Nobelium - tube bass preamp project

[vigilante398]

really appreciate you posting this schematic i’m assuming this is the c2c nobelium?
i’m about to start building this one. very excited.
i don’t mean any disrespect to anyone involved in this circuit, but i’m just trying to comprehend why one would choose a 4n7 coupling cap on a ‘clean’ preamp intended for bass (on the 1/4” out) seems like it would cut a fair bit of low end..
or maybe it’s because that stage is supposed to overdrive if it gets a lot of volume? i guess i could always try a 10n or 22n and see if there’s much change

C8 isn't a coupling cap, it's part of the HPF for the "rolloff". The SPDT there jumpers over the cap, removing it from the circuit. Tweaking the value will adjust the corner frequency of the filter.

 

[owlexifry]

C8 isn't a coupling cap, it's part of the HPF for the "rolloff". The SPDT there jumpers over the cap, removing it from the circuit. Tweaking the value will adjust the corner frequency of the filter.

i saw that - that one makes sense. i was talking about the other one (C5). but it’s all good, it’s not hard to try different values

 

[vigilante398]

i saw that - that one makes sense. i was talking about the other one (C5). but it’s all good, it’s not hard to try different values

Ah gotcha. C5 is a little different because it's not a simple triode gainstage, that's part of the white cathode follower. Still nothing wrong with trying different values, but they behave a little differently.

 

[owlexifry]

it's not a simple triode gainstage, that's part of the white cathode follower.

ahhhh cool, i just read up on what these actually are. some sort of an oddball push-pull output driver arrangement. pretty nifty

 

[matmonster58]

What is the minimum voltage rating for C13 and C14?

I'm looking a Panasonic caps that only go up to 10V

also 2mm or 2.5mm lead spacing?

 

[vigilante398]

What is the minimum voltage rating for C13 and C14?

I'm looking a Panasonic caps that only go up to 10V

also 2mm or 2.5mm lead spacing?

The rule of thumb I was taught for cathode bypass cap voltage is B+/mu of the tube, which in our case is right around 240V / 20 = 12V. You might be okay with 10V caps, but it's a toss-up.

Lead spacing of C13 and C14 is 100mils (2.54mm).

 

[matmonster58]

Thanks! What is the purpose of C13? I can't imagine 220uF vs 221uF makes much of a difference

 

[vigilante398]

Thanks! What is the purpose of C13? I can't imagine 220uF vs 221uF makes much of a difference

I'm with you there my dude, no idea. But that's what was in the original unit I traced.

 

[Lekky]

Thanks! What is the purpose of C13? I can't imagine 220uF vs 221uF makes much of a difference

I suppose it's for suppressing oscillation when two consecutive gain stages share a cathode resistor. Maybe an additional parallel small-value bypass cap increases stability compared to just one big bypass cap.

 

[vigilante398]

I suppose it's for suppressing oscillation when two consecutive gain stages share a cathode resistor. Maybe an additional parallel small-value bypass cap increases stability compared to just one big bypass cap.

Very possible. The designer of the original has a lot more university degrees than I do, I'm sure there's something to it.

 

[matmonster58]

I know hifi guys will put a fast discharging cap in parallel with a bigger cap to prevent smearing of high frequencies but that's outside the range of most instruments. Noble strikes me as coming from the hifi world so perhaps it's a carry over.

 

[dra9his]

You're missing the 6k8 and trimpot before you hit the transformer, but otherwise looks good. The 6k8 inline with the signal is in the original, the trimpot was my addition because I don't like the idea of a fixed level output on a preamp and have heard some people complain the original's output is too hot.

View attachment 65135

Just curious, is Nobelium a reverse engineered Noble or just a Noble wannabe? (beside that's 9V run)
Is this schematic of a Noble's reversed engineered by you?

Also, does anyone know what's mains transformer? manufacturer and model?

