U67 PSU from standard tube microphone PSU for SCT800

[mihi_fuchs]

Here is an option for a cheap mod of an existing PSU. It gives you the required voltages and clean power to supply your U67 clones.

You can find all info under the following link:
https://www.dropbox.com/scl/fo/00g1...ey=n513xh6lkvxsf1a578b0cdn9d&st=jc4sicnv&dl=0

Best regards,
Michael

 

[Khron]

Some of those traces are worryingly squeezed together... C101+ carries, what, 250V DC? Is there even half a mm between the corner of the positive pad, and that trace going to "H-"?

 

[Khron]

https://resources.altium.com/p/using-an-ipc-2221-calculator-for-high-voltage-design

In table 6-1, let's look at column B4 ("External conductors, with permanent polymer coating") - 101-300V requires a minimum spacing of 0.4mm. Don't know about you or anyone else, but i find that (the board layout / routing) kinda scary. And i noticed a similar thing on your 251 board, the B+ trace is literally butted right up against the heater trace I'm actually surprised the board fab was able to manufacture the boards, with trace / pad spacing THAT tight.

If you'd rather not redo the boards, at the very minimum least you should add plenty of thorough disclaimers everywhere, denying any and all possible liability for damage or bodily harm. Just saying...

 

[rock soderstrom]

Yes, that is indeed very close, and without any need, you have enough space to lay the traces safely.

Otherwise, handy project!

 

[Khron]

Just started thinking whether tube-based projects (and/or even just power supplies) should possibly be vetted / reviewed by someone qualified (not necessarily me). Some of the things i've seen around here (in general, not just this project) raise serious questions...

 

[gyraf]

as this is all diy, I don't think we can or should intervene in any other ways than by just pointing out the problems when we see them..

It would be a tremendous task to supervise everything, and would require detailed knowledge of local safety rules all-over the world

/Jakob E.

 

[Khron]

as this is all diy, I don't think we can or should intervene in any other ways than by just pointing out the problems when we see them..

It would be a tremendous task to supervise everything, and would require detailed knowledge of local safety rules all-over the world

/Jakob E.

I figured as much - well, just a thought...

 

[dmp]

Nice project! Is there a cheap place to buy this size PSU enclosure?

And a question, how does the mic SNR measure compared to the traditional PSU design? With the fixed bias design in the U67, the mic SNR is sensitive to the H-. The traditional PSU uses 20v AC and a 15V zener, followed by passive filtering, while your design starts with 9.5vAC, drops ~1.88v through the first two resistors and then uses the LM337 regulator. Also, what heater AC voltage does the H- start to crap out? If you drop from 9.5vAC I expect you'll hit the minimum voltage drop of the LM337. Typically, you want to design for the regulator to drop a few volts minimum so it can maintain regulation (it cannot regulate down to 0v across it), but obviously you don't have much Volts to work with starting with 9.5vAC

Also, it looks like you used a heat sink insulator. If you disconnect the heat sink pads from the ground plane, you can omit the insulator, which gives better dissipation and a lower regulator temp (but you are not dropping much so not a big deal).

 

[mihi_fuchs]

https://resources.altium.com/p/using-an-ipc-2221-calculator-for-high-voltage-design

In table 6-1, let's look at column B4 ("External conductors, with permanent polymer coating") - 101-300V requires a minimum spacing of 0.4mm. Don't know about you or anyone else, but i find that (the board layout / routing) kinda scary. And i noticed a similar thing on your 251 board, the B+ trace is literally butted right up against the heater trace I'm actually surprised the board fab was able to manufacture the boards, with trace / pad spacing THAT tight.

If you'd rather not redo the boards, at the very minimum least you should add plenty of thorough disclaimers everywhere, denying any and all possible liability for damage or bodily harm. Just saying...

Hey,

yeah, definitely the PADs are a problem there. I put them with the 0.4 mm from the IPC2221. However they should be double.

The traces are actually separated according to the IPC (0.4 mm). That's also the PAD distance. As said that needs change.
Same thing on the ELA251 and C12 (same PSU).

However, if you find that kinda scary, which NORM do you suggest for boards like that, as I actually checked the distance with the IPC (the PADs excluded - my mistake).

