Nobelium - tube bass preamp project

[Schisme]

Hey There,
Built a Nobelium kit and was very happy with it until I noticed a buzz in the high frequencies (enhanced if I turn the high pot up or the volume pot up). Noticed it was a ground problem because it would stop as soon I I touched pretty much anything : strings, pots etc. I am sure of my sound chain, isolated everything, no ground problem anywhere else.

It was a tight build and the PCB touched the enclosure. So opened it up, put the PCB outside the enclosure, turn it on and now it sounds like an overdriven tube and same buzzing problem. I am absolutely puzzled.

Do you have any idea ?

Thanks so much.

 

[Schisme]

 

[vigilante398]

Hey There,
Built a Nobelium kit and was very happy with it until I noticed a buzz in the high frequencies (enhanced if I turn the high pot up or the volume pot up). Noticed it was a ground problem because it would stop as soon I I touched pretty much anything : strings, pots etc. I am sure of my sound chain, isolated everything, no ground problem anywhere else.

It was a tight build and the PCB touched the enclosure. So opened it up, put the PCB outside the enclosure, turn it on and now it sounds like an overdriven tube and same buzzing problem. I am absolutely puzzled.

Do you have any idea ?

Thanks so much.


View attachment 70453

In this picture it looks like you have your ground lifted (the switch on the side by the XLR jack), do you get the same result with the XLR grounded? Also is this occurring through both the 1/4" jack and the XLR, or just one or the other?

 

[Schisme]

Hey,
Thank you for your answer, it occurs with jack and XLR, I should have precised. I mainly use the unit as a preamp to drive a power amp.
As a DI, the switch of course affects the sound but doesn’t change the problem.

 

[vigilante398]

Hey,
Thank you for your answer, it occurs with jack and XLR, I should have precised. I mainly use the unit as a preamp to drive a power amp.
As a DI, the switch of course affects the sound but doesn’t change the problem.

What power supply are you using with it?

 

[Schisme]

I have tried 3 different ones : one spot 9v (1700ma), Harley Benton power plant iso 2 (2000ma I think) and my trusted DC10 link. Always 9v.

I have tried in different rooms with wall outlets that are not in series in case I was picking up something like the fridge. Same everywhere. Tried two different amps.

 

[vigilante398]

I have tried 3 different ones : one spot 9v (1700ma), Harley Benton power plant iso 2 (2000ma I think) and my trusted DC10 link. Always 9v.

I have tried in different rooms with wall outlets that are not in series in case I was picking up something like the fridge. Same everywhere. Tried two different amps.

Any of those power supplies should be fine, you just need to make sure you're getting 600mA or more, so you're set.

Nothing is really jumping out at me from looking at the pictures, I'm afraid the best I can do is general advice:

• check HV rail voltage, should be around 240V or so
• check input voltage after the schottky (1N5817), should be just under 9V but not less than 8V
• reflow solder joints, particularly around the tube sockets and tube PCB pins
• check everywhere for solder bridges

I wish there's more I could do, but unfortunately when there isn't something visually obvious then that's about all I can do.

 

[Schisme]

I perfectly understand. Thank you for the informations.

The situation has updated though. Now all I get is overdriven sound (tube overdrive). The volume pot works as a volume, the bass as a kind of weird tone and the high doesn’t seem to work at all.

I had problems with this pot as he wasn’t out enough of the enclosure to screw the nut and I must admit I forced a tiny bit. The PCB seems fine but could all this come from a defective potentiometer ?

I am reading 8,86v at the schottky and 236,8v at R9.

EDIT : with the multimeter the pot seems to offer resistance and it varies as I turn it.
Bad tube ?

 

[vigilante398]

I perfectly understand. Thank you for the informations.

The situation has updated though. Now all I get is overdriven sound (tube overdrive). The volume pot works as a volume, the bass as a kind of weird tone and the high doesn’t seem to work at all.

I had problems with this pot as he wasn’t out enough of the enclosure to screw the nut and I must admit I forced a tiny bit. The PCB seems fine but could all this come from a defective potentiometer ?

I am reading 8,86v at the schottky and 236,8v at R9.

EDIT : with the multimeter the pot seems to offer resistance and it varies as I turn it.
Bad tube ?

If you're getting distortion no matter what then you may want to take a look at your tubes if you have others to try out.

 

[Schisme]

Swapped tubes, that’s not it. Same problem, I am now getting overdriven sound even with correct voltage readings

UPDATE EDIT :
I ordered my parts from Tayda electronics. And I am astonished to see that the 1M resistors measure 500K… Checked with 2 multimeters. I have difficulties to understand how I was reading correct voltages when turned on.

EDIT 2 : I also have weird readings on electrolytic capacitors (100uf that measures 170uf). So seems like I am in a desoldering party.

 

[Schisme]

Final update :

All works finally fine. What was the culprit ? I was running the Nobelium in the power section of an EBS HD 350 which was faulty overnight, without having been moved or anything. One capacitor in the drive section had one leg lifted from the pcb. So that was total surprise about the amp and that the drive section had an impact even when plugged to the return jack of the FX loop.

Long story short. It is fine. Thank you for your help and patience !

