- This topic has 4 replies, 3 voices, and was last updated 1 year, 6 months ago by Anonymous.
March 19, 2019 at 7:49 pm #4892AnonymousInactive
I recently finished an MKC pedal. While it sounds fine basically, I noticed that it doesn’t have a lot of gain, especially when comparing it to another Klon-style pedal I borrowed from a friend.
So I hooked up my oscilloscope and found out that the gain stage IC1B only has a maximum gain of 3! According to this site it should have a gain of 100 at 1kHz:
I already checked all resistor and capacitor values, changed the opamp, checked for continuity and resoldered some solder points. I also tried lifting R13 and R7, but still the same gain.
Here are the voltages at IC1:
Any idea what might be the problem?
Thanks!March 20, 2019 at 8:00 am #4894BillyModerator
It should be a low to mid gain pedal R15 is the resistor to change you can do the ultra drive mod you’d wire in a dpdt switch with the stock 47K on one side and a 4K7 on the other which gives you masses of gain
So check R15 for cold joints etc initiallyMarch 20, 2019 at 9:18 am #4895AnonymousInactive
Hey Billy, thanks for your reply!
I am really talking just about the second Opamp (IC1B) here. With a gain of 3 it hardly pushes the Ge-diodes into saturation. R15 is just a summing resistor which has nothing to do with the gain of this stage, so changing it would just make the preceding signal louder and push the second opamp into saturation (which is not what we want, bearing in mind that the original had an even higher headroom in this stage!).
I know it’s not supposed to be a high gain distortion pedal, but between 3 and 100 there’s quite some difference! The circuit around IC1B uses basically the same values as the original, so it should at least be in the ballpark….March 20, 2019 at 12:55 pm #4896wilkie1Moderator
The circuits are not the same. The GPCB MKC adds a 100K resistor at R26 which changes the max gain of that stage to about 5. (1+ 470K ((R10))/ 100K ((R26))+15K((R9))=5.1.
Obviously, if you wish to increase the gain of that stage, you may reduce or jumper R26.
The circuit should still exceed unity output level with the stated values. If it does not, then there be a different issue.March 21, 2019 at 10:34 am #4905AnonymousInactive
Thank you very much! I overlooked that R26 affects the gain of this stage, now it all makes sense…
I jumpered R26 and now it sounds like a Klon to me! 😉
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