Author Topic: Help with chunk chunk mod?  (Read 302 times)

jman77

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Help with chunk chunk mod?
« on: September 15, 2021, 10:41:46 PM »
So I’ve recently revisited my chunk chunk build. I noticed I socketed r9 which currently has a 40k in it. Build doc calls for 470k. For the life of me I can remember why I did this, and I can’t find any info searching. Any help?

JackSkellington

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Re: Help with chunk chunk mod?
« Reply #1 on: September 16, 2021, 02:05:40 AM »
40K? Weird value for a reseistor.
Anyway, if that resistor is smaller than stock 470k let pass more signal, so more gain, maybe a bit of more treble.
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Zerro

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Re: Help with chunk chunk mod?
« Reply #2 on: September 16, 2021, 04:06:31 AM »
Any buildplan for considering? I mean schematics.
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madbean

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Re: Help with chunk chunk mod?
« Reply #3 on: September 16, 2021, 05:50:02 AM »
So I’ve recently revisited my chunk chunk build. I noticed I socketed r9 which currently has a 40k in it. Build doc calls for 470k. For the life of me I can remember why I did this, and I can’t find any info searching. Any help?

R9/R10 form a voltage divider to reduce the output of Q2 going into Q3 (R10 is also part of the bias scheme for the JFET). So, a lower value of R9 means a bit more signal getting to the Gate of Q3. I don't recall anyone ever discussing modding the 470k value but it might have happened. You can change it back to 470k. It's should not require resetting the Drain trimmer, AFAIK, but that only takes a minute anyway.

jman77

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Re: Help with chunk chunk mod?
« Reply #4 on: September 16, 2021, 08:31:41 AM »
Any buildplan for considering? I mean schematics.
You can find everything for it in the archive page at the top of the shop page. Chunk chunk listed under the dust/boost/od section

jman77

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Re: Help with chunk chunk mod?
« Reply #5 on: September 16, 2021, 08:54:06 AM »
So I’ve recently revisited my chunk chunk build. I noticed I socketed r9 which currently has a 40k in it. Build doc calls for 470k. For the life of me I can remember why I did this, and I can’t find any info searching. Any help?

R9/R10 form a voltage divider to reduce the output of Q2 going into Q3 (R10 is also part of the bias scheme for the JFET). So, a lower value of R9 means a bit more signal getting to the Gate of Q3. I don't recall anyone ever discussing modding the 470k value but it might have happened. You can change it back to 470k. It's should not require resetting the Drain trimmer, AFAIK, but that only takes a minute anyway.
Following the schem I was able to see the signal flow into q3. I had no trouble biasing the build years ago. I also found the 470k labeled chunk chunk mod in my spare parts box. I built this 5-6 years ago. I could’ve sworn I read about that mod on some forum but it’s been a long time. Heck I think it was the last pedal I built until now. I’m getting back into it. The pedal I’m currently building has 2n5457s, preamp build. I bought a matched quad set. Although I can’t biasing them down to the proper voltage with the 20k trimmer listed for the build. I had a spare 100k so I dropped it in and was able to biasing q1 of that build to 4.25v. Now these 2n5457s are through hole. I have smds mounted to adapter boards on the way. I’m gonna drop one of them in and see if i can bias  it to 4.5-4.25 without changing trimmer. The packaging (great quality anti static) and the stats listed for each jfet has me thinking these through holes are genuine. Or else someone took the extra time and money to make me think they are. This got me to dive deeper into jfets. Which has me wanting to build a jfet matcher. I figured I’d try the j201s out in it when I do. Long story short, that’s the reason I opened up the chunk chunk.
https://i.imgur.com/aIXKw02.jpg

Zerro

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Re: Help with chunk chunk mod?
« Reply #6 on: September 16, 2021, 11:17:15 AM »
Maybe you could take a chance on BF245 too, for it's low noise figure. Setting of output electrodes DC values with bias trimmers will be very similar - cca 4-4,2V. For me, I would substitute those bias trimmers with fast resistors with closest higher values after setting proper bias for all transistors. Coz trimmer is mechanical contact, that will allways add some noise to the signal, especially after some time, when this mechanical contact will oxidate, simply getting old. Setting of couple Q4-Q5 is about setting 1/2 of Vb at Source of Q5. So don't care, what will be at Q4 Drain.
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