Newbie Questions; Left in dark in another forum

geoffrey

Member
Hi All!
So some of you saw my first ever post and first PedalPbc build (VHS) in the build section the other day. This was my second successful build; my first build was with a board purchased from a competitor (who shall remain unnamed). Even though the build went well, I had some questions about the finished product; and so I did what most people would: went to their forum with my questions. Well, either their forum is dead or nobody wishes to help me, so since I received such a warm welcome from you guys the other day, I feel like I bring a few general beginner questions here and maybe get some help.

So this competitor’s circuit is meant to emulate
3 old fender amps of yore and simply has volume and drive controls. The drive packs a heck of a punch, as the description in the instructions state. But the description also says that it ranges “from a clean boost to overdrive”, however the thing is there’s nothing clean about even the lowest drive or volume settings on the one I built: it ranges from light dirt to pure filthy breakup. Don’t get me wrong, it’s pretty, but I wish it would start out as a clean boost as stated, and I’m wondering why it doesn’t. So my questions are 3:

1. The build contains 3 trim pots. Could how I set my trimmers have an affect on this issue? I know I installed them correctly, but I had no idea what to do regarding setting them afterword. I have never worked with trimmers before, so I just fooled around with the settings until I found the one I thought sounded the best.

2. Could switching transistors affect how clean/dirty it sounds? I used the 2N547 listed in the build instructions, but the instructions also suggested possibly experimenting with a J113, MPF102, or J201. How would switching the transistor up affect the sound?

3. And finally, could have I have done something wrong with the build to cause this?

forgive the long post and possible dumb questions. Keep in mind I’m just a beginner :). I’m really disappointed I was left in the dark in this competitor’s forum. I’m not sure if I’ll shop there again. PedalPcb and this forum have really impressed me thus far, so I think I might stick around.

Thanks in advance for your time and help! And if I should have posted these questions elsewhere in the forum, please kindly let me know
 

Robert

Reverse Engineer
Staff member
Without knowing the circuit / seeing a schematic it's tough to say how you should set the trim pots.

In a lot of cases you start by adjusting the trim pots so the drain of each corresponding JFET (2N5457, J201, etc) measures a particular voltage, usually 1/2 VCC. (4.5V for a 9V pedal, 9V for an 18V pedal, etc)
 

giovanni

Well-known member
+1 to what Robert said. Hard to say without seeing the circuit but my guess is that if the transistors are not properly biased you may get more dirt. Also where did you purchase the transistors? There are many counterfeit JFETS out there.
 

geoffrey

Member
Without knowing the circuit / seeing a schematic it's tough to say how you should set the trim pots.

In a lot of cases you start by adjusting the trim pots so the drain of each corresponding JFET (2N5457, J201, etc) measures a particular voltage, usually 1/2 VCC. (4.5V for a 9V pedal, 9V for an 18V pedal, etc)
The instructions stated to set between 4.5 and 6.5. But no matter where I adjust the trim pots, I couldn’t get near those numbers, which is where my ignorance fully comes out into the open. Now I fear I may be testing or using my DMM incorrectly
 

geoffrey

Member
+1 to what Robert said. Hard to say without seeing the circuit but my guess is that if the transistors are not properly biased you may get more dirt. Also where did you purchase the transistors? There are many counterfeit JFETS out there.
I’m now embarrassed to say I purchased my 2N5457s from Amazon

My J201s are from Mouser. I haven’t tried one here yet
 

geoffrey

Member
What transistors did you install? Where do you connect DMM?
I installed 3 2N5457s.

I was told to put one connector on the middle leg of the trimmer and then the other on one of the other legs, and then repeat to see if I get the seem reading with each, which I do, but I am checking ohms here not volts with this process…at least
That’s what I was told to do by a veteran builder :unsure:

Also, should all 3 transistors be adjusted to the same setting?
 

geoffrey

Member
Low and behold, I finally just got a response from the other forum. Coincidence? Anyway here is the reply I got:
“Ideally you want to bias the 2N5457s to between 4.5v and 6.5v using those trimmers and a multimeter. You’d power the pedal up and place the multimeter probe on the Drain of each 2N5457 the adjust the trimmer up or down until you get the correct value – the correct value depends on the 2n5457 (generally around 6.0v) and your taste (you could find that 5.9v is the magic tone or 6.2v is way better). The way the board is laid out the Drain is the leg of the transistor closest to the bottom of the board. You would have to do the same thing if you were using J113s, MPF102s or J201s.”

