Tone Bender MKI - biasing and odd note decay

[gmr1]

Working on trying to get the MKI right. When using it with a Les Paul on just the treble pickup it has a bit of the velcro thing on lightly picked notes, and a wavery note decay - not "farty" more an odd shimmery treble thing. If both pickups are used, it's fine.

Is this all bias related? transistors not getting enough signal? It's a shame as it's really nice sounding when picked normal, it just decays so oddly. I'm afraid re-biasing the circuit to get rid of the odd treble response on the decay is going to leave it dull sounding.

I replaced the 180K with a 470K as I've read it has little effect the tone, and it makes the circuit easier to bias. Did seem to help.

I know it's a finicky circuit. anyone experience anything like the above? Thoughts as to a plan of attack?

Thanks!

[tcobretti]

Yep , it's a bias issue.  My Mk I isn't as bad as you describe, but it does pretty much gate the signal if I stop playing.  You might try plugging in different transistors to see if you have one that works better (this circuit works a bit better with slightly leaky transistors).  Otherwise I'd replace the resistors on the collectors with trimpots to find an appropriate value for the resistor there. 

[gmr1]

Yep , it's a bias issue.  My Mk I isn't as bad as you describe, but it does pretty much gate the signal if I stop playing.  You might try plugging in different transistors to see if you have one that works better (this circuit works a bit better with slightly leaky transistors).  Otherwise I'd replace the resistors on the collectors with trimpots to find an appropriate value for the resistor there. 

Yep (I also use the word "yep" all the time... odd!), I figured as much. I've built a lot of MKII's, and this is a different sound then I've dealt with - usually it's the typical mis-biased farting noise. I'm trying to find the right balance of a little bit of white noise, and the trans. not getting choked. The transistors have a little bit of leakage, and are in the ballpark of good gain range. I know the circuit has some shortcomings, but I really like the rawness of this one - I might just have to deal with a little noise. I tried the JMI reissue, and that was way too noisy for my liking. Thanks! - Greg

[mac]

Q3 biasing is similar to Q1 in a TB MKII, but is intended for higher leakage transistors. Tweak the 8k2, best voltage is around 7v to 7.5v.
Q2 is also a problem since the fuzz pot can send bias too high or too low. And pray for Q1 emiter to be above 2v or 3v.
Not easy to fine tune, but when you succeed it's a nice fuzz...

mac