Insight into "U47-inspired" with EF12 tube?

[zebra50]

I don't think it's really a problem. Just making sure you know.

I know a lot of people here who visit both forums. And the PSW mic forum can be stricter than here!

Anyway, back to the topic...

 

[soapfoot]

Also, Stu--

I noticed that on your schematic for the "STU47," the EF12 shows three grids, two of which are connected together and to the plate.  

In light of the EF12's internal connections (grid 3 to cathode and metal case), how did you manage to do this?  Or is it a misprint on the schematic (or am I misunderstanding something)?

Thanks,

 

[zebra50]

My bad - just a copy and paste error from another schematic. It would be grid 3 to cathode, grid 2 to plate.

I did make a mic around an EF40, and played with the grid connections. It was an experimental build, but I couldn't hear a difference between connecting grid 2 to plate or cathode. Noise was about the same in each case too.

 

[electroslut]

It seems you have got way more help than I ever have had on this very informative forum! You have every thing you need here except
a couple caps and resistors! weather you power the heater from the B+ or its own supply will not effect the sound at all if its nice and clean. trying to lower the B+ to the heater voltage to make it more U47 is just silly. Now that you know the proper way to power and bias the tube
the last little tweaks I would do by ear and test gear. also see if you can borrow a U47 (if you don't have one) and get it as close as you can!
by the time you get all the hard work done (building the rest of the mic) you will have more info then you will need! 

 

[scott_humphrey]

Here is another option for you from Oliver Archut:

http://www.tab-funkenwerk.com/sitebuildercontent/sitebuilderfiles/AMIU47alttubeschematic.pdf

I recently built one using his U47 metalwork and transformer with a K47 capsule. I sounds great to me, but I don't have access to the real deal to give any kind of fair comparison. I'm sure that it won't sound like the original  ...but then again, and API pre does not sound like a Neve, but they are both effective.

I ended up using a Telefunken EF802 tube. You can still find these on the German eBay. One deviation from the schematic that I did was to use two 1N4148 diodes in series to bias the cathode (they are connected from the cathode directly to ground). I tried this after reading about it from Brian at Fox Audio Research:

http://www.foxaudioresearch.ca/The47.htm

Having a fixed bias for the tube seems to be an important part of the sound from what I've read, and in my experience I like the fixed bias sound better    ...but who knows?

I tried Oliver's schematic as is and also tried it with the diodes for fixed bias at the cathode. The end result sounded the same to me (as was the final voltage on the cathode), so I left the diodes in the circuit rather than the resistors since that was the last configuration that I tried. I'll post pics at some point.

Happy building!

 

[gary o]

I read that fox diode stuff..... would like to learn more about the diode bias thing.....I dont fully understand the different biasing schemes ....I am a little slower than others but I get there in the end...

The mic Fox was modding had IR red diodes.......is that LEDs ?....

 

[soapfoot]

Another question about the STU47:

why the .33uf film capacitor?  The original value is .5, and I've heard of some people going as high as 1uf.  All the stuff I've read about EF12 versus VF14 has indicated that the EF12 sounds similar except the low-end sounds a little "relaxed."  This is not first-hand info, just from reading on the net.

I'm curious what led you to the .33 value.

Thanks!

 

[scott_humphrey]

You can use either two 1N4148 diodes in series or a single infrared LED to achieve a cathode bias of 1.1V. Brian Fox has done formal testing and reports that the IR LED doesn't stay "open" at lower current draws, so maybe the 1N4148 diode is a better option here. I don't want to misquote Brian, so I'll send him an e-mail to see if he can provide a more accurate description of what's going on.

The basic idea, though, is to replace the cathode resistor/capacitor with an LED or diode. Just like with having a resistor running from cathode to ground there is a voltage drop that occurs across the LED or diode that provides the biasing voltage for the cathode. The difference, though, is that with the LED/diode the voltage stays at the same level (it is fixed) regardless of the current draw of the tube. When using a resistor, the voltage changes as the current draw of the tube changes (due to Ohm's law V=IR) so the voltage is not fixed with that configuration.

The key to having this work is to find an LED or diode that will provide the correct cathode bias voltage.

As to whether or not this is "better" it is really just a matter of taste, so try it out for yourself and see. I personally think that it sounds much better than the typical cathode bias with the R/C. Punchier and more phase coherent is how I would describe it, but maybe I'm kidding myself.

