K47 with EF42 tube ?
« on: October 25, 2010, 01:07:39 PM »
Hello all! Here's what I posted and the responses from Klaus's forum:
Well, here we go....I am new to the forum and have read a ton of material here. I feel very outclassed here being a lowly computer electronics tech whose knowledge is out of date. I have built and modified a ton of effects pedals and even one piece of studio gear. The studio that I'm involved in has purchased a mic bottle, a NOS K47 capsule re-diaphragmed by Neumann, a NOS EF42 tube, and a Hauffe transformer. Also, a PSU spec'd for 105 volt output.
I was wondering if anyone had a schematic that would give me a starting point in order to build this thing? Maybe even a pcb layout, or point me to someone who could sell me pcb's for this project?
From what I understand, the EF42 is a pin for pin replacement for the EF12. I would like to post photos and have a lot of input from the forum about completing this project.
Questions and comments are welcome, as I feel a bit overwhelmed by the information out there and the different ways people construct these things. In fact, there's just not a lot of comprehensive info. Maybe with my photos and some input, we could have a thread that provides such info, and in the process, I'll be able to finish this mic.
Any help will be greatly appreciated!
Thank You
Sean Harris

Tim Campbell's response:
There has been a lot of mention on the internet about the use of EF42 tubes being similar to EF12/VF14's. This must be a case of one person misidentifying a tube and others repeating this until it's perceived as fact.

I have tried to use EF42's as audio tubes and found it impossible. They have some wonderful qualities for audio but are inherently so microphonic as to render them unusable. If you like the EF12 use an EF12 or try an EF40 or the EF184.

Klaus's response:
Sean, I admire and encourage your explorations. It is through tinkerers like you that the conventional approach to designing good recording microphones will one day be eclipsed.

However, why try to re-invent the wheel, and use parts and design approaches that plenty of vintage mic cloners already pursue? Why not try something new?

If your intent is to build a decent copy of a U47 mic, there are easier and less costly ways. Numerous manufacturers offer either all the parts to do that, or will sell you pre-assembled copies that contain most of the parts you mentioned, and at reasonable prices. (By the way: Neumann does not reskin its capsules, so I am not sure what was sold to you as such.)

If your intent is to make a superior recording mic, I would venture to say that using the same copy parts that are commonly used to build a passable mic that is not quite capable of delivering the sonic quality of the real thing also makes little sense to me.

Either the wheel is perfectly round, and all we can do is tinker at the edges, or new designs are waiting to be explored and tested. I hope you can be a part of the group that is trying the latter.

Best of luck, and lots of courage!

My response:
Thank you Tim and Klaus your responses. I will see that we acquire an EF12, EF40, or EF184 tube. This acquisition of parts was a very good deal from a monetary standpoint. There are many parts including several mic bottles, wima caps, several chinese capsules and various transformers (1 hauffe and 2 chinese). I stand corrected Klaus. I re-read the e-mail, and the claim was that it was serviced by a Beumann Germany technician, not by the Neumann itself. I am dubious about that claim as well, but who knows? Anyway, I have accepted this task as a challenge and it is my intent to make a decent mic using the k47 capsule / hauffe transformer and mic bottle. I realize through reading on these forums that there are many kits available, but I kind of have to make mics out of what I currently have. There are schems on these forums for a k47/e12 mic, and from what I've read, the results are pleasing to most ears. Re-capturing "the original" is impossible without the k7/bv8/vf14 combination. (please forgive if my numbers are incorrect...I'm fresh here..)I also read through some posts talking about Hauffe tranformers being used as replacements by Neumann, but I'm sure that this is a new transformer, so sonically it won't be the same as a nos replacement or the original.
I just want to build a decent quality recording mic for my friends who have through went great trials and tribulations on a shoe string budget to build a small studio. I'm not doing this to re-invent the wheel...or exactly copy a U47.
When I get the parts in hand, I'll photograph them, and we'll see what kind of mess I got myself into...
The question now is...does anyone have a pcb layout for the k47 / e12 combination? Or, any other layouts? I have a sinking feeling that I'm going to spend a lot of time on a breadboard...
Any help is greatly appreciated!

Tim's response:
This isn't a mic builder's forum. Try asking the same questions here http://www.groupdiy.com/ . I'm sure you'll find the right kind of help and encouragement

Any help is appreciated!
Sean Harris


Re: K47 with EF42 tube ?
« Reply #1 on: October 25, 2010, 01:17:42 PM »
Check the microphone METAs:


Lots of reading!  ;D

There are just a few microphone projects with PCBs, the G7 and the Royer mod.
You have to ask for them in the black market.

Two other interesting projects is the MK7 and MK47 kits from ioaudio:
« Last Edit: October 25, 2010, 01:19:58 PM by Sredna »
Hmm... in which direction do the electrons actually go?


