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This was a question from a user no longer subscribed (see below)
I’m building a pedal with the Boogie 57 and the PURP. The build doc for the PURP says to use a 9v battery as the power source. The Boogie 57 build doc just says add power to the circuit. It does not specify that it has to be a 9v battery. I don’t plan on having a battery in the finished product. Do I need to use the battery to bias the PURP or will it work okay if I just plug the pedal into the wall?
I had the Boogie 57 plugged into the wall and was able to get Q1, Q2 and Q4 very close to 5v using the trimmers, but with Q3 I couldn’t get it below 8.1v. Will that be a problem and why do you think that’s happening? Is it a bad trimmer?
Thanks!
The only reason to mention a battery in the build document is so that people do not use a depleted battery when biasing. Honestly it should say “Do not ever use a battery”.
On the Boogie are you sure you are biasing the correct matching pair (trimmer / transistor).
Also how does it sound?
Okay. Thanks for clarifying that you should not use a battery when biasing.
On the Boogie, I am sure I’m biasing the correct matching pair because I can see the voltage change when I rotate the trimmer. It just seems weird that when I turn it as far as it will go, it won’t drop below 8.1 volts even though the other three will.
It sounds really good. My metal head friend, the guy I’m building it for loves it. I just wonder if it would sound even better if the voltage on Q3 were correct, around 5v.
You can check the trimmer adjusts resistance
It’s wired as a variable resistor, with no power to the circuit check the resistance between legs 1 and 3 the two legs next to each other, turn it fully CCW then CW and see if the trimmer resistance changes
You could also try swapping transistors if you’ve socketed them to see if it’s potentially a problem with Q3 transistor
Just swap any Q that biased ok with Q3 and see if you can get it to bias
Hey, I just had a tranny or 2 that wouldn’t come down past 8!!!. I had a bad B1M pot. Once the gain pot was replaced, all was good with the biasing.
Thanks for posting that Robert. I went back to test the pots and found that one of the leads from the B1M gain pot was not soldered onto the board. I soldered that on and it works now. I tested the pots anyway, just to see, and found that the A100k bass pot was at 50.1k and the gain pot was only .732M, but when I removed them and tested them again they were within spec. So I soldered them back onto the leads and they were back to .723M and 50.1k, but all the rest of the pots are where they should be and Q3 still won’t go below 8.32v. Any ideas why that might be?
You will probably not get a correct resistance measurement with the component wired into the circuit. It will be measuring other resistances in series or parallel with the resistance you are measuring. You must isolate the component before measuring.
Hello!
I recently completed the Boogie and it works except the volume. The pot is working and i hear the “correct” sound but its way to quiet.
Q1-Q3 (J201) are biased about to 5V but the trimmer for Q4 does not work and keeps at 8,6V.
Any Idea what problem it could be? Thank you!
Be sure you are biasing Q4 with the Q4 TR4. I say this since Q5 is literally right beside the TR4 Trimmer.
It is an easy mistake to make.
If that is not it then I suggest trying another JFET.
It may be prudent to verify all of your resistor values by color band. You can try a different type of JFET like J201, J113, MPF102, 2N5485 etc. which all have the same orientation. Also how does it sound as it is? Do you notice a difference in sound with the trimmer adjusted from CW to Center to CCW?
Low output volume could be a number of faults
Q4 drain does need to be biased to between 4.5 to 6v
Q5 is an output buffer and as you know doesn’t require biasing
Check the code on your trimmer is 104
TR4 is to set the bias voltage on Q4 drain pin it doesn’t directly affect the volume but it will if Q4 is not biased correctly and doesn’t amplify the signal as it should
I think you have your FET pins mixed up on your voltage readings,
Your drain and source pins are the wrong way around
If Q4 is incorrectly biased it will also affect Q5
I did a wee bit of work on this version to get a better range across the gain pot a while back so the circuits good
You can also check TR4 is working with no power to the circuit check the resistance value between the 2 outer pins and that it changes as you turn it
All your component values and orientation that I can see look correct
I found a v1 board batch same as yours and used J201 JFETs from a recent 1,000 piece batch that I sell.
You can see I am able to adjust the voltage.
Please be sure you are using the correct trimmer to adjust Q4 as it is real easy to grab the wrong one.
Also be sure you are using a 104 code Trim pot.
Hey i finally figured out that the TR4 is an 100 instead of a 100K pot… so this must be it right?
more obvious than expected to look for a wrong pot value… next time i ll triple check the stuff haha
So i ll order some new trimpots now, try it and give report if it worked out.
Thank you so much anyway without this forum i may had given up on this.
Greetings from Germany
Yeah that’ll do it Patrick once you get the correct trimmer if everything else is as it should be I’m sure with Q4 biased correctly it’ll amplify the signal and bring your volume up to where it should be
Problem solved! Sounds great! Thank you all again!
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