how do I use a 50v bias tap for a 48vDC mic phantom power?

[mike_mccue]

bump... in hopes someone can offer some comment. :-)

[tubeswell]

I am also curious about the hi voltage power supply... I think I have predicted the correct voltage values on the secondary now that I have placed the 100uf 500vDC cap at the beginning of the filter stage. Does it appear that I'm making that estimation correctly?

Hi Mike

I've never seen a 12Y3GT datasheet, but if you can find one, it should tell you what the current output of that tube is and the voltage drop as well as the maximum recommended reservoir capacitance (intuitively the 100uF seems to be too high - the 'Y3GT' nomenclature suggests to me it is likely to be in the same ballpark as a 5Y3GT, for which 40uF is pushing it - but I could be way wrong about this). Aside from that, I'm sure you are aware that with the CLC filter you've changed it to, you don't need as-big-a-choke to get to your desired B+ (as you would with a choke- input filter), and your B+ will also be higher. Using Merlin b's rule of thumb for CLC filters you could even use 20uF reservoir and 20uF output cap with a 5H choke and get excellent filtering results. JM2CW

[Steve Conner]

OK, here are some comments:

I don't believe there is any such tube as a 12Y3GT. And if there was, it would have a 12V filament, so it wouldn't work off a 5V heater winding. You probably meant a 5Y3 of some sort. For a tube mic preamp, the 5Y3is probably overkill, the 6X5, EZ80 or whatever is probably more suitable. Unless you're going true vintage style, with a push-pull line output stage powered by 6V6s, that puts out enough voltage into 600 ohms to double as a cattle prod.

The 70V DC is still over the rating even for a LM317HV. This means that if you short the phantom power, the LM317 will blow. It will most likely fail short and put 70V onto your mic. You need a zener diode or dropper resistor in series with the input of the LM317, to drop some voltage. I showed how to do this in the schematics I posted earlier.

Let's check your calculations for the LM317 resistor values. First calculate the adjust pin current with Ohm's law: 1.25v/220 ohms = 5.68mA.

Now multiply this by the other resistor: 0.00568 * 8228 = 46.75V.

Finally add 1.25 to get the actual output voltage: 48V. Yay!

[mike_mccue]

Thanks!!!

I fixed the 12YGT typo to read 5YGT. Thanks for catching that.

I'll consider the comments and be back later... I just wanted to settle the issue with the typo.

best regards,
mike

[mike_mccue]

I added the dropper resistor to the schematic:



Now I'm off to re think the hi voltage power supply.

Thanks again everyone,
mike

[tubeswell]

Well with a 5Y3GT off 270VAC-0-270VAC, you'll only get something like 300VDC. I would only go to 40uF max with a reservoir cap for your CLC filter with this tube. But if you use a GZ34, you'd get around 350VDC and you could easily go to 60uF for the reservoir (2 & 1/2 cents worth this time)

[mike_mccue]

Hi,

It took me a while to get back to this. Thanks for the reply.

I'll spec C01a at 40uF.

How about the other filter caps? I realize 500uF caps are excessive but had planned for them to entertain the notions of a previous mentor who had an interest in stereo and bass amps. I do have the 500uf caps here and paid for, but am willing to downscale if that's for the best.

thanks again,
mike

[mike_mccue]

Hi,
I'm just bumping in hopes that anyone with better sense than I can offer, repeat, or make note of any un acknowledged opinions before I start drilling into a chassis and building a layout board.

:-)

Thanks again for all the help.

best regards,
mike

[defaced]

I'd probably go overboard and make the 100 ohm resistor 2 watts. Right now you have a 100 ma fuse, assuming the resistor will see the full 100 ma in a fault condition, it'll dissipate 1 watt. Since you generally want a margin of error on power ratings, and the next common size is 2 watts, I'd use 2 watts.

As for the 500 uf caps, I'd at least try them. They'll probably be pretty big physically, so you may want to get something smaller that will work just as well.

[mike_mccue]

I had come up with the same reasoning... I'll spec a 2 watter for now.

thanks,
mike