Re: K47 with EF42 tube ?
« Reply #2 on: October 25, 2010, 02:04:13 PM »

   iirc, it was Oliver no less who mentioned it on the Neumann forum. He also suggested that Telefunken EF80 and EF802 were direct descendants

   Kindest regards,

« Last Edit: October 25, 2010, 02:09:25 PM by strangeandbouncy »
. . . . RUH ROH . . . . .

Re: K47 with EF42 tube ?
« Reply #3 on: November 01, 2010, 09:07:47 PM »
So it turns out that a NOS EF12 Tube is on it's way as well for this project...

I have searched and searched... Does anyone have a schem or layout for the K47 capsule / EF12 / BV8 combination?

I am ordering the transformer in light that I have a EF12 on the way...

Any help is appreciated...

Sean Harris


Re: K47 with EF42 tube ?
« Reply #4 on: November 01, 2010, 09:30:54 PM »

Found this one somewhere, dont know anything about it.  :P
Hmm... in which direction do the electrons actually go?

Re: K47 with EF42 tube ?
« Reply #5 on: November 01, 2010, 10:27:16 PM »
Thank You. I don't know if this was Oliver's schematic, but thank you the same.

I'm still looking for his schem.


Re: K47 with EF42 tube ?
« Reply #6 on: November 23, 2010, 02:22:55 PM »
Well ,the parts have arrived and I've spent many hours on here reading posts and technical information.
Here's what I have acquired:
1 X NOS EF12K Telefunken Tube
1 X NOS EF14 RFT Tube tested on a Tesla Tube tester
Wima caps and some 1/2 watt 1% metal film resistors
4x Matched Telefunken EF86 Tubes
4x UF42 Tubes unknown origin
3x PSU'S with 7 pin connectors 9 patterns (they are identical to the Peluso PSU unit)
3 x Chinese Mic capsules and mounts of unknown Quality
1 x Neumann K47 Capsule of unknown pedigree (this capsule apparently been re-skinned by a qualified Technician)   
3 X Microphone Bodies (Identical to Peluso Mic bodies)
1 x Hauffe BV8 equivalent transformer
1 x TAB K47 Capsule mount
1X TAB T47 PCB mount transformer
3x Chinese transformers
3 carrying cases for mics/psu's (Identical to Peluso Cases)
I am fully going to document my journey  here in the spirit of www.diystompboxes.com
Here are pictures:

Re: K47 with EF42 tube ?
« Reply #7 on: November 23, 2010, 03:38:40 PM »
First, I'm going to build a K47 / TAB T47 / EF14 tube combination using the tested RFT tube.
I will use the schematic by Oliver posted here:

Now, it seems that I cannot find information about these PSU'S, so I will measure the voltages from all of the pins.

I will need 105V, 5.05V, 0V Ground, 0V Filament, and 2 outputs from the transformer.
Here is the pin out from the PSU (WITH NO LOAD):
Pin 1 182.4 V
Pin 2 6.71V
Pin 3 PATTERN ...19.59V,38.6V,57.9V,77.2V,96.9V,117.3V,138.0V,159.5V
Pin 4 To Pin 7 and Pin 1 XLR
Pin 5 To XLR Pin 3 (Mic Transformer)
Pin 6 To XLR Pin 2 (Mic Transformer)
Pin 7 to Pin 1 XLR (Ground)

So, we have a problem Houston.
I need to modify the PSU. From what I remember, dropping voltage with resistors increases current. mmm. I don't quite understand what to do here.
Any help is appreciated. I'll keep looking.


Re: K47 with EF42 tube ?
« Reply #8 on: November 23, 2010, 04:28:17 PM »
I have read this thread concerning the PSU problem:

I know it applies to the Gyraf mic, but, E=I x R works pretty well. haha

Gyraf states that reducing the PSU voltage will reduce the voltage to the tube as well. That's in the G7.

I need 105V for the tube and capsule. And of course, 5.05 V for the heater.
I will not use the pattern switch on the PSU for this mic. I will probably just disconnect pin 3 on the 7 pin connector.
I wish I had 8 pins so I could implement the polar pattern switch at the PSU.
Any ideas on what switch I could use in the Mic Body? I don't really like the idea of drilling holes in the body.
Any thoughts are appreciated.
So, I will disassemble this PSU and measure to determine the resistive values of the voltage divider resistors.
I realize that I cannot make resistance measurements in circuit, so I will try to read the bands...what was that colour code ryhme that I learned in college?
I may have to de-solder them and measure to be on the safe side.
Either way, voltage measurements are in order before and after the resistors.
Till later..



Re: K47 with EF42 tube ?
« Reply #9 on: November 23, 2010, 05:25:34 PM »
If I were you, I'd try the psu loaded. tube mic psu's are most of the time simple CRCRC... networks, you could be surprised about how much the voltage can drop with a load attached . try 100K to 200K between pin 1 and 7, it will suck about 1 or 2 mA's, witch I believe is in the good ballpark to emulate the mic load.
Most of the time, the patern voltage is derived from the high voltage. so it will drop with it. this can also apply to the heaters voltage . EF14 datasheet shows 6.3v at 470 mA for the heaters. not sure this PSU is made for underheating tubes tho.
In any case, opening the PSU and change or add a few resistors would always be my first choice before trying to implement a switch in the body..



