Q3.
If you stick to hfe near 70 it should work with no further tweaks.



Enjoy!

mac

[jrod]

This looks cool! Thanks for sharing!

I had been messing around with the original circuit on the breadboard without much luck. I was getting terrible noise during note decay. I can't wait to give this one a try!!!

[superferrite]

Bless your heart, Mac!
I have a fat sack of low-gain Silicons

NOW, for the Mk II, por favor!

[mac]

Please note that even for Si, bias are highly dependent on hfe because the emiter is grounded.
Let me know your experiences.

NOW, for the Mk II, por favor!

jaja ok!
The TB Silicon Trilogy 

mac

[stinky]

Hi. I've tried the circuit but got a problem. When hitting the strings hard the sound was very compressed and dark at first but as the notes ring it opens up and gets louder. I don't notice it when lowering the guitar volume or playing softer. I really want to know what's wrong and how to fix it.
I used transistors with hfe 60-70 in Q1 (e 2v) and Q3 (c 8.5v)  and hfe 90-100 in Q2 (c 8.5v-5.8v).

Thanks for sharing the circuit!

[R.G.]

Can I get one of you guys who are furiously soldering these together to try my idea of adding leakage by soldering a reverse biased germanium junction across the collector-base of a silicon transistor?

I *think* this provides the leakage of a germanium transistor equal to the leakage of the diode. This is then amplified by the transistor, etc, etc. I'm up to my eyeballs in other stuff and can't try it out in any real way right now.

[rousejeremy]

Subbed for sound samples. My germanium MKI is noisy and annoying.

[mac]

Hi. I've tried the circuit but got a problem. When hitting the strings hard the sound was very compressed and dark at first but as the notes ring it opens up and gets louder. I don't notice it when lowering the guitar volume or playing softer. I really want to know what's wrong and how to fix it.
I used transistors with hfe 60-70 in Q1 (e 2v) and Q3 (c 8.5v)  and hfe 90-100 in Q2 (c 8.5v-5.8v).

Maybe it is the 2.2uf cap interacting with the fuzz pot path. Make it smaller, say 0.22uf.
Or set Q3 collector a little lower.

mac

[mac]

Can I get one of you guys who are furiously soldering these together to try my idea of adding leakage by soldering a reverse biased germanium junction across the collector-base of a silicon transistor?

I *think* this provides the leakage of a germanium transistor equal to the leakage of the diode. This is then amplified by the transistor, etc, etc. I'm up to my eyeballs in other stuff and can't try it out in any real way right now.

RG,
I have nothing to do the rest of the year except going to the beach, so I'll try your idea 

Since this circuit calls for leaky germs I think that new germ diodes have little leakage for it, most likely it will work with old ones.
BTW, a reversed diode and its leakage current can be considered like a big mega ohm resistor from B to C?

mac

[R.G.]

I have nothing to do the rest of the year except going to the beach, so I'll try your idea 

Go ahead, rub my face in it. 

Since this circuit calls for leaky germs I think that new germ diodes have little leakage for it, most likely it will work with old ones.
BTW, a reversed diode and its leakage current can be considered like a big mega ohm resistor from B to C?

Taking the question first: no, not quite. A reversed diode acts more like a constant current source (or leak) than it does a resistor. The leakage on a reverse biased semiconductor junction rises over a small-ish few volts of reverse bias, then flattens out a LOT. It's not perfectly flat, but the leakage remains reasonably constant, not rising linearly with reverse voltage like a large resistor would. It was this idea that actually kicked off the Millenium Bypass. It's very, very difficult to get resistors that are consistent to any degree, and much larger than 1M, which is what the Millenium circuit needed.

That being the case, going to an external device to supply "appropriate"  leakage is much simpler. You can choose diodes for leakage, using old unsuitable germanium devices for their collector base or base-emitter junction. You can use two (or more) modern germanium diodes with low leakage for germanium, but still 1000x the leakage of silicon til you get the leakage your circuit likes.

This offers a degree of freedom in using transistors that picking among old germaniums does not.

In combination with piggybacking to lower gain, it may well be possible to trim in a generic silicon transistor to sound like an old, low gain and leaky germanium.

Wow. Who'd have ever thought that there would be a use for deliberately making things worse. 

