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Pertaining to the P.U.R.P. I built this a couple of weeks ago to be part of a multi effect drive. I tested it, and I really liked it, and set it aside waiting to be needed for the rest of the build. Yesterday, I wired it up with the other components I am using, and I noticed a couple of problems. First, the pot seems to work backwards. IE: If you turn it counter clockwise you get full output, and clockwise completely mutes the pedal. Also, the boost is much less or virtually non existent. Although the three way tone seems to be working. This is not consistent with how I remember this working. I don’t recall this circuit ever attenuating the main signal, but I could be wrong, and I’m pretty sure that I would remember the pot working backwards. Since I have a solid -9v rail and the bias voltage is @-7.1, I guess that I have managed to fry the transistor. I don’t have anymore 2n3906’s on hand, so I can’t “shotgun” it at this point. I did order replacements and they will be here this week.
Am I recalling correctly that the P.U.R.P. Offers a pretty significant boost that doesn’t attenuate and increases with a clockwise motion?
Do you think I’m on the right track with thinking the issue is the transistor? I surmise that the signal is traveling through the feed back rather than through the gain stage.
A picture would help with determining an issue.
Yes, the bias was too high. shoot for 4.5 to 6
Sounds like the boost pot was soldered backward (guessing without an image)
There is an overly detailed analysis of the PURP on the Guides Page.
Analysis of the PURP – Pump’d Up Rangemaster by BIG O
Thanks for your swift reply. I had review the build guide before building and after and it says the bias should be between 6.8- 7.8 My bias was at 7.1. So what would you recommend as the ideal bias for a 2n3906?
I had the pot on the solder side of the board with the shaft above the circuit board, like most of your other pcbs.
For troubleshooting, just now I soldered the pot so that it is still on the solder side but facing down (toward the foot switches.) Below I have included a picture of this after the change. I also changed the bias to 6.9. I tested again, and now the pot work in the desired way but there is no boost, with the pot fully open. There is also no grit. Also, turning the pot all the way down causes no signal to pass. All of these issues cause me to believe that the transistor is “toast.” I will remind you that I had tested this a couple weeks ago and everything was working and sounding great, as I recall. I really hope that the pot is backwards in the picture below, as it won’t fit in my enclosure if this is how it is supposed to go. Thanks again for your help and insight!
Addendum, should have read you reply more thoroughly, I will lower the bias even more.
The picture above is correct for pot orientation.
Also, there is a little number 1 to indicate lug 1 on the back of the board where the Boost pot is installed.
Make sure there is no contact between the back of the pot and the board solder.
If you need to you can always hand-wire a potentiometer to get it to fit in with your current project. Just use some heavier gauge wire to hold the potentiometer and PCB in place.
I would agree that the transistor is dead which would cause the rest of your issues.
Well as it turns out, I have this resolved. I retested with bias at 4.6 and all I got were “farting sounds,” (an industry term) In frustration I just started playing and turning the trimmer. Suddenly, I hit a point where great tone and boost broke forth. For giggles I A/B’ed it with my vintage pedal and I think that it sounds a bit better! The pot direction is still squirrely IMHO, but I have a work around for that. Now I can get this buttoned up and sent to its new home. Thanks for your help! BTW, the bias is now really low, about -2.1vdc.
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