Son of Ben (Benson) low voltage on Q2 and Q3

[Stephenthegun]

I didn't figure the thread on how to bias the Son of Ben trim pots was the place for this question, so I thought I would start a new thread. I have finished the Benson, but when it came to Q2 and Q3 I have some odd voltage readings. Neither Q2 nor Q3 when all the way up reaches anywhere near 9v. I am using 50k pots as specified and at 100% Q2 is 3.27v and Q3 is 5.26v. I biased Q3 down to 4v and left Q2 maxed out. I must say, it sounds great, but if I could make it sound better I would like to. Would there be an easy explanation as to why I am not reaching 9v or near it with these trim pots? And, if there is something I can do to fix it what might that be?

Not sure how relevant this is, but I did set Q1 to 2K per Chuck's instructions and the voltage is reading 2.58.

Thanks for your help!

 

[spi]

Shouldn't Q2 and Q3 be set to 4v?

 

[benny_profane]

First, you won’t be able to get 9V at the drains from a 9V supply because there’s a 10k series resistor. Second, where did you get your JFETs? Q1-drain seems rather low if you have the trimmer set to 2k.

 

[Stephenthegun]

Shouldn't Q2 and Q3 be set to 4v?

View attachment 15933

Yes, the problem is I can’t get Q2 up to 4v and Q3, even though it was more than 4v still seemed weaker than it should be when maxed out.

 

[Stephenthegun]

First, you won’t be able to get 9V at the drains from a 9V supply because there’s a 10k series resistor. Second, where did you get your JFETs? Q1-drain seems rather low if you have the trimmer set to 2k.

i got my J201’s from Banzai. They are through hole, but they are working in the pedal just with low voltages, though Q3 is set to 4v.

I thought I had read on the “how to bias” thread that they should be near 9v when the trim pot is maxed out.

I will check to make sure I am still at 2k on Q1.

 

[Boba7]

If I remember correctly Q1 should be biased at 5.1v (I'd trust that voltage instead of the 2k resistance) and Q2/Q3 at 4v.
I use a 50k trimpot for Q1 and 100k trimpots for Q2/Q3. Maybe try a 100k for Q2 and see if you can bias it at 4v.

Yes, the problem is I can’t get Q2 up to 4v and Q3, even though it was more than 4v still seemed weaker than it should be when maxed out.

I don't understand "Q3 seemed weaker than it should be".

 

[Stephenthegun]

If I remember correctly Q1 should be biased at 5.1v (I'd trust that voltage instead of the 2k resistance) and Q2/Q3 at 4v.
I use a 50k trimpot for Q1 and 100k trimpots for Q2/Q3. Maybe try a 100k for Q2 and see if you can bias it at 4v.



I don't understand "Q3 seemed weaker than it should be".

I will re-bias Q1. Thanks for the advice.

In the trimpot adjustment thread, Chuck mentions that "At one end of the trimpot travel, you should get close to 9V. At the other end, you should get substantially less, maybe something close to zero. If that isn't happening, there is something wrong with your pedal." This concerned me as neither of my trimpots got anywhere near 9v.

 

[Boba7]

I will re-bias Q1. Thanks for the advice.

In the trimpot adjustment thread, Chuck mentions that "At one end of the trimpot travel, you should get close to 9V. At the other end, you should get substantially less, maybe something close to zero. If that isn't happening, there is something wrong with your pedal." This concerned me as neither of my trimpots got anywhere near 9v.

As long as you get about 4v on both transistors (Q2 and Q3), you shouldn't worry about it.

 

[Stephenthegun]

As long as you get about 4v on both transistors (Q2 and Q3), you shouldn't worry about it.

Okay, thanks for that. I was concerned that something else was wrong, this puts me at ease.