One of my older vero layouts for the Benson Preamp that I posted on Guitar FX layouts forum a while back - updated due to the bass and treble outer lugs being the wrong way around...
I have previously posted the point-to-point version, but I forgot to post the vero layout at the time. I never actually boxed this one up, as I prefer the Fairfield Barbershop as far as JFET overdrives go.
BENSON PREAMP - VERO LAYOUT
Hi, I just built this and it works great, thanks for your work, lots of cool stuff here. Have a question, the pedal works fine but i would like to know how to fine tune Q1, with the 12k resistor I get almost 7v on the drain, which is maybe correct but i think its a bit high. Looking the PedalPCB instructions they use 10k res plus a 50k trimpot and they suggest setting it to 12k so the 12k resistor makes sense here but I think there must be some parameter that can be measured to make a more exact adjustment. Do you have any suggestions? I don't understand why Q2 and Q3 have suggested voltages (4v) and Q1 doesn't...
ReplyDeleteChas Benson himself provided the "right" voltages.
Deletehttps://www.freestompboxes.org/viewtopic.php?p=276522#p276522
Q1 should be 5.2 while Q2 and Q3 should be 4V
Another crucial thing for getting the right voltages is using SMD J201 as many through hole J201 are unfortunately fake or out of spec.
Is the PedalPCB layout the same as the one posted in FSB?
For Q1 you could add a 100k trimpot instead of the 12k resistor in order to get the voltage of ~5V. Another option would be to try as many J201 as possible in that position to get the right voltage.
Deleteand finally: I just installed the 100k trimpot as I had a fixed 12k resistor in that position as well and dialed in the 5.2V while I had 6.2V before. I does make a difference, one I like :-)
DeleteMaybe a general advise for this build, use multiturn trimpots unless you have a very steady hand.
Hello. You can still buy J201 from mouser.
Deletehttps://au.mouser.com/ProductDetail/InterFET/J201?qs=OxRSArmBDfyvD8SbvWteMw%3D%3D
They’re just a bit expensive. I believe the through hole J201 have the same specs as the SMD and will vary just as much. Do not buy J201 from Tayda or any sellers in China on eBay.
A 20k / 25k trimmer should do the job. 100k will work, it just has a much larger range making it slightly harder to dial in.
Thanks for the info! with a 22k resistor it gives me 5.4v, it adds more sustain and has a better decay. As you guys say a trimpot is a good idea and then adjust by ear, maybe 5.2v is a good starting point for Q1. Thanks for the help.
DeleteHello. Q1 is a boost stage, where Q2 & Q3 are where the dirt happens. Ideally Q1 should be pretty clean, so it's biased a little differently. Just depends on how you like it really.
ReplyDeleteNormally I either include trimmers to bias on the vero board, or I breadboard it and test it before building (both by ear and on the scope)
Might be worth trying a 15k resistor. Good luck