Insight into "U47-inspired" with EF12 tube?

[Gus]

I was trying to give some advice about the web.  I have built a few microphones and changed a few circuits around in microphones (some call this simple stuff modding)designed solid state and tube gain stages for microphones for myself so what do I know?  I have also repaired a number of microphones for friends Lomos, Neumanns, Oktavas, china ones etc.   I have built both solid state and tube circuits and mixes.

My point is not too believe anything you read about microphones until you hear one or test one or build/mod 50 or more circuits.
I have heard some "web respected" microphone that I did not like  some >$3,000.  I have also heard some "web respected" microphones I thought were very good.

FWIW one example I bought a GT44 years ago because a "web guru" posted about how good it was.  That taught me to not believe what a certain person posted ever again on the web.  Don't forget people are often trying to sell you something.

Biasing matters but what matters more?   I heard two of one companies I believe 47 like "web respected" microphones that sounded like they were cathode biased.  I did not look inside the microphones because they were not mine.  They both had the m7 /47 capsule grill sound but were a little different sounding than a fixed bias circuit.  

But as always the Web exaggerates the sound differences in microphones.

 

[scott_humphrey]

Hi Gus,

It's good to be back after my 5 year hiatus. Did I miss much? Has anything new happened in the last 5 years in this forum?

I did have fun searching for your schematic. I believe that you are referring to this one:

You're using a resistor connected to the LED presumably to keep the current that flows through the LED from shutting off. This would make the LED operate at the flat portion of its curve (so the current through the LED doesn't shut off and screw up the cathode's bias voltage).

Do I understand this correctly?   ...and, are you connecting that resister to the heater voltage supply?

I don't think that you would need the smoothing capacitor that you've drawn in parallel with the LED, but let me know if you think otherwise.

 

[Gus]

Try with and without the cap bypass.  Also you need to be mindful of the semiconductor material and the curves

http://en.wikipedia.org/wiki/Light-emitting_diode

 

[gary o]

Sorry for confusion Skylar......interesting about capsule sound noticed similar in my few DIY mics.
I doubt I could here a neuman badge but I no what I like when I hear it so i'll experiment ask questons copy old circuits till I hear it hopefully i'll learn as I go

thanks for interesting info chaps.

 

[Gus]

Soapfoot

Google, Bing, Ask etc
Ohms law
power in circuits
Thevenin
Thevenin and Norton
Triode

If you are the same soapfoot I noted you got wrong answers to some questions at Klaus's.

A u47 is not cathode biased(a very small amount if one wants to be 100% correct but not to any extent it really matters to the operation of the tube) it might look that way but think about it.

 

[soapfoot]

Thanks.

The bias in a U47 comes off the heater supply, correct?

 

[electroslut]

yes.

 

[electroslut]

since R3 is there, there is still 1.2 volts above ground.

 

[soapfoot]

OK, here's a draft I made.  Any thoughts/corrections/advice etc.?

 

[rodabod]

Are you aiming for 1.5V bias? You could swap that 60R resistor for a trim pot.

 

[soapfoot]

I did some searching around looking for the "sweet spot" for EF12 bias.  I found one schematic for an EF12 tube mic, and the bias voltage was 1.67v.  This made sense to me, because the EF12 is an ancestor to the EF86, and I've read (earlier in this thread, actually) that 1.68v or thereabouts is the 'sweet spot' for an EF86.

So I just figured I'd start there, and experiment if I had to.

 

[mad.ax]

>Soapfoot
The biasing of the U47 is not about the value of the 29 ohms resistor, but more about the current flowing through it...
My advice: Study how Max did it in his MK7...

Axel

 

[soapfoot]

>Soapfoot
The biasing of the U47 is not about the value of the 29 ohms resistor, but more about the current flowing through it...
My advice: Study how Max did it in his MK7...

Axel

Axel,

Thanks for your comments.  I'm not sure what you mean, however.  The voltage divider created by the 60R and 29R in my schematic above should give about 1.6-ish volts if my calculations are correct, which was my target.  What do you think is incorrect about this arrangement?

 

[mad.ax]

I didn't say it is incorrect!
I just want to point out that 29 ohms is not a 'magical' number, neither would be 1.1, 1.25, pr 1.6V bias...
This famous 'sweetspot' may be 1.6... Or more... Or less... (Like in the MK7 schematic...)
The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

 

[gary o]

Im gonna be making a U47 ish mic soon was thinking of have a trim pot on the outside of the mic case and a DVM connected so I can sing n record the mic and alter the bias as I go......cant wait for my parts to arrive....very interesting stuff.

 

[soapfoot]

I didn't say it is incorrect!
I just want to point out that 29 ohms is not a 'magical' number, neither would be 1.1, 1.25, pr 1.6V bias...
This famous 'sweetspot' may be 1.6... Or more... Or less... (Like in the MK7 schematic...)
The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

I see what you're saying now.  Yeah, that seems like a good idea.

 

[soapfoot]

OK so I've just started thinking about designing a regulated power supply for this thing.  I've been reading the power supply meta and it's a bit overwhelming.  A lot to digest there.  I've also been looking at other microphone power supplies for examples.

Seems that some people think regulation is a good idea to ensure consistent sound from day-to-day, and some think it's totally unnecessary.  At the very least it seems it would help filter out that last tiny bit of ripple.

I'm always down to learn something new. 

Stewart, designer of STU47: what did you use for a power supply?  Regulated or unregulated?  Do you have a schematic?

 

[gary o]

Well Im gonna knock this up in a hurry and gonna modify to give me 120VDC for another mics HT

oops forget to post the link

http://www.groupdiy.com/index.php?topic=41379.0

 

[boogietube]

I think this is a good place to start the psu:
http://www.twin-x.com/groupdiy/albums/userpics/MK7_psu.JPG

 

[boogietube]

There is a BOM, and the transformer can be bought here:
http://www.edcorusa.com/default.aspx

This was their reply to my e-mail inquiry:

That is a PWRC100-1 cost is $23.03 each.  You can call in at 800-854-0259 to place your order as this item is not on the website.

Phyllis Weston
EDCOR Electronics Corporation
7130 National Parks Hwy.
Carlsbad NM 88220
Phone (575) 887-6790
Fax (575) 887-6880
www.edcorusa.com