Nobelium - tube bass preamp project

What is the reason for having cathode bypass capacitors C13 and C14 in parallel? Maybe someone asked before, but I haven't found anything.
 
Smrtokvitek said:
What is the reason for having cathode bypass capacitors C13 and C14 in parallel? Maybe someone asked before, but I haven't found anything.
Click to expand...
Honestly I'm not really sure. I wouldn't expect any noticeable difference at all between a 220uF bypass cap and a 221uF bypass cap, and the cathode bypass capacitor calculator on Ampbooks reflects this. I left it in because it was in the original I traced, and unless there's something I honestly consider bad design, I'll leave it in to avoid people saying "THAT'S NOT HOW IT IS IN THE ORIGINAL!" even if it doesn't particularly make sense to me.

All that said, Jack (designer of the Noble) is more educated than I am, maybe he just knows something that I don't.
 
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That power supply is great for experimenting with, I ended up using that circuit to power a 1 watt amplifier pedal I made.
I used the power amp part of Rob Robinette's 1 watt Bassman 5F6A-M amplifier with that same circuit power supply but I made it all onto one PCB which fitted into a 1590BB box with the same slots as the C2CE drill files, yep, I copied the heck out of the C2CE parts that worked for me.
I really like that small power amp pedal but I will make a better version one day with a bit more gain, I only used 1 triode of the 12AY7 tube.


Robs amp (right down the bottom of the page)

Bassman Micro

Bassman Micro Amplifier
robrobinette.com
 
vigilante398 said:
Honestly I'm not really sure. I wouldn't expect any noticeable difference at all between a 220uF bypass cap and a 221uF bypass cap, and the cathode bypass capacitor calculator on Ampbooks reflects this. I left it in because it was in the original I traced, and unless there's something I honestly consider bad design, I'll leave it in to avoid people saying "THAT'S NOT HOW IT IS IN THE ORIGINAL!" even if it doesn't particularly make sense to me.

All that said, Jack (designer of the Noble) is more educated than I am, maybe he just knows something that I don't.
Click to expand...

That makes sense. I thought it was going to be this. :D Cudos for keeping it closer to the original. There is no downside if you don't think about the added cost. :)

I know about use cases in HiFi audio where people do this, but with a different type of caps because of lowering the ESR in specific applications. The ratio between them is usually also lower.

This is basically less than a 0.5% difference in the total capacity. That is an order of magnitude from the tolerance if you use really fancy 5% caps. :D
 
Smrtokvitek said:
That makes sense. I thought it was going to be this. :D Cudos for keeping it closer to the original. There is no downside if you don't think about the added cost. :)

I know about use cases in HiFi audio where people do this, but with a different type of caps because of lowering the ESR in specific applications. The ratio between them is usually also lower.

This is basically less than a 0.5% difference in the total capacity. That is an order of magnitude from the tolerance if you use really fancy 5% caps. :D
Click to expand...
Which makes me wonder if he did in fact use a film cap for the 1uF. I didn't take anazing notes during the tracing process, I just drew a schematic, so it's possible it could have been and I just don't remember. As cool as it would be to keep the original unit around to reference, that's a big investment to just have around to look at.
 
I was studying the preamp a bit, and I have another question. Why is the XLR out volume 500k and not a smaller value? The 500k is too big for the transformer that expects much lower impedance, so it easily could be 50k, for example. The loading of the second triode wouldn't be a problem, but the C7 would have to be increased to 470n or something like that to keep the bass response.

I get that it is how it was in the original unit, but let's take this from an educational perspective.
 
Smrtokvitek said:
I was studying the preamp a bit, and I have another question. Why is the XLR out volume 500k and not a smaller value? The 500k is too big for the transformer that expects much lower impedance, so it easily could be 50k, for example. The loading of the second triode wouldn't be a problem, but the C7 would have to be increased to 470n or something like that to keep the bass response.

I get that it is how it was in the original unit, but let's take this from an educational perspective.
Click to expand...
The original does not have an XLR out volume control, it is fixed at what would be "maximum" of a Nobelium. I wanted to make sure the pot value was significantly higher than the impedance of the transformer's primary so it wouldn't impact the transformer's response, and I thought 500k seemed like a reasonable choice.
 
Thanks, now I look at the transformer datasheets, and it should be fine. I was under the impression that the primary impedance is usually about the order of magnitude lower. With the Jensen, it should be fine to use 500k XLR pot with the 250k in parallel, as it expects 200k according to the datasheet, and the worst case should be 125k at 50% of the volume setting.
 
Smrtokvitek said:
Thanks, now I look at the transformer datasheets, and it should be fine. I was under the impression that the primary impedance is usually about the order of magnitude lower. With the Jensen, it should be fine to use 500k XLR pot with the 250k in parallel, as it expects 200k according to the datasheet, and the worst case should be 125k at 50% of the volume setting.
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Hey Smrtokvitek... I had someone ask me to build a Nobelium with a Jensen transformer... where did you find this almost mythical beast?
 
SamuelG said:
Hey Smrtokvitek... I had someone ask me to build a Nobelium with a Jensen transformer... where did you find this almost mythical beast?
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The only place I've ever been able to get them was direct from Jensen. I had to call their sales team on the phone and set myself up as a customer, then email a request for quote, agree to the quote, and pay the invoice. This was for an order of 3 units. It doesn't seem like they like catering to small builders.
 
SamuelG said:
Hey Smrtokvitek... I had someone ask me to build a Nobelium with a Jensen transformer... where did you find this almost mythical beast?
Click to expand...
I've been in this same situation for the past few months.
Short answer: You can't get them right now

I've talked to Jensen, radial, and every other distributor and no one has them. I even talked to Jack from Nobel and he's having a hard time getting a hold of them. There is a Rode branded one on reverb in Australia right now but I haven't been able to get in contact with the seller.

What I ended up doing was just waiting until I found a JDI that was cheap enough and then I pulled the transformer out of it.
With how hard they are to find, I just wouldn't offer a Jensen transformer as an option. Cinemag, Unagi, and LBA are all great alternatives.
 
SamuelG said:
I’ve messaged the seller some days ago, but I haven’t had a response.
Click to expand...
I should have read further down the comments :)
The Unagi unit is my go-to, but the guy loved the “Jensen” name. Has fallen through now in any case. I have just bought a FWBIM v3 so I’m selling my nobelium to fund more parts for another C2CE build :)
 
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