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Vintage threads from the first ten years

EH Small Stone & CS Tonebender problems

8/15/1999 9:55 AM
Arnaud	EH Small Stone & CS Tonebender problems Hi ! We started a discussion with R.G. about the GCB-95 buffer stage (see "Crybaby GCB-95 buffer?") which deviated to other effects questions (to be true, I was the only one to ask questions... R.G. knows about everything about effects). I just copy(and correct mistakes ...) the ones I posted at the end of the last message. 1) About the Small Stone. I have already understood the basic phase stage(with a phase spliter). I have also understood the MXR trick with the JFET drain-source resistor controlled by the gate-source voltage. Thanks to your article, I have finally understood the Small Stone concept (my sleep has been disturbed for a long time...) with this "negative resistor" simulation. I haven't thought a very long time since I read your article but I would have simply shown a part of the input voltage to the negative input of the OTA (which would have "converted" it into a current so that it would be equivalent to the inverted input voltage applied to a resistor). It seems that both input voltage and OUTPUT voltage are equally showed to the negative pin. Would it work without the resistor between the output of the buffer and the negative input ? I have written the (output volatge)/(input voltage) function (how do you call it in english?) and it doesn't work like a usual phaser: an interval of frequencies are cancelled and the input current of the OTA controls the highest frequency of this interval. But I'm probably wrong... 2) About the TB First, I must tell you I protoed it with AC 128's and I use the base-emitter of a AC 128 instead of the 1N270 diode (it sounded really bad with an non-matched Ge diode. Not like assymetrical clipping at all. More like a very bad noise gate at low guitar volume). My problem is that I admit the base-emitter diode with the 1N270 looks like a classical "back-to-back diodes" clipping section. But the transistor is not biased. Its base receives more or less current(controlled by the 'Fuzz' variable resistor) of the input signal. So when the input voltage is positive, the current goes in the 1N270 (so nothing on the collector because the transistor is blocked) and when it's negative, it goes in the transistor base (so a current appears in the collector). THIS IS NO CLIPPING AT ALL. IT'S JUST A TERRIBLE MESS! In fact, I'm sure my explaination is totally wrong. Help me. I also measured -35mV at the Q3 base. WHY ??? The transistor is not biased at all!!! Thank you for your help

8/15/1999 9:55 AM

Arnaud

EH Small Stone & CS Tonebender problems

Hi !

We started a discussion with R.G. about the GCB-95 buffer stage (see "Crybaby GCB-95 buffer?") which deviated to other effects questions (to be true, I was the only one to ask questions... R.G. knows about everything about effects). I just copy(and correct mistakes ...) the ones I posted at the end of the last message.

1) About the Small Stone.

I have already understood the basic phase stage(with a phase spliter). I have also understood the MXR trick with the JFET drain-source resistor controlled by the gate-source voltage. Thanks to your article, I have finally understood the Small Stone concept (my sleep has been disturbed for a long time...) with this "negative resistor" simulation. I haven't thought a very long time since I read your article but I would have simply shown a part of the input voltage to the negative input of the OTA (which would have "converted" it into a current so that it would be equivalent to the inverted input voltage applied to a resistor). It seems that both input voltage and OUTPUT voltage are equally showed to the negative pin. Would it work without the resistor between the output of the buffer and the negative input ? I have written the (output volatge)/(input voltage) function (how do you call it in english?) and it doesn't work like a usual phaser: an *interval* of frequencies are cancelled and the input current of the OTA controls the highest frequency of this interval. But I'm probably wrong...

2) About the TB

First, I must tell you I protoed it with AC 128's and I use the base-emitter of a AC 128 instead of the 1N270 diode (it sounded really bad with an non-matched Ge diode. Not like assymetrical clipping at all. More like a very bad noise gate at low guitar volume).

My problem is that I admit the base-emitter diode with the 1N270 looks like a classical "back-to-back diodes" clipping section. But the transistor is not biased. Its base receives more or less current(controlled by the 'Fuzz' variable resistor) of the input signal. So when the input voltage is positive, the current goes in the 1N270 (so nothing on the collector because the transistor is blocked) and when it's negative, it goes in the transistor base (so a current appears in the collector). THIS IS NO CLIPPING AT ALL. IT'S JUST A TERRIBLE MESS! In fact, I'm sure my explaination is totally wrong. Help me. I also measured -35mV at the Q3 base. WHY ??? The transistor is not biased at all!!!

Thank you for your help

8/16/1999 10:58 PM
marrk	Regarding the Tonebender, double check the polarity of your diode placement, and make sure your Fuzz pot is 250K, not 2M as indicated on the schematic. The resistor on Q2 emitter is 3.3k, it looks like 33k, but it isn't. Also, try a different transistor, AC128 are notoriously variable in quality. From what R.G. has said, all the fuzz happens with the diode and Q3. Try swapping that one first. The way Q1 and Q2 are arranged, you might even try silicon (heresy!) devices. While we are discussing the Tonebender, I've been trying to find a way to make the circuit more responsive to the guitar's volume knob. It seems whenever volume is set below 7 on the guitar, the sound gets stuffy and doesn't clean up too well. Can anything be done, or should I just keep using my fuzz face