[LucifersTrip]

Can I get one of you guys who are furiously soldering these together to try my idea of adding leakage by soldering a reverse biased germanium junction across the collector-base of a silicon transistor?

after doing that, can we still use your test to get accurate results?
http://www.geofex.com/FX_images/xstrtest.gif

[R.G.]

Actually, yes. That test measures the current flowing in the collector in two conditions: (1) with only the transistor's internal leakage into the base, and (2) with an added external bias current added.

If there was a semi-internal leakage current added to the device under test from base to collector, it acts the same as an internal leakage, in that it funnels charge carriers into the base region.

There are probably micro-differences where this is not exactly the same, but I speculate that it will pick up much of the effect, just like piggybacking does a good job of lowering the effective gain.

[mac]

I've just discovered that the C2, C6, C4, C7 and C5 are a positive feedback path, and/or in combination with the resistors to gnd they are much like an oscillator.

Updating the schematic to avoid potencial oscillations or motorboating. Removing C6 and C7 an adding a cap across 47k (R12) to gnd.
Or making R12 a 50kB pot, and a cap between the wiper and gnd, as variable treble cut.

mac

[R.G.]

That would be very odd if the rest of the circuit is right. The big difference in size with C6 and C7 and the others makes it unlikely to oscillate unless there is something else helping it.

Is it grounded correctly, and is the power supply bypassed correctly?

[stinky]

Hi. I've tried the circuit but got a problem. When hitting the strings hard the sound was very compressed and dark at first but as the notes ring it opens up and gets louder. I don't notice it when lowering the guitar volume or playing softer. I really want to know what's wrong and how to fix it.
I used transistors with hfe 60-70 in Q1 (e 2v) and Q3 (c 8.5v)  and hfe 90-100 in Q2 (c 8.5v-5.8v).

Maybe it is the 2.2uf cap interacting with the fuzz pot path. Make it smaller, say 0.22uf.
Or set Q3 collector a little lower.

mac

Thanks, will try this when I've got some time for myself. The sound of fuzz pedal debugging is not appreciated by everyone.

[mac]

Stinky,
I tried hard playing, which I rarely do , and lowering the cap helps, loosing lows.
The problem is not the cap but Q2 hitting Q3 too hard. I added a 4k7 resistor between them and it works a lot better IMO.
This pedal has the sound of misbiased transistors 

That would be very odd if the rest of the circuit is right. The big difference in size with C6 and C7 and the others makes it unlikely to oscillate unless there is something else helping it.

Is it grounded correctly, and is the power supply bypassed correctly?

RG,
I did everything to get oscillations but you were right.
I only had problems when I put the metal cover under the breadboard, but oscillations stopped when I grounded the cover.

No need to remove the rool off caps.

mac

[stinky]

Adding a resistor after Q2 helped. I'm thinking a pre-gain would be a good addition to this circuit.
Now I want to try it with germ diodes as RG suggested and wonder if I got it right.
Is this how you simulate a leaky, low gain transistor?

[R.G.]

Is this how you simulate a leaky, low gain transistor?

That's the first iteration, OK.

Mac reports that the distortion changes on a first attempt. There will probably be some tinkering with bias as the leakage from the diode affects the bias point - as it's supposed to.

[stinky]

I have tried a germ diode instead of the extra resistor with Q3. I can't get Q3s collector voltage low enough, it's 9.2v (power supply 9.4v) and stays there even if I change diode. When measured according to http://www.geofex.com/Article_Folders/ffselect.htm the leakage varied from 100-300 uA with different diodes and a hfe 100 transistor. A germ transistor I tried (hfe 110, leakage 300) gave me collector voltage 8.5v. It feels like I've missed something.

[LucifersTrip]

I have tried a germ diode instead of the extra resistor with Q3. I can't get Q3s collector voltage low enough, it's 9.2v (power supply 9.4v) and stays there even if I change diode. When measured according to http://www.geofex.com/Article_Folders/ffselect.htm the leakage varied from 100-300 uA with different diodes and a hfe 100 transistor. A germ transistor I tried (hfe 110, leakage 300) gave me collector voltage 8.5v. It feels like I've missed something.

just curious...what happens if you use the ge transistor that gave you 8.5V as the diode (eb or bc) ?