Telefunken 251 Clone finished

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

deveng

Well-known member
Joined
Jan 11, 2005
Messages
371
Location
California
Gents,

Here's my finished mic based on the AC701 schematic for the Telefunken 251.  Its got an under-heated Heinz Kaufman 5840 connected as a triode, CEK-12 capsule, T14/a transformer and a Telefunken 251F power supply with a adjusted B+ and filament voltages for the 5840.  Of course the plate and cathode resistors were changed.  Polystyrene caps for the plate and capsule shunts, low impedance electrolytic for Ck and polypropylene for the series output capacitor.  Its wired for cardioid only.

The body is closer to a C12 diameter but I use the double mesh on the head basket like the 251 (excluding the top mesh).  Worked on first power up and is very quiet.  Only had 2 5840's so I think I got lucky with this one being low noise.  I've only recorded 3 vocal tracks as yet so the verdict is out on how it sounds overall.  The vocal tracks were warm, fat and not sibilant with a smooth high end.  But, I'll reserve my full opinion until after its tested with multiple instrument sources with more high frequency content than voice.

All high impedance parts are mounted just under the capsule on a disc through hole board which could be changed out fairly easily with a different disc with correct components for a 6072 tube.  Mic body length is setup to allow room for the 6072 if I ever go that route.

More pics to follow

Regards,
Jeff
 

Attachments

  • T251_7.jpg
    T251_7.jpg
    762.7 KB · Views: 232
I want to hear this puppy! Why did you underheat the 5840, and can you elaborate on what you changed values to resistor-wise in the mic? Also which transformer? I don't know a T14/a.
 
Thanks.  I was originally going to build a C12 circuit.  The material was whatever I could find at the local hardware store and had the head basket milled by my brother years ago.  The body, head basket and internal structure sat on a shelf for 10 years.  Yes, that's right.  Recording projects, mic preamp builds, compressor builds etc, etc kept pushing this to the bottom of my list of to do's.  When it finally came up in the que to build, I changed my mind and went for the 251 circuit.  I suspect because the head basket is the smaller size (more like the C12) it would change the acoustics of the capsule.    I've left the second, finer mesh loose so I can remove it and test the sound with and without.  So since the circuit is a "251" but the head basket is a different shape, it will probably have different characteristics from both the 251 and C12.  More of a Frankenfunken.  I'm not a purist.  As long as the mic sounds excellent and is versatile that's what I'm shooting for. 

Interesting note for those who have not built a tube mic.  When completed, it feels much more personal than any other gear I've put together (and that's a lot).  There's something magic about the first step in the recording chain.  Moving from the acoustic world to the electronic realm, and getting it right, is a very satisfying trip!

Finishing my MK47 has now moved up in priority and when done will certainly not be the last tube mic I build!!

regards,
Jeff
 
The main reason for under heating is to extend the tube life.  Plate resistor was changed to 100k, cathode to 750.  The voltage divider for the capsule polarization voltage needed to be tweaked a bit to get the 60v required for the CEK12.  I first bread boarded the entire circuit with a 100pf polystyrene in place of the capsule and verified all the working voltages.  I also used a 33Meg grid resistor.  I may experiment with higher values just to see how the bottom end, and noise levels change.  The transformer is a Peluso.    I'll try to post some sound samples when I've got the thing in use and have finished my "break in" analysis.

I should probably take out the "clone" in the subject.  Its a bit misleading!
 
When completed, it feels much more personal than any other gear I've put together (and that's a lot).  There's something magic about the first step in the recording chain.  Moving from the acoustic world to the electronic realm, and getting it right, is a very satisfying trip!

Yes, I can't agree more!
Do you also have the experience that your last build always sounds better than anything you have build before?
I'm afraid it must have something to do with psychology, but maybe you hear the love you have put into a project...
(A French chef once told me: "You do not only taste the ingredients, but you also taste the love that is put in!")

 
Well, I try to be objective when I finish something and listen with an open ear.  But, that's difficult when you've spent many hours designing, sourcing parts, building, debugging.  I do have an excitement that comes with each build and the first listen with musical sources.  Some of the excitement is tempered because I always run bench tests for SNR+noise, THD, frequency response, etc.  Then some generator tones to just listen to it for any unusual characteristics that need attention (or not) on the bench. 

I try to keep my emotional response for later.  I can't always say I've been successful at doing that though!!  If this mic felt the love I put into building, its gotta sound good!

