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

[Baderjoe]

I built the microphone to Dany's (Poctop) specs so there wasn't any modifications. I was assuming it is either:
A. issues with the HiZ section (I have no way to measure that part)
B. Capsule is damaged
C. Some kind of short at the relays.

I was able to inject 1k sine wave into the grid so I am thinking the amplifier section is functioning. I am not sure what to do next.

Thanks Baderjoe for your help!!

That sounds good. You have proven the amplifier is working. The issue is, the output impedance of a signal generator is low (about 50 ohms), so it can drive almost anything. The output impedance of the capsule is extremely high, so it can’t drive a low impedance input. Do you have a value resistor in series between the grid and the capsule’s output lead? Generally, this is a 1 Meg ohm resistor.

 

[berret]

Do i see a connection between 2 and 7 on the tube? And a connection from 7 to earth?
If so it won't work. Disconnect 2 from 7, leave 7 to earth.

 

[gearfreq]

That sounds good. You have proven the amplifier is working. The issue is, the output impedance of a signal generator is low (about 50 ohms), so it can drive almost anything. The output impedance of the capsule is extremely high, so it can’t drive a low impedance input. Do you have a value resistor in series between the grid and the capsule’s output lead? Generally, this is a 1 Meg ohm resistor.

The only resistors I see in series are R2 and R3 (500k + 500K= 1MOhm) From the schematic and PCB diagram it looks to divide the B+ Voltage in 1/2. Forgive me as I am a newbie to tube microphone applications. Below I attached the PCB diagram to show the basic signal path and values. The only resistors I see between the grid and front capsule is R6 (30M 1/2W) and R7 (8M). Although I maybe reading this wrong. I appreciate you helping me out in this learning process.

 

[gearfreq]

Do i see a connection between 2 and 7 on the tube? And a connection from 7 to earth?
If so it won't work. Disconnect 2 from 7, leave 7 to earth.

I don't believe pin 2 (grid) and pin 7 of the tube are connected in anyway (although it looks like it is on the Trans4funks1 schematic) Here is the PCB diagram of to show the layout. I maybe totally wrong as I am a newbie to tube and high impedance applications. I appreciate any help you may have.

 

[Baderjoe]

I don't believe pin 2 (grid) and pin 7 of the tube are connected in anyway (although it looks like it is on the Trans4funks1 schematic) Here is the PCB diagram of to show the layout. I maybe totally wrong as I am a newbie to tube and high impedance applications. I appreciate any help you may have.

I would desolder the three relays from the board. If the board matches the schematic, that will keep only the front diaphragm on the polarizing voltage and the front backplate connected to to grid of the active side of the tube. If the board matches the schematic, you should get audio.

 

[vmanj]

I welcome everyone.
What type of polarization filter capacitor - 4800pF (ceramic, film) is preferable to use in this microphone?

 

[Purplenoise]

I welcome everyone.
What type of polarization filter capacitor - 4800pF (ceramic, film) is preferable to use in this microphone?

Most use polystyrene. Original was tubular ceramic

 

[vmanj]

Thank you.

 

[Greenie]

Just wondering what the values are on the Resistor/Capacitors circled on the photo?

 

[Drosselmeier]

Just wondering what the values are on the Resistor/Capacitors circled on the photo?

8Mohm & ... 5nF

 

[Greenie]

8Mohm & ... 5nF

Thanks.

 

[vmanj]

Is it possible to use zener diodes in the power supply of this microphone so that the B+ voltage is more stable?
Could this degrade the sound or add noise?

 

[Greenie]

Seven pin mic contact to PCB:
Pin 1: A+
Pin2: A-
Pin 3: Omni
Pin 4: Heater H+
Pin 5: B+
Pin 6: F8, Figure of 8.
Pin 7: Ground \ pluss to shield.

 

[Greenie]

Seven pin mic contact to PCB:
Pin 1: A+
Pin2: A-
Pin 3: Omni
Pin 4: Heater H+
Pin 5: B+
Pin 6: F8, Figure of 8.
Pin 7: Ground \ pluss to shield.

On the pic here it seems like A+/- is going from 5 and 6 to the 3 pin XLR, but in the mic end it's pin 1 and 2? In the schematics pin 5 is B+ and 6 is F8? I'm a bit lost here?

 

[Greenie]

I also wonder if the schematics included here is the valid one to use? I have omtitted the writing in blue, but is that the intention? I can't get the 9 volt to reach the output, so I am thinking about building a turret type board instead.

 

[Delta Sigma]

On the pic here it seems like A+/- is going from 5 and 6 to the 3 pin XLR, but in the mic end it's pin 1 and 2? In the schematics pin 5 is B+ and 6 is F8? I'm a bit lost here?

Is this pic from the first page? It doesn't look right. Looks like 5/6 to me. It only matters that you use the corresponding pinout in the mic as well. Just make sure your heater and 0V are using the larger gauge wires in your cable. Audio pair can go directly to your 3 pin XLR.

 

[Delta Sigma]

I also wonder if the schematics included here is the valid one to use? I have omtitted the writing in blue, but is that the intention? I can't get the 9 volt to reach the output, so I am thinking about building a turret type board instead.

Do you mean your 9VAC is not getting to the PCB? Or your 6VDC is not getting to your 9 pin? There are a lot of places to measure along the way to troubleshoot where your circuit is failing. -ve DVM lead to 0V then the other lead can check for voltage along the way.

 

[neowalla]

Does anyone have a part number, make/model suggestion for the transformer for the power supply? I've done some searching for dual output power transformers but I'm not finding anything. I'm sure there must be a go-to trans for these ElaM/C12 style power supplies.

 

[Delta Sigma]

Most are using the transformer in the TPS 1000 power supply. Dany's boards for this project fit this unit.

https://store.studio939.com/product/tps100-tube-microphone-power-supply

 

[Greenie]

HI. I am having som issues with the 9v in the vintage mic pcb for the D 251. I get 9/4.5 allt he wat till r8, from one en to the other the voltage is 4.5 v and on the other side it is 2v'ish. So at the voltage regulator it is only 2 volt and at the other end it is mV's? R8 gets very hot. In the schematics included, the wattage is 1w on these 4.7Rs, but I have seen 5w indicated, (which is in my PSU), is 1w enough?

Also in the the includde schematics there is parts in blue print that is said to be omitted, is that so?