 

[JoeKewl]

I have a question about capacitor selection.
When not specifically marked on the BOM, what Voltage should I be looking for? I have been trying to figure this out and wait for someone else to ask. But decided to just go ahead ask. Sorry for the inexperience.

I have been modifying my own copy the original PDF with links to the products on the line I would be happy to share once I am done.

 

[vigilante398]

I have a question about capacitor selection.
When not specifically marked on the BOM, what Voltage should I be looking for? I have been trying to figure this out and wait for someone else to ask. But decided to just go ahead ask. Sorry for the inexperience.

I have been modifying my own copy the original PDF with links to the products on the line I would be happy to share once I am done.

I updated the BOM a while ago with links to suggested components, but if there is no voltage specified for caps I typically say 50V for film caps, 16V for electrolytics.

 

[JoeKewl]

I updated the BOM a while ago with links to suggested components, but if there is no voltage specified for caps I typically say 50V for film caps, 16V for electrolytics.

Perhaps I am using the BOM link incorrectly. Thank you for the reply.

I found the BOM with the links inserted. Thanks again

 

[fmcc]

I've ended up ordering these for the inductors: https://uk.rs-online.com/web/p/leaded-inductors/2400869 (and ended up with a few of them for this + other C2CE/Sushibox builds) - the footprint and lead size seem to be a wee bit bigger than those from the BOM (https://www.taydaelectronics.com/100uh-power-inductor-2-1-a-radial.html) but should they otherwise be OK if they fit?

The inductors ended up being the trickiest part for me to source in the UK, which is perhaps in part because I'm not confident with part substitutions for them. Does it form part of a filter for the power supply and would other types of inductors work?

 

[vigilante398]

I've ended up ordering these for the inductors: https://uk.rs-online.com/web/p/leaded-inductors/2400869 (and ended up with a few of them for this + other C2CE/Sushibox builds) - the footprint and lead size seem to be a wee bit bigger than those from the BOM (https://www.taydaelectronics.com/100uh-power-inductor-2-1-a-radial.html) but should they otherwise be OK if they fit?

The inductors ended up being the trickiest part for me to source in the UK, which is perhaps in part because I'm not confident with part substitutions for them. Does it form part of a filter for the power supply and would other types of inductors work?

You may have to do some creative lead bending as that's a 7.5mm pin spacing compared to a 5mm footprint on the PCB. But if you can make it fit then it should work.

 

[soutkari]

I'm a bit confused by R1 and C1 on the small board with the XLR jack. Are these components not in the BOM?

 

[vigilante398]

I'm a bit confused by R1 and C1 on the small board with the XLR jack. Are these components not in the BOM?

Whoops, you're correct they are not on the BOM. I'll get that taken care of. Those components are for the ground lift, so if you're not using the ground lift the preamp will still operate without them, but obviously you'll want them in there.

 

[Tassieviking]

G'day all
I asked this on Talkbass but no one seemed to be interested, maybe a pedal forum might be more interested in it.
I am about to start my Nobelium build very soon, just waiting on some Tayda parts to arrive and my tubes and some caps from Evatco (AU).
I was wondering if anyone has tried different capacitors for C8 in the HPF part ?
When I work out C8 and R18 4n7 and 220k I get a frequency of 154Hz at a roll off of 6dB, the original "claims" it is 90Hz and 6dB roll off.
Everyone seems to like it as it is on Talkbass so I presume it sounds great as it is.
If I change the HPF switch to ON-OFF-ON and add one cap across the switch I can get 2 different HPF frequencies (if I cut the track between post 2-3 on the switch).
I thought if you have a normal bass and an upright you might be better off with 2 separate frequencies for the HPF, and a small cut on the PCB and 1 extra capacitor to the build cost would be worth it. (You have to order a different switch though)
I am leaving my schematic and a list of frequencies for anyone who might be interested in what I worked out, but please be advised I might be completely wrong in my thinking.
I'm old, have a degenerative bone disease that leaves me in permanent pain so I ALWAYS have narcotic painkillers in my system and a screaming noise in my head. It gets a little bit hard to concentrate sometime is all.
Cheers
Mick

 

[fmcc]

When I work out C8 and R18 4n7 and 220k I get a frequency of 154Hz at a roll off of 12dB, the original "claims" it is 90Hz and 6dB roll off.

I'd had the same thought, and got the same corner frequency. My understanding is that the 90Hz/6dB numbers are -6dB/oct, and describe the slope of the filter. The slope of C8/R18 is 77Hz at -6dB/oct - I don't know how to produce a bode plot(?) for the frequency response of the filter, but I think that this probably does tally with the noble spec sheet saying: "Low cut switch drops 6dB/octave below 90Hz

If I change the HPF switch to ON-OFF-ON and add one cap across the switch I can get 2 different HPF frequencies (if I cut the track between post 2-3 on the switch).
I thought if you have a normal bass and an upright you might be better off with 2 separate frequencies for the HPF, and a small cut on the PCB and 1 extra capacitor to the build cost would be worth it. (You have to order a different switch though)

The schematic looks good to me - I didn't end up doing this, but I'd thought about using this approach to make it switchable between stock and something more like 40Hz. With it being a first order filter, I didn't think it'd really have the effect I wanted, so might build a separate HPF unit to run before it.