Thanks for your check - highly appreciated,
BR
Michael

 

[mihi_fuchs]

Just started thinking whether tube-based projects (and/or even just power supplies) should possibly be vetted / reviewed by someone qualified (not necessarily me). Some of the things i've seen around here (in general, not just this project) raise serious questions...

Actually - would love that ... but, I am of the opinion that Im responsible for my designs. Not anyone else. And I do not think the forum has the resources to do as you suggested, nor it would be wise with legal responsibility, I guess ...

But, as gyraf said - these hints are highly appreciated and help a lot, as many read those.

 

[mihi_fuchs]

Sorry for being late with getting back on the remarks - it's been busy times here, and I forgot to hit the watch button ... So I missed it, my apologies ...

 

[Khron]

The traces are actually separated according to the IPC (0.4 mm). That's also the PAD distance. As said that needs change.
Same thing on the ELA251 and C12 (same PSU).

European mains have up to ~330V between live and neutral - when's the last time you've seen mains input PCB traces separated by the bare minimum 0.4mm?

Especially when there's plenty of room on the board, why not use it?

However, if you find that kinda scary, which NORM do you suggest for boards like that, as I actually checked the distance with the IPC (the PADs excluded - my mistake).

As much as i can get away with - in a U67 (re)design i did within the last year, i spaced the B+ traces away from anything else by a good 2-3mm or so on every side. Call it "worst case scenario" thinking...

 

[mihi_fuchs]

Nice project! Is there a cheap place to buy this size PSU enclosure?

And a question, how does the mic SNR measure compared to the traditional PSU design? With the fixed bias design in the U67, the mic SNR is sensitive to the H-. The traditional PSU uses 20v AC and a 15V zener, followed by passive filtering, while your design starts with 9.5vAC, drops ~1.88v through the first two resistors and then uses the LM337 regulator. Also, what heater AC voltage does the H- start to crap out? If you drop from 9.5vAC I expect you'll hit the minimum voltage drop of the LM337. Typically, you want to design for the regulator to drop a few volts minimum so it can maintain regulation (it cannot regulate down to 0v across it), but obviously you don't have much Volts to work with starting with 9.5vAC

Also, it looks like you used a heat sink insulator. If you disconnect the heat sink pads from the ground plane, you can omit the insulator, which gives better dissipation and a lower regulator temp (but you are not dropping much so not a big deal).

I think the voltage I measured from the transformer was 11 V. I guess, that is a bit higher. To be honest, I can measure once I'm home, what voltage drop I had. I did not determine in the end, as far as I remember.

Noise was not a problem. The only problematic Mic I ever had with these PSUs was with the U47 clone/similar mic whatever using the ET806 tube. there the LM317 and also others (I tried a few) had serious noise issues. But there's an easy fix for that around here in the forum.

Regarding the U67, probably the filtering cap in the microphone does maybe help regarding the noise issue (C11 in the original schematic) ...

I had the PSU from the mics. However as far as I know it's the same as the alctron PSU T11A. Quite common and cheap to find. And you can reuse transformer, fuse and XLR connectors etc. I got mic+PSU for as low as 60 €.

Actually, and to be honest, I had to recheck - the Heatsink is not connected to GND. So you can omit the insulator.

I can measure next week the voltage drop over the LM337, if that's of help to you?

BR
Michael

 

[mihi_fuchs]

European mains have up to ~330V between live and neutral - when's the last time you've seen mains input PCB traces separated by the bare minimum 0.4mm?

Especially when there's plenty of room on the board, why not use it?

The plenty of room is a point I cannot deny . As the PADs need a rework it's no big deal.

As much as i can get away with - in a U67 (re)design i did within the last year, i spaced the B+ traces away from anything else by a good 2-3mm or so on every side. Call it "worst case scenario" thinking...

That's actually roughly the value with uncoated traces according to the IPC ... So you apply the same to both? Or treat them differently?

Thanks again,
BR
Michael

 

[Khron]

That's actually roughly the value with uncoated traces according to the IPC ... So you apply the same to both? Or treat them differently?

Why push my / your luck, when dealing with high voltages, especially when not completely forced to? "Absolute bare minimum" is a bit...