Tonebender MKI Questions

[nation]

Dear All

I am in the process of building a Tonebender MKI and have a few questions:

(i) When my attack pot is turned fully CCW I get a volume pedal swell effect. When I strike my strings the volume swells quickly then drops off quickly, the remaining sustain is at a low volume. The intensity of this changes if I change transistors. Rolling down the guitar volume below 7 the pedal goes very quiet. If 7 'clock is fully CCW, the problem persist until I turn the attack pot above 8 oclock after which swell effect disappears.  I have checked the B50k pot and it is working correctly. My wiring and board look in good shape. Where else could I look to trouble shoot this?

(ii) With my attack pot above 9 oclock,  as I roll off the guitar volume below 7 I start getting large amounts of sizzle and gating in my note decay and notes die rather quickly to a scratchy end. I'm not sure how a MKI Tonebender is supposed to sound and I have read mixed comments on the MKI's behavior under volume roll off and youtube isn't helping much given the variety in MKI builds. I know I won't get the same clean up qualities of a Fuzz Face but is the MKI prone to more gated effects when the guitar volume is rolled off?

Specs of the transistors I'm using are:

Q1 - 62/57ua
Q3 - 127/140ua
Q3 - 82/130ua

I have checked voltages with the attack pot at 0 and full on and the C numbers are approx 0.5V higher than those posted for good working units. This is the layout I am using:

[thermionix]

"positiveve"

[nation]

"positiveve"

[thermionix]

"+ve Ground" on the layout.  I don't know what the extra "ve" is for.

[nation]

"+ve Ground" on the layout.  I don't know what the extra "ve" is for.

Yes, that's what I though you meant  Yes I have wired it positive.

Interestingly I was just testing an FZ-1 circuit I have built and it has the same volume swell effect with the Attack knob but a lot more extreme. Anywhere between full CCW and 12 oclock results in volume swelling.

Is it just a case of needing to keep trying different transistors?

[mac]

Did you try with different Q2,3 with more (or less) leakage?

mac

[nation]

Did you try with different Q2,3 with more (or less) leakage?

mac

I think this is what I need to do. Unfortunately the transistors I have are either low hfe/low leakage or high hfe/high leakage, I don't have any on hand circa 100hfe and in the 100uA to 300UA range. I did try a transistor at 180 hfe and 190UA in Q2 and Q3 and the swell effect was still there but better. Just want to eliminate a build problem as the cause of the volume swell. 

[duck_arse]

transistor voltages, circuit diagram, and photos of yr build, please?

25uF between Q1 and Q2? check the voltage at either end of that cap at both extremes of the gate attack pot.

[nation]

transistor voltages, circuit diagram, and photos of yr build, please?

25uF between Q1 and Q2? check the voltage at either end of that cap at both extremes of the gate attack pot.

Circuit diagram as per first post. No subs. Pictures of my build below.

Transistor voltages as follows with a 9.24V power supply:

Attack Pot full CCW        
Q1:   
E   0
B   -45mV
C   -9.20V
Q2:   
E   -283mV
B   -343mV
C   -9.24V
Q3:   
E   0V
B   -15.4mV
C   -9.09V

Attack Pot full CW        
Q1:   
E   0
B   -161mV
C   -5.4V
Q2:   
E   -280mV
B   -348mV
C   -9.24V
Q3:   
E   0
B   -207mV
C   -8.6V

My Q2 voltages are the same when the attack pot is on full CCW and full CW. I was under the impression that the attack post biases Q2?

Voltages on the 25uF cap between Q1 and Q2 are -323mV on the negative side and -161mV on the positive side.

[duck_arse]

Circuit diagram as per first post. No subs.

no. there is a layout diagram first post, no sign of the circuit diagram.

[EBK]

Circuit diagram as per first post. No subs.

no. there is a layout diagram first post, no sign of the circuit diagram.

Duck, is "circuit diagram" the Australian term for schematic?

[duck_arse]

um. well. if you said circuit diagram to me, I'd know what you meant. if you said schematic to me, I'd still know what you meant. do you think I ask "can I see a circuit diagram, please" too much?

[EBK]

um. well. if you said circuit diagram to me, I'd know what you meant. if you said schematic to me, I'd still know what you meant. do you think I ask "can I see a circuit diagram, please" too much?

