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).