I also switched the circuit in my diy Telefunken-Ela-M251E-style mic so that it now uses an infrared LED (and an old Blue Kiwi capsule):

http://www.treblebooster.com/m251e/telefunken_ela_m251e_circuit.htm

Again, I like the sound better, but your mileage may vary....

 

[soapfoot]

An unbypassed cathode resistor is a form of negative feedback, correct?  So using the diodes for bias would have a similar effect to removing negative feedback from the circuit, right?

 

[Gus]

First Scott good to see you post.  I remember when you first posted about your microphone build


Years ago I gave away a LED bias tube circuit and the schematic was even posted here.  The correct way to use LED or diodes for biasing.  Have fun looking for it.  Just using a LED alone in the cathode leg can sound worse than resistor cathode bias.

I wonder if the circuit is like the circuit I posted years ago?

EH tubes?

 

[gary o]

Well Im just interested to learn & if I can maybe make something that sounds good clone or otherwise most likely tweaked

Ill have a search for led biasing in mics...

Scott lovely DIY mic thanks for info very interesting

An unbypassed cathode resistor is a form of negative feedback, correct?  So using the diodes for bias would have a similar effect to removing negative feedback from the circuit, right?

ing

very interested to learn about the above too

Thanks to all posting here for yr knowledge.

 

[soapfoot]

A few words on biasing from Klaus Heyne that I found.  The context was that he was discussing a U47 on eBay that had been converted to cathode bias by way of a bypassed resistor, and fitted with a dedicated regulated heater supply for the VF14.

changes in tube biasing substantially alter the sound of this or any other condenser mic, as anyone who has ever played with the two types of biasing will confirm.

In response to the question: "How would it sound when the U47/VF14 circuit is changed to cathode biasing?"

In a word: anemic.

The heft of the midrange is gone, and replaced with more of a hifi sound that adds clarity and speed at the price of removing texture and musicality.

By the way, most VF14 to EF14 conversions, which usually are done with cathode bypass, suffer from the same thing.

 

[Gus]

The best way to learn is to build circuits and try them in a microphone.

I always question what I read on the web.

Tube microphones are so yesterday.

 

[Skylar]

I always question what I read on the web.

+1
especially the quotes from two posts above

 

[soapfoot]

Just passing on what Klaus said.  I have no first-hand experience either way-- probably should've pointed that out.  But Klaus's opinion on the U47 is pretty well-respected, so I figured I'd pass it along.

 

[gary o]

I always question what I read on the web.

+1
especially the quotes from two posts above

Im 2 post above..........what dont you believe?

 

[Skylar]

Gary, you're 3 posts above my previous post.

I was talking about the Klaus quotes.
He's saying that changing your circuit from fixed bias to cathode bias will "substantially alter the sound of this or any other condenser mic"
Question that.

I've designed/built 6 different tube mics over the past year...some have fixed bias, some cathode-biased. Some are steel tubes, some glass, some sub-miniature. Each mic uses a different transformer. Despite all of those differences, as I move the same capsule between each of these mics, the sound follows the capsule.
Even changing the value (and type) of the output capacitor doesn't change the overall tone as much as changing the capsule.

BUT, please don't believe me (or Klaus for that matter); try it for yourself and see.

[size=10pt]In a word: anemic.

The heft of the midrange is gone, and replaced with more of a hifi sound that adds clarity and speed at the price of removing texture and musicality.

Oh, give me a break. This is that typical audiofool BS. Is he talking about a microphone or a Monster cable?

 

[milesjackson]

Oh, give me a break. This is that typical audiofool BS. Is he talking about a microphone or a Monster cable?

C'mon Skylar, Klaus isn't going far enough.  To preserve the texture and musicality of a tube mic, we must use $600 power supply cables! 

 

[soapfoot]

I think it might just be the case that what Klaus's considers to be "drastic" might not seem so to most of the rest of us.  However, he's probably heard, handled, worked on, and been inside more U47s in his lifetime than almost anyone in the last 40 years.  That counts for something, in my opinion.

 

[e.oelberg]

I think the most interesting about Klaus Heyne is how he established his web reputation. If I read his posts I'm always fascinated by his delicate language, his humble attitude, his ears are the best you'll find on the web, I'm sure he hears the color of the Neumann Badge.

Nicholas