Telefunken ELA M 251 Clone Tube Microphone Build Thread (D-Ela M 251E)

[Tahoebrian5]

I think he said he checks them on a tube tester but also listens to them. I’m assuming he has some sort of amplifier that he uses to listen to them but I didn’t press for details. He came highly recommended on the realgearonline forum.

 

[TLRT]

I think he said he checks them on a tube tester but also listens to them. I’m assuming he has some sort of amplifier that he uses to listen to them but I didn’t press for details. He came highly recommended on the realgearonline forum.

Proper test I make is trying in a "virtual mic" box, which the elam mic preamp with dual sockets,so I can listen the noise in real circuit and evaluate the noise in spectrum analyzer. Tube testers haven't got high grid resistors,max 1M, I test with 220Mohm, that will kick out all bad tubes.

 

[Purplenoise]

Understood. The guy I talked to indicated he tests them all to make sure they are suitable for a mic. The 12A’s go for around $65 compared to $300 is fairly appealing. I just wanted to make sure it wasn’t going to change the character of the mic too much as I don’t want to go through all the trouble to build it and not have it sound like an Ela family mic.

On the other hand I can’t help wondering if I should go ahead and get a good NOS 6072 while it is still an option at all.

Which capsule are you using?

 

[dmp]

How should one measure tube noise?

I've measured SNR of tube mics I've built (which is a combined measurement of all components + preamp). Of about 6 tube mics I measured SNR of 18-19 dB (including one with a GE 6072 *****)
In comparison dynamics would measure higher >22dB
I'm not sure this way of measuring is very good as it's dependent on the test setup.

 

[Tahoebrian5]

Which capsule are you using?

Beez Neez, AMI transformer, Apex body

 

[Purplenoise]

Beez Neez, AMI transformer, Apex body

Ok. The beesneez capsule is a “dark” and more mid forward variation of the brass ck12. If you want to be closer to a 251 here is what I would do.

-6072 ge 5 star OR a bright 12at7 like telefunken, Siemens and sylvania. My personal favorites are telefunken and Siemens. All those 12at7s tend to be noisy so get a mic grade one from a seller that can check them. I would recommend BOWIE and TLRT.

- keep grid resistor at 30M

- cathode capacitor keep at 22uf but use an axial metal cased tantalum ( nos Siemens is really good). You can use sockets so that you can audition different values and types.

- for output (coupling) cap use 1uf preferably wet tantalum. 125v should be fine. They pop up on eBay from time to time. I’ve seen sprague 1.7uf wet tantalums a few times which is also good.

 

[Tahoebrian5]

Does the smaller value of output cap block more low frequencies than higher values? Or the opposite?

 

[Purplenoise]

Does the smaller value of output cap block more low frequencies than higher values? Or the opposite?

Yes, smaller values less sub lows but sometimes because of the resonant point moving higher might give you the impression of a fuller “chestier” sound on vox. The originals rolled off at around 100hz.

 

[Tahoebrian5]

Yes, smaller values less sub lows but sometimes because of the resonant point moving higher might give you the impression of a fuller “chestier” sound on vox. The originals rolled off at around 100hz.

Thanks, can you also remind me how the cathode bypass cap affects freq response? I used to know this stuff but it’s been too long!

 

[Purplenoise]

Thanks, can you also remind me how the cathode bypass cap affects freq response? I used to know this stuff but it’s been too long!

It affects primarily the low end. Higher values more low end. It’s not uncommon to see 47uf in originals…even 100uf. You should try different values and types there to hear the difference. That’s why I suggested the sockets for easy swap.

 

[TLRT]

How should one measure tube noise?

I've measured SNR of tube mics I've built (which is a combined measurement of all components + preamp). Of about 6 tube mics I measured SNR of 18-19 dB (including one with a GE 6072 *****)
In comparison dynamics would measure higher >22dB
I'm not sure this way of measuring is very good as it's dependent on the test setup

Open the mic, ground the tube grid turret with a small alligator clip. Hum will go away immediately and you can listen the tube noise only.

 

[dmp]

Open the mic, ground the tube grid turret with a small alligator clip. Hum will go away immediately and you can listen the tube noise only.

And do you measure it as a dB level? What is a good level to be below for a mic tube?

 

[TLRT]

And do you measure it as a dB level? What is a good level to be below for a mic tube?