8/16/1999 10:58 PM

marrk

Regarding the Tonebender, double check the polarity of your diode placement, and make sure your Fuzz pot is 250K, not 2M as indicated on the schematic. The resistor on Q2 emitter is 3.3k, it looks like 33k, but it isn't. Also, try a different transistor, AC128 are notoriously variable in quality. From what R.G. has said, all the fuzz happens with the diode and Q3. Try swapping that one first. The way Q1 and Q2 are arranged, you might even try silicon (heresy!) devices. While we are discussing the Tonebender, I've been trying to find a way to make the circuit more responsive to the guitar's volume knob. It seems whenever volume is set below 7 on the guitar, the sound gets stuffy and doesn't clean up too well. Can anything be done, or should I just keep using my fuzz face

8/17/1999 5:01 PM
Arnaud	Thank you for your help, Mark ! My Tonebender works (nearly) perfectly. I just don't know how it works. I think all the Q1,Q2 part works fine. It just amplifies the guitar signal. Maybe your problem with guitar volume below 7 comes from this stage but I would be surprised. I'm going to check it. The clipping stage is absolutely strange. Since I don't have an oscilloscope at home, I can't check how it works. I think we must consider the base-collector junction to understand it. A transistor can be seen as 2 diodes with a common cathode (in our PNP case) which is the base of the transistor. It looks more like a classical clipping section,now! But I'm not sure of it. I tried to check the bias of Q3. I unplugged all the caps and the Fuzz control. Referring to ground, I found -38mV on base, -2.4V on collector and -3.8V at the other side of the 18K resistor. SO THERE IS A CURRENT THAT COMES INTO THE COLLECTOR AND I DIDN'T PUT ANYTHING ON THE BASE !!! I swapped with another AC128 and I had about the same result. I don't know why a voltage appears on the base. Is the collector current a leakage current ? I heard Ge transistors had a lot of leakage currents. I only have AC128's as Ge transistor. Other Ge transistors are nearly impossible to find (in France, I mean ...). HELP ME, PLEASE !!! About your Fuzz Face, I think the TB is totally different. The TB is made to make a lot of noise (really huge with all controls at max, isn't it) and is not as subtle as the Fuzz Face. I think you can keep it !

8/17/1999 5:01 PM

Arnaud

Thank you for your help, Mark !

My Tonebender works (nearly) perfectly. I just don't know how it works.

I think all the Q1,Q2 part works fine. It just amplifies the guitar signal. Maybe your problem with guitar volume below 7 comes from this stage but I would be surprised. I'm going to check it.

The clipping stage is absolutely strange. Since I don't have an oscilloscope at home, I can't check how it works.

I think we must consider the base-collector junction to understand it. A transistor can be seen as 2 diodes with a common cathode (in our PNP case) which is the base of the transistor. It looks more like a classical clipping section,now! But I'm not sure of it.

I tried to check the bias of Q3. I unplugged all the caps and the Fuzz control. Referring to ground, I found -38mV on base, -2.4V on collector and -3.8V at the other side of the 18K resistor. SO THERE IS A CURRENT THAT COMES INTO THE COLLECTOR AND I DIDN'T PUT ANYTHING ON THE BASE !!! I swapped with another AC128 and I had about the same result. I don't know why a voltage appears on the base. Is the collector current a leakage current ? I heard Ge transistors had a lot of leakage currents. I only have AC128's as Ge transistor. Other Ge transistors are nearly impossible to find (in France, I mean ...).

HELP ME, PLEASE !!!

About your Fuzz Face, I think the TB is totally different. The TB is made to make a lot of noise (really huge with all controls at max, isn't it) and is not as subtle as the Fuzz Face. I think you can keep it !

8/17/1999 5:48 PM
marrk	Arnaud, I am glad to hear that it is working for you. I am a newbie with regard to the theoretical stuff. Perhaps some of the other folks here can help you out with that. I think you are right about Q3's function here, and perhaps it doesn't need to be biased when used in this manner. Both the base and collector are connected to the battery via small resistors. The fuzz pot is varying not only signal, but voltage as well. Leakage is probably a factor here also. I am trying to play more, and solder less these days. It is hard to resist the temptation to experiment. Enjoy your fuzz

8/17/1999 5:48 PM

marrk

Arnaud, I am glad to hear that it is working for you. I am a newbie with regard to the theoretical stuff. Perhaps some of the other folks here can help you out with that. I think you are right about Q3's function here, and perhaps it doesn't need to be biased when used in this manner. Both the base and collector are connected to the battery via small resistors. The fuzz pot is varying not only signal, but voltage as well. Leakage is probably a factor here also. I am trying to play more, and solder less these days. It is hard to resist the temptation to experiment. Enjoy your fuzz

8/17/1999 6:12 PM
Arnaud	Thank you for your encouragement ! I just want to add one thing: I don't think the fuzz pot varies the voltage on the base. Capacitors don't let continuous voltage pass. Only alternative voltages (like your guitar signal) pass. To study a bias point, replace all the caps you see on a schematic by an open-circuit. That's all PS: Think I should play more, too...

8/18/1999 1:04 AM
marrk	"Capacitors don't let continuous voltage pass". See what I mean about not knowing (or forgetting)stuff like this! Thanks, Mark

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