It's not too much.  Just a subtlety that made me pause.  I read "circuit diagram" as an informal, slightly ambiguous phrase, and when you politely insisted that a circuit layout diagram wasn't a "circuit diagram", it made me wonder if, to you, "circuit diagram" was a formal, unambiguous term.  Just a bit of curiosity, that's all.   

[bluebunny]

Just for the record, I'd never heard of "schematic" until I came here.  It's been a "circuit diagram" in the motherland for centuries.    You funny Murcans and your crazy terminology!   

[Electric Warrior]

transistor voltages, circuit diagram, and photos of yr build, please?

25uF between Q1 and Q2? check the voltage at either end of that cap at both extremes of the gate attack pot.

Circuit diagram as per first post. No subs. Pictures of my build below.

Transistor voltages as follows with a 9.24V power supply:

Attack Pot full CCW        
Q1:   
E   0
B   -45mV
C   -9.20V
Q2:   
E   -283mV
B   -343mV
C   -9.24V
Q3:   
E   0V
B   -15.4mV
C   -9.09V

Attack Pot full CW        
Q1:   
E   0
B   -161mV
C   -5.4V
Q2:   
E   -280mV
B   -348mV
C   -9.24V
Q3:   
E   0
B   -207mV
C   -8.6V

My Q2 voltages are the same when the attack pot is on full CCW and full CW. I was under the impression that the attack post biases Q2?

Voltages on the 25uF cap between Q1 and Q2 are -323mV on the negative side and -161mV on the positive side.

"Q1" is actually Q2. 

[mac]

"Q1" is actually Q2. 

The first transistor's emitter voltage should be higher IMHO.

And Q3 base and collector should be "constant", unless temperature changed between readings.

Similar circuit,
http://www.geofex.com/Article_Folders/zonkmach/zonkbst.pdf

mac

[duck_arse]

um. well. if you said circuit diagram to me, I'd know what you meant. if you said schematic to me, I'd still know what you meant. do you think I ask "can I see a circuit diagram, please" too much?

It's not too much.  Just a subtlety that made me pause.  I read "circuit diagram" as an informal, slightly ambiguous phrase, and when you politely insisted that a circuit layout diagram wasn't a "circuit diagram", it made me wonder if, to you, "circuit diagram" was a formal, unambiguous term.  Just a bit of curiosity, that's all.   

oooohhhh, no, I'd never [please don't search my posts] say "circuit layout diagram". we're quite cosmopolitain and open minded here downunder, we've heard the word schematic, just don't use it. you'd prolly better check w/ Rob Strand and Slowpoke for second opinions.

and still no sight of the schem in question.

[ljudsystem]

I've got a mk I breadboarded. I've been trying to get it to sound good for a few days now, almost there. The sustain is pretty good but there's still a bit of gating.

I have the same problem as nation, I only got low leakage russians or high leakage chineese ac128s.

This is my board for the moment:

Q1: 70 hfe, very low leakage
Q2: 100 hfe, same
Q3: 60 hfe, same

The leaky germs created horrible crackling noises.

I've read in allot of forums that this circuit needs leakage. What I don't get is why 

How does the leakage effect the circuit?

Plus, does the hfe of q1 really matter? It looks like a unity gain buffer to me...

[ljudsystem]

Forget the question about leakage, I read Keen's article on the Zonk Machine.

[Electric Warrior]

I've got a mk I breadboarded. I've been trying to get it to sound good for a few days now, almost there. The sustain is pretty good but there's still a bit of gating.

I have the same problem as nation, I only got low leakage russians or high leakage chineese ac128s.

This is my board for the moment:

Q1: 70 hfe, very low leakage
Q2: 100 hfe, same
Q3: 60 hfe, same

The leaky germs created horrible crackling noises.

I've read in allot of forums that this circuit needs leakage. What I don't get is why 

How does the leakage effect the circuit?

Plus, does the hfe of q1 really matter? It looks like a unity gain buffer to me...

The leakage should help with the bias. These were probably first made with three OC75s, which are usually quite leaky. OC75s sure worked for me.

If I recall correctly, q1's hfe made an audible difference to me. Experimenting with different hfes/leakages for all three transistors certainly will pay off with this circuit. It's very interactive and tricky to get right.

Are you aware that Sola Sound/Gary Hurst tweaked the resistor between Q1's Q2's base and -9V while building these? They would cut out the resistor and solder a new one in or solder in an additional resistor in parallel. That should help setting the range on the attack pot and getting the crackle fixed. You could try picking a resistor that will set Q2's collector to half the supply voltage or even lower with the pot maxed.