I use rme digicheck and I have reference tubes. Mid frequencies should be below -100dB,RMS better than -76dB,and tube should not be microphonics and no pops/clicks. Digicheck can hold the peaks so I can test for days if needed(ac701 really needs 24-48h tests)

 

[Tahoebrian5]

Is millmax sockets the way to go if I wanted to try auditioning a few components?

anyone know which is the correct size?

 

[Purplenoise]

Is millmax sockets the way to go if I wanted to try auditioning a few components?

anyone know which is the correct size?

Yes those are the ones I used. Just get single strips so you can cut single pieces. It’s the same size as the sockets for IC op amps

 

[Tahoebrian5]

Okay another q, sorry! does the stock Apex head basket affect the tone? Do you guys have a recommended replacement?

 

[Purplenoise]

Okay another q, sorry! does the stock Apex head basket affect the tone? Do you guys have a recommended replacement?

There is a headbasket replacement that is closer to a 251 at “studio 939”. No top ring, dual layer.
Headbasket size, material, layout do affect sound to an extent.

 

[Purplenoise]

So after building a point to point 251 as close as I could to the original I decided to experiment a little with the d-251 changing the tube to 12at7. Here are some personal and unscientific observations that some might be interested in.
- With the 12at7 the mic is kinda bass and low mid heavy when properly biased with a 47k plate resistor. This was common to all at7s I tried and transformers. For the capsule I used ( akg nylon ck12 pulled from a c12vr) I ended up with:
-75k plate resistor
-30M grid resistor ( nos beyschlag)
-0.25uf PIO output cap ( even 0.47uf was too bassy)

——a little observation about the output cap. With the 12at7 output caps in the range of 0.47 to 2uf made the mic sound “bloated”. I guess that was a combination of resonant point and capsule? Using a 0.25uf moved the resonant point up I guess giving a nice “chesty” low end to male vocals in combination with a cathode bypass cap in the range of 2-10uf.
Interestingly enough when using a 3.4uf wet tantalum out cap the response was pretty balanced even with a 22uf cathode cap.

——————-


-10uf/22uf cathode bypass cap
-Ami t14

As far as capacitors for the cathode bypass I used a Nos axial metal cased tantalum. Metal cased tantalums sound very different than the regular yellow ones. Also electrolytics, wet tants etc sound different. Voltage rating plays into it too.
For the 4800pf I used a 4700 styrene in parallel with a tubular 100pf ceramic. Also the 100pf high cut is nos ceramic. In the 251 build I prefer the sound of ceramics to styrenes ( I know I know blasphemy). I used nos tubular dog bone ceramics.
All these are personal observations from listening and certainly not measuring. Fun stuff to experiment with.

 

[Tahoebrian5]

So after building a point to point 251 as close as I could to the original I decided to experiment a little with the d-251 changing the tube to 12at7. Here are some personal and unscientific observations that some might be interested in.
- With the 12at7 the mic is kinda bass and low mid heavy when properly biased with a 47k plate resistor. This was common to all at7s I tried and transformers. For the capsule I used ( akg nylon ck12 pulled from a c12vr) I ended up with:
-75k plate resistor
-30M grid resistor ( nos beyschlag)
-0.25uf PIO output cap ( even 0.47uf was too bassy)
-10uf/22uf cathode bypass cap
-Ami t14

As far as capacitors for the cathode bypass I used a Nos axial metal cased tantalum. Metal cased tantalums sound very different than the regular yellow ones. Also electrolytics, wet tants etc sound different. Voltage rating plays into it too.
For the 4800pf I used a 4700 styrene in parallel with a tubular 100pf ceramic. Also the 100pf high cut is nos ceramic. In the 251 build I prefer the sound of ceramics to styrenes ( I know I know blasphemy). I used nos tubular dog bone ceramics.
All these are personal observations from listening and certainly not measuring. Fun stuff to experiment with.

Did you change the power supply for the 12at7 also? Dropping resistors I think they are called if I remember correctly?

in general looking at load lines even for the 6072 I’m wondering why the B+ isn’t up in the 300 volt range. Is it just too hard to do in a small area of the mic body?

 

[Purplenoise]

Did you change the power supply for the 12at7 also? Dropping resistors I think they are called if I remember correctly?

in general looking at load lines even for the 6072 I’m wondering why the B+ isn’t up in the 300 volt range. Is it just too hard to do in a small area of the mic body?

Yes changed those. From 75k I believe to 47k.
300v would be overkill in a mic no? The only mic I know with a high b+ is the u67 at 210v