Do you guys agree that this is the process I’m missing here? Makes sense, but is completely new information for me
 

temol

Well-known member
Connect black connector to GND (for example negative connector of the power supply or barrel of the input/putput jack), red connector to drain of the transistor. Measure voltage at marked points - 1, 2, 3.
1669239702975.png
 

giovanni

Well-known member
I would be wary of transistors purchased on Amazon. If you can’t bias them properly, they may be grossly out of specs. You need a transistor tester to verify them or some other method for measuring their specs.
 

geoffrey

Member
I would be wary of transistors purchased on Amazon. If you can’t bias them properly, they may be grossly out of specs. You need a transistor tester to verify them or some other method for measuring their specs.
Yes, Giovanni, rookie mistake that won’t happen again. Then again, other than this “clean boast”
issue I’m on about here, the overdrive does sound pretty darn good, so maybe I got lucky this time

I’m going to try out those mouser J201s later because at least I know those are legit. And yes, a transistor tester sounds like a must buy for me.
 

geoffrey

Member
Most of the mods there, are members here. Nice folks. They're probably just busy with the holidays.
You’re right, Fig. My excitement for my new hobby has led to impatience, which in turn has possibly caused me to come off the wrong way. I’m really not “that” guy. Scouts honor

If I offended anyone or sounded uppity by my “being left out in the dark” attitude, I apologize.

I’m very, very grateful for all your help!
 

Diynot

Well-known member
Have you verified the values of the Source resistors (the 1k)? They are also responsible for biasing the 2n5457. If you aren’t getting the voltages to come into spec with just the trimmers wondering if there might be something amiss with those.
 

geoffrey

Member
Have you verified the values of the Source resistors (the 1k)? They are also responsible for biasing the 2n5457. If you aren’t getting the voltages to come into spec with just the trimmers wondering if there might be something amiss with those.
Yes, I verified all resistors before installing them, but now that you bring it up I’m going to double check.

Thanks for bringing this up…more knowledge for me to absorb

I’ll keep you all posted. Probably won’t get to it until this weekend unfortunately because I want to get on it right now :LOL:
 

temol

Well-known member
One general commet regarding transistors.
For example - J201 counterfeits from ebay are most often rebranded other models. I have a lot of them, they work for me, but I don't use them where J201 is needed. They are ok as source followers (in buffers). So even if you have transistors from an unreliable source, don't assume that the circuit is not working because of the transistors. If it's JFET, the pinout is preserved, then there should be something on the output.
Of course, there is still the matter of setting the appropriate drain voltage - if nothing changes despite rotating the trimpot - maybe the measurement is made on a different transistor, or the wrong trimpot is set? Or an incorrectly mounted transistor. Or a problem with the source resistor. Or....
If the range of drain voltage changes is insufficient (the voltage cannot be lowered enough) - increasing the value of the trimpot may help. But this may suggest that the transistor has parameters other than required in this particular circuit. If the voltage can't be raised enough - maybe there is a resistor in series with the trimpot (and minimal voltage is set by resistor value).
 
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geoffrey

Member
One general commet regarding transistors.
For example - J201 counterfeits from ebay are most often rebranded other models. I have a lot of them, they work for me, but I don't use them where J201 is needed. They are ok as source followers (in buffers). So even if you have transistors from an unreliable source, don't assume that the circuit is not working because of the transistors. If it's JFET, the pinout is preserved, then there should be something on the output.
Of course, there is still the matter of setting the appropriate drain voltage - if nothing changes despite rotating the trimpot - maybe the measurement is made on a different transistor, or the wrong trimpot is set? Or an incorrectly mounted transistor. Or a problem with the source resistor. Or....
If the range of drain voltage changes is insufficient (the voltage cannot be lowered enough) - increasing the value of the trimpot may help. But this may suggest that the transistor has parameters other than required in this particular circuit. If the voltage can't be raised enough - maybe there is a resistor in series with the trimpot (and minimal voltage is set by resistor value).
Fascinating stuff, Tremol. I’m really enjoying learning everything I can. This will be duly noted
 
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