Benson Preamp help!! [SOLVED]

[tome.talevski]

Moderator: Moved from the Benson preamp thread.

Hello - hope its ok to revive an older thread.. I've attempted this layout on Vero and for some reason can't get it to work! Im confident I populated the board correctly and fairly confident that my cuts are clean and that there are no solder bridges etc.. this is my 5th Vero build.. all the others have been successful. Here's what happens:

- the only signal/sound I get is when I turn the gain off/CCW and then turn it CW perhaps 5%.. theres a white noise scratch sound.
- This sound can me altered by the volume (louder/softer obviously) but also by the treble (sharper/duller)... the bass pot doesn't produce any audible difference.

Any help would be really appreciated!
Cheers!

[Cigano23]

- the only signal/sound I get is when I turn the gain off/CCW and then turn it CW perhaps 5%.. theres a white noise scratch sound.
- This sound can me altered by the volume (louder/softer obviously) but also by the treble (sharper/duller)... the bass pot doesn't produce any audible difference.

Any help would be really appreciated!
Cheers!

I suspect transistors and/or biasing is not correct. I recommend using trimpots instead of fixed drain resistors or trying more JFETs. I would definitely try adding the trimpots, first of all.

[tome.talevski]

Hey, thanks for the reply. I have two 50k trimpots installed as per the vero layout. I’ve also used the transistors on the layout - J201. Are you suggesting I reinstall another set of transistors? Thanks again!

[tome.talevski]

- the only signal/sound I get is when I turn the gain off/CCW and then turn it CW perhaps 5%.. theres a white noise scratch sound.
- This sound can me altered by the volume (louder/softer obviously) but also by the treble (sharper/duller)... the bass pot doesn't produce any audible difference.

Any help would be really appreciated!
Cheers!

I suspect transistors and/or biasing is not correct. I recommend using trimpots instead of fixed drain resistors or trying more JFETs. I would definitely try adding the trimpots, first of all.

Also, and this a newbie question, can incorrect biasing create what I’ve described above? I don’t have a multimeter handy (will get one across the next few days) so can’t accurately measure anything atm.. I naively thought that the bias trimmers would act rather dynamically and alter the properties of the pedal. But I think that’s nit the case and in fact incorrect biasing can prevent it working all together?

[tonyharker]

You could have fake J201s. They are not being made any more. You really need some test equipment if you are going to DIY and a multimeter is essential.

[tome.talevski]

You could have fake J201s. They are not being made any more. You really need some test equipment if you are going to DIY and a multimeter is essential.

Getting my hands on a multimeter today hopefully so will aim to post all the necessary numbers/values

[tome.talevski]

You could have fake J201s. They are not being made any more. You really need some test equipment if you are going to DIY and a multimeter is essential.

Getting my hands on a multimeter today hopefully so will aim to post all the necessary numbers/values

So, ive managed to get a multimeter and get some figures that might be useful in resolving this dilemma. FYI, I pulled out the j201 I had I there, soldered in sockets, and put a new set of J201 and the same issue remains.

So, in the interest of being thorough, I essentially replicated the Vero layout in this thread... im quite certain its accurate. Im also quite certain there are no unintended bridges across cuts or strips.. i used the MM to confirm this. I haven't changed any of the values of anything ... its, to my eyes, exactly like the layout. FYI, im using a circuit test pedal that to test this circuit... it definitely works so no issues re switches etc.

I measured the J201's, ground the black probe and pressing the red probe against each of the pins and the diode:


Q1:
D: 9.17
S: 1.3
G: 0

Q2:
D: 9.17 when trimpot 1 is CCW /// 9.07 when trimpot 1 is CW.
S: 1.25
G: 0

Q3
D: 8.45 when trimpot 2 is CCW /// 8.94 when trimpot 2 is CW.
S: 1.17
G: 0


D1
A (anode, the non-band end) = 0
K (cathode, the banded end) = 9.19


Again, any help is much appreciated... im hoping to learn quite a bit form this trouble shooting experience!! cheers



Here are a couple of pics:

[tome.talevski]

Hello,

I’ve posted this on the end of the benson preamp thread however just realised it’s probably better suited here.

I've attempted this layout on Vero, the layout in the benson thread, and for some reason can't get it to work! Im confident I populated the board correctly and confident that my cuts are clean and that there are no solder bridges etc.. this is my 5th Vero build.. all the others have been successful. Here's what happens:

- the only signal/sound I get is when I turn the gain off/CCW and then turn it CW perhaps 5%.. theres a white noise scratch sound.
- This sound can me altered by the volume (louder/softer obviously) but also by the treble (sharper/duller)... the bass pot doesn't produce any audible difference.

So, ive managed to get a multimeter and get some figures that might be useful in resolving this dilemma. FYI, I pulled out the j201 I had in there, soldered in sockets, and put a new set of J201 and the same issue remains.

So, in the interest of being thorough, I essentially replicated the Vero layout in this thread... im quite certain its accurate. Im also quite certain there are no unintended bridges across cuts or strips.. i used the MM to confirm this. I haven't changed any of the values of anything ... its, to my eyes, exactly like the layout. FYI, im using a circuit test pedal that to test this circuit... it definitely works so no issues re switches etc.

I measured the J201's, grounding the black probe and pressing the red probe against each of the pins and the diode:


Q1:
D: 9.17
S: 1.3
G: 0

Q2:
D: 9.17 when trimpot 1 is CCW /// 9.07 when trimpot 1 is CW.
S: 1.25
G: 0

Q3
D: 8.45 when trimpot 2 is CCW /// 8.94 when trimpot 2 is CW.
S: 1.17
G: 0


D1
A (anode, the non-band end) = 0
K (cathode, the banded end) = 9.19


Again, any help is much appreciated... I've read some stuff about twisting the pins on the JFETS?

im hoping to learn quite a bit form this trouble shooting experience!! cheers

Here are some pics:

[The G]

It looks like the trimpots are not doing anything, so I'd check the way they are wired. Unfortunately the second pic (which I presume contains the trace side) is not readable.