 

[vigilante398]

Just curious, is Nobelium a reverse engineered Noble or just a Noble wannabe? (beside that's 9V run)
Is this schematic of a Noble's reversed engineered by you?

Also, does anyone know what's mains transformer? manufacturer and model?

Yup, the schematic of the Noble was traced by me, that's where Nobelium's design came from. I opted for an SMPS instead of the power transformer mostly for simplicity but also for cost.

The power transformer Noble uses is custom wound for the purpose and is not an off-the-shelf available part.

 

[dra9his]

Yup, the schematic of the Noble was traced by me, that's where Nobelium's design came from. I opted for an SMPS instead of the power transformer mostly for simplicity but also for cost.

The power transformer Noble uses is custom wound for the purpose and is not an off-the-shelf available part.

Do you know, who is manufacturing it and what's the output voltage? I don't understand from the pictures if it has HW and heaters or HW and 9V for PSU and another voltage regulator to get 6.3 for heaters or all 3 on 3 taps?

Thanks,
Andy

 

[vigilante398]

Do you know, who is manufacturing it and what's the output voltage? I don't understand from the pictures if it has HW and heaters or HW and 9V for PSU and another voltage regulator to get 6.3 for heaters or all 3 on 3 taps?

Thanks,
Andy

No idea, I already sold the unit I had purchased to trace. If I were to design it with a power transformer I would use a 12VAC tap straight to the heaters which I can then rectify and regulate down to 9V. Again I don't know for sure if that's what he did, but that's what I would do.

 

[dra9his]

No idea, I already sold the unit I had purchased to trace. If I were to design it with a power transformer I would use a 12VAC tap straight to the heaters which I can then rectify and regulate down to 9V. Again I don't know for sure if that's what he did, but that's what I would do.

I thought you might have some detailed pictures from inside. Maybes someone else that still has one might help me.
Thanks

 

[vigilante398]

I thought you might have some detailed pictures from inside. Maybes someone else that still has one might help me.
Thanks

I took some pictures when I first picked it up, I'll have to see if I can find them. My recollection is that nothing is labeled on the transformer, and since my plan from the beginning was to use an SMPS instead of a transformer I wasn't interested in checking voltages of the taps.

EDIT: I'm on a plane so I don't have anything better to do than look through my old pictures, and I don't see the pictures I remember taking. The one I traced for Nobelium is actually the second Noble I've owned, I bought one a few years ago and the first one was the one I got a lot of pictures of but didn't end up tracing. I got this phone in Spring 2021, and the pictures aren't here, so they must have been from my old phone, which means they're around somewhere on my computer.

 

[dra9his]

I took some pictures when I first picked it up, I'll have to see if I can find them. My recollection is that nothing is labeled on the transformer, and since my plan from the beginning was to use an SMPS instead of a transformer I wasn't interested in checking voltages of the taps.

EDIT: I'm on a plane so I don't have anything better to do than look through my old pictures, and I don't see the pictures I remember taking. The one I traced for Nobelium is actually the second Noble I've owned, I bought one a few years ago and the first one was the one I got a lot of pictures of but didn't end up tracing. I got this phone in Spring 2021, and the pictures aren't here, so they must have been from my old phone, which means they're around somewhere on my computer.

Oh, that's a commitment. thank you very much. it happens to me too, lots of pictures I thought I took are nowhere to be found.

 

[vigilante398]

For anyone that's been wondering, I have more Nobelium enclosures on order, I hope to have them back in stock in a week or so.

 

[vigilante398]

Nobelium enclosures are back in stock, I have a bunch now.

Pre-Drilled Enclosure

This option is to include a pre-milled enclosure for your PCB project. Enclosures are bare aluminum, no finish, CNC-milled. I try to get these done as quickly as possible, but please allow up to 3 …

c2celectronics.com

I also got my new daughter boards in that are compatible with the Unagi transformers, but I haven't had a chance to test them yet. As soon as I do I'll get an additional drop-down option to add a transformer when you buy a Nobelium PCB.