Re: K47 with EF42 tube ?
« Reply #10 on: November 23, 2010, 06:02:18 PM »
I will not use the pattern switch on the PSU for this mic. I will probably just disconnect pin 3 on the 7 pin connector.
I wish I had 8 pins so I could implement the polar pattern switch at the PSU.

You can do it with a relay as its done in the MK47:


If you think you're out of pins, you could always free up pin 4 or 7. I think one ground will do....

If you combine the schematics of U47 and U48, you'll be able to implement omni, cardioid and eight pattern.
Hmm... in which direction do the electrons actually go?


Re: K47 with EF42 tube ?
« Reply #11 on: November 23, 2010, 06:35:41 PM »
You are going to need a good heater power supply for the Oliver circuit you linked.

First Ohms law will help with the adjustment of the B+.  Work out the plate current for the u47 like circuit and size the power supply dropping resistors values and power rating for the desired voltage or if it is zener shunt regulated google etc zener shunt regulation.

Not having been inside that china supply I don't know what the heater circuit is and this is the section that might cause issues with the linked schematic.

« Last Edit: November 23, 2010, 06:39:25 PM by Gus »

Re: K47 with EF42 tube ?
« Reply #12 on: November 23, 2010, 06:52:08 PM »
You were partially correct. The High voltage part has no regulation, but, the heater has this:

L7806CV is a 6 volt positive voltage regulator. I may have to change this to get 5 volts.
I'm thinking the L7805CV would do the trick. I won't even have to change the heat sink.

Since pins 4 and 7 are tied together at the PSU XLR connector, electrically, they're the same. I will implement the pattern switch with pins 3 and 4 or  3 and 7.
I am awaiting some more components for this build, so I am halted designing a PCB for this. I have a preliminary design using this program:
http://diy-fever.com/software/ (the 1.X version)

 I would guess a week at most. I won't be able to try it under load until then. Using a simulated load is a great suggestion, but one I'm uncomfortable with. I will use a dual 68pf cap pair to simulate the capsule because I don't want to damage the K47.

As far as three or more pattern switching using a specialized relay, that's a great idea. I have no idea where to acquire the relay, though.
ioaudio appears only to sell kits.  As well, I want to do baby steps on this one. My 100+ hours of research hasn't taught me "experiment" as of yet.
My knowledge is limited to making and modifying a few dozen guitar effects pedals, and a digital electronics education circa 1987.

I haven't looked at the rest of the PSU circuit yet, so I'll report back when I have that figured out.

Re: K47 with EF42 tube ?
« Reply #13 on: November 23, 2010, 07:07:33 PM »
Thanks Gus!

I have attached these photos of the PSU:

I'm really glad to have input here!



Re: K47 with EF42 tube ?
« Reply #14 on: November 23, 2010, 07:29:30 PM »
the photo doesn't show exactly what's in the heatsink but if it's a 3 terminal regulator for the heater it has no chance of working noise free with the linked schemo, as Gus mentioned.

Re: K47 with EF42 tube ?
« Reply #15 on: November 23, 2010, 07:41:01 PM »
Ok , bockaudio.

Why? It is a three pin 1.5amp 6volt regulator.

I have linked the pdf in a previous post.

Please be more specific.
Like I said in a previous post, this will be in the spirit of diystompboxes where everyone helps each other.
If you have a suggestion...fire away!


Re: K47 with EF42 tube ?
« Reply #16 on: November 23, 2010, 07:55:28 PM »
Shunt regulation...zener diodes....the answers are coming



Re: K47 with EF42 tube ?
« Reply #17 on: November 23, 2010, 07:58:38 PM »
This circuit (the U47 you referenced) uses a "fixed bias" to the cathode which is tapped off of the heater supply and any noise or ripple from the regulator will leak into the cathode circuit which will then cause modulation of the grid with-respect-to the cathode... ie; It makes it hum.

Check out the mk7 PSU here (not the modded one but the from-scratch one): http://www.groupdiy.com/index.php?topic=30124.0

You might be able to tear out and rebuild the heater portion of the PSU to match this more closely to get 5.05V and even the Ht section for your 105V and remote switched patterns. Probably just as easy to breadboard it and use the chassis you have.

Music is everything!
Audio is everything else!

Re: K47 with EF42 tube ?
« Reply #18 on: November 23, 2010, 08:46:04 PM »

Re: K47 with EF42 tube ?
« Reply #19 on: November 23, 2010, 10:48:49 PM »
I'm thinking about snatching the voltage after the diode rectifier. Then, assembling the rest of the heater supply as per the ioaudio schem.
I believe I will have to change resistive values in the rc network in order to drop the voltage. Which one (s) should I look at?
I really need help here folks....


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