 
It is always good to be realistic. But sometimes you create a 'lucky combination'. (Usually not...)
Some time ago I made something like a 'C12-FET'. Complete with remote pattern switching and all. It sounded fantastic!
Also extremely low self noise.
It is not that I am always happy with my creations, but this one was really good.
So I knew what I could expect, but this week I used the microphone again and was (again!) pleasantly surprised.
I think those moments give you the real satisfaction of DIY!
 
I did some basic tests on this mic today.  Nothing scientific just general stuff.  I mic'd a Taylor acoustic and it sounded very good.  Moving the guitar around so the mic faced the fretboard and then back to the bridge was all good.  The mic captures all the details and harmonics.  The response of the mic reminds me of the reason I bought the Taylor.  Even across all the strings with a rich top end and full but unexaggerated bottom.  The results were better than most other mics from my locker. 

I also did a quick noise test.  Set up a U87i and the 251 into a pair API512's full open.  Opened up an FFT analyzer and found them visually just about the same.  Granted the U87 is the older version without the DC to DC converter.  Its supposed to have about 3dB more noise than the U87Ai version (which is 12dB (A)).  The U87i picked up a bit more low frequency rumble but that could be because it was not in a shock mount.  There was no evidence of any 60Hz hum (and I didn't hear any either).  Body grounding and head basket shielding appears to be fine.  Mic is quiet so no need to test another 5840!  This one will do.  I may build another HiZ circuit pcb with a spare 5840 so in the event of a tube failure, I could replace the circuit in 20 minutes and be up and recording again.

Overall I'm extremely pleased with this mic build so far!

Regards,
Jeff
 
RuudNL said:
Some time ago I made something like a 'C12-FET'. Complete with remote pattern switching and all. It sounded fantastic!

Tell us more! I've always had the idea to do one of Tim's C12 capsules on a FET mic but never had the time to do the necessary research..
 
Gents,

I've been using this mic on a regular basis and I'm am absolutely loving it.  Shortly after completing this mic I also completed an MK47 as well.  Now the MK47 sounds fantastic.  But, there's something about this 251 clone that just blows me away.  I installed a Peluso CEK12 capsule I got used from Dan Deurloo and a T14/1 output transformer from Peluso.  It sounds just beautiful on everything I've used it on.  I'm not sure why.  Probably just the right combination including the use of a 5840 tube.  I have the parts to build a second MK47 and will do so.  I also now want to build a second 251 clone to match this one.

Gotta love it when good things happen!

Regards,
Jeff
 
Looks great!
Love to see an original looking mic here, instead of going to great lengths to look just like an original.
I have built a number of mics with the 100k / 5840 / 750r combination and they sound great. I first saw that in Royer's tape op design. There was no Ck in his design, however, interestingly.
I haven't explored using a shunt cap across the transformer as in the 251. Have you tried with and without? They added that cap to try to make the mic more U47ish, what ever that is (midrangey?)
I'm just finishing up a mic now with this basic design and will compare against the others I have to explore some of these design choices.
 
Dmp,

I followed the Telefunken 251 vs1 schematic (with AC701) as close as possible but with the changes required for a 5840.  I do have the shunt 400pF cap on the plate just before the series output cap and the 4800pF shunt at the capsule just as it shows in the original schematic.

I played around with the grid resistor and output blocking cap size and ended up with the 33Meg grid and 1.6uF output cap. 

I find this mic to have a bit less bottom end than the MK47.  I left it this way to have a choice.  Top end is similar on both, smooth with no sibilance.  I love how this mic sounds so I don't want to change anything!  Especially since I'm using it a lot.  I may experiment on the next one. 

I also like the second mesh installed.  I originally left it "loose" so I could try the headbasket with and without the second fine mesh.  I like the sound with both in.  I've gone ahead and silver epoxied it in place so no going back now. 

Wonderful mic.  Sometimes you get lucky and things just work well.  I made decisions on the build ahead of time based on my simulations, the original schematics and committed to them.  It turned out right for me.  I do wonder what this would sound like with a Campbell capsule!

Regards,
Jeff
 
Now that I look at it again, I believe you're right.  It does appear to be 100pF.  I just did a quick simulation and the difference is 400pF rolls off around 49kHz and 100pF rolls off at 192kHz in my version of the circuit.  I'll have to dig into this a bit more.  I may need to change that cap to 100pf.  If the simulation is correct, I can't hear that difference but it would be more accurate to the schematic.

Regards,
Jeff
 
Back
Top