[tome.talevski]

It looks like the trimpots are not doing anything, so I'd check the way they are wired. Unfortunately the second pic (which I presume contains the trace side) is not readable.

Thanks for the response. I’ve wired them in as they are shown in the vero layout.. is there something obvious I’ve missed here?

[The G]

Repair the second image link, maybe we can spot something on the trace side.

LATER: Could you provide a link to the vero layout? I didn't find it at a quick glance.

EVEN LATER: Is it viewtopic.php?p=276534#p276534 ?

[tome.talevski]

Repair the second image link, maybe we can spot something on the trace side.

LATER: Could you provide a link to the vero layout? I didn't find it at a quick glance.

Here's the trace side and another image off the component side..

[tome.talevski]

Repair the second image link, maybe we can spot something on the trace side.

LATER: Could you provide a link to the vero layout? I didn't find it at a quick glance.

EVEN LATER: Is it viewtopic.php?p=276534#p276534 ?

yep, thats the one

[The G]

Probably you already did it: first check for shorts near the trimpots:

Then I'd try turning the trimpots all the way clockwise (so they're at full resistance), then I'd short the 10k resistors going to the drains of the JFETs, and then I'd turn s l o w l y (to not endanger the JFETSs) the trimpots counterclockwise until the drain voltage reaches 5V or so.
Wrong reasoning, see below.

[tome.talevski]

I'd try turning the trimpots all the way clockwise (so they're at full resistance), then I'd short the 10k resistors going to the drains of the JFETs, and then I'd turn s l o w l y (to not endanger the JFETSs) the trimpots counterclockwise until the drain voltage reaches 5V or so.

Interesting..! Im willing to give it a go.. by short you mean remove? Apologies, im only just beginning to grasp the methods and ideas that go into circuits and pedal building.

Also, do you think that could explain the why the circuit at the moment doesn't pass any signal/sound a part from the minor scratch sound within the first 5% of the gain sweep?

[The G]

I'd try turning the trimpots all the way clockwise (so they're at full resistance), then I'd short the 10k resistors going to the drains of the JFETs, and then I'd turn s l o w l y (to not endanger the JFETSs) the trimpots counterclockwise until the drain voltage reaches 5V or so.

Interesting..! Im willing to give it a go.. by short you mean remove? Apologies, im only just beginning to grasp the methods and ideas that go into circuits and pedal building.

Also, do you think that could explain the why the circuit at the moment doesn't pass any signal/sound a part from the minor scratch sound within the first 5% of the gain sweep?

By shorting them I mean connecting a wire between the legs of the resistors so they become 0Ω. I suspect your JFETs need smaller drain resistors for a good bias.

And not being corectly biased is the reason the circuit does not pass any signal - the JFETs (which are the active parts) function as amplifiers under the conditions provided by the build.


LATER: I cannot recommend enough that you build an audio probe, it makes following the signal through the circuit so easy.

[tome.talevski]

I'd try turning the trimpots all the way clockwise (so they're at full resistance), then I'd short the 10k resistors going to the drains of the JFETs, and then I'd turn s l o w l y (to not endanger the JFETSs) the trimpots counterclockwise until the drain voltage reaches 5V or so.

Interesting..! Im willing to give it a go.. by short you mean remove? Apologies, im only just beginning to grasp the methods and ideas that go into circuits and pedal building.

Also, do you think that could explain the why the circuit at the moment doesn't pass any signal/sound a part from the minor scratch sound within the first 5% of the gain sweep?

By shorting them I mean connecting a wire between the legs of the resistors so they become 0Ω. I suspect your JFETs need smaller drain resistors for a good bias.

And not being corectly biased is the reason the circuit does not pass any signal - the JFETs (which are the active parts) function as amplifiers under the conditions provided by the build.

Thanks again! ill give that a go... however, after passing the multimeter across those two sections you pointed out I think I may have found something.. the yellow circled area and the row above connect according to the MM.. although I can see any bridge or obvious connection on the trace side (I know it might look like there is on the picture..that actually the pin on an angle).

[The G]

[...]... however, after passing the multimeter across those two sections you pointed out I think I may have found something.. the yellow circled area and the row above connect according to the MM.. although I can see any bridge or obvious connection on the trace side (I know it might look like there is on the picture..that actually the pin on an angle).
[...]

That would not explain why does the first JFET behave in the same way, so I'd expect for that to not be the issue. Remember to turn the trimpots all the way clockwise, so they have the maximum resistance. You should have 50k between those rows.

[tome.talevski]

[...]... however, after passing the multimeter across those two sections you pointed out I think I may have found something.. the yellow circled area and the row above connect according to the MM.. although I can see any bridge or obvious connection on the trace side (I know it might look like there is on the picture..that actually the pin on an angle).
[...]

That would not explain why does the first JFET behave in the same way, so I'd expect for that to not be the issue. Remember to turn the trimpots all the way clockwise, so they have the maximum resistance. You should have 50k between those rows.

Update: just tried shorting the resistors and slowly turning the trimpots CCW however nothing really happened.. like you said earlier, its like the trimpots aren't really doing much..?

[The G]

Hmm, I reread what I wrote and it was blatantly wrong. I apologize. To make the drain voltage smaller we need to increase the trimpot resistance, not to decrease it .

I'd undo the shorting and replace the trimpot at the first JFET with a 1M potentiometer. If that works for the first JFET, you can try it at the second too.

I'm off to make a cofee...