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

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Purplenoise

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Hey electron wizards…question. I have enough parts to fool around with this kit so I am thinking about building a variation with 6201 tube ( possibly 12av7 too) and an M7 capsule ( omitting the 100pf cap).
Since ck12 has separate backplates ( 4 wires) and m7 is single backplate (3 wires) is there a way to use the patterns? Do I just connect the single backplate to FB ( front backplate)?
Thanks
 

vmanj

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Greetings to all.
If you use 12AT7, the value of the anode resistor will be 47K. Do I need to change the value of the 1.8K cathode resistor or can it be left as is?
 

doqmemory

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Since ck12 has separate backplates ( 4 wires) and m7 is single backplate (3 wires) is there a way to use the patterns? Do I just connect the single backplate to FB ( front backplate)?
Figure 8 will not work without isolated backplates in this PBC. @Matador has a hybrid version that uses the C-12 approach for patterns from the PSU using a single (or electrically connected) backplate.

If you want to use @poctop 's kit, just connect the single M7 backplate to FB, you can get cardioid and omni.
 

Purplenoise

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Figure 8 will not work without isolated backplates in this PBC. @Matador has a hybrid version that uses the C-12 approach for patterns from the PSU using a single (or electrically connected) backplate.

If you want to use @poctop 's kit, just connect the single M7 backplate to FB, you can get cardioid and omni.
Thank you so much, that’s what I was suspecting. Cheers.
 

vmanj

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There is another question about connecting a capsule (ONLY A CARDIOID IS OF INTEREST).
If I understand correctly, in ELA M 250 the capsule is connected as in the Yu-47, only the other way around. The backplate of the capsule is connected to the grid, and in the u47 the membrane is connected to the grid?
 

doqmemory

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There is another question about connecting a capsule (ONLY A CARDIOID IS OF INTEREST).
If I understand correctly, in ELA M 250 the capsule is connected as in the Yu-47, only the other way around. The backplate of the capsule is connected to the grid, and in the u47 the membrane is connected to the grid?
Yes, the backplate goes to grid and the polarization is applied to the membrane in ELA M250E, as shown in the schematic on the first page of this thread.
 

doqmemory

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I see, I just thought I made a mistake and misunderstood the diagram.
One of the reasons I have read for not polarizing the membrane is to help prevent attracting dust via static charge.

Perhaps someone with more knowledge about servicing vintage microphones might chime in here about the following question: Do old ELA M 250/251 capsules (membrane polarized) collect more dust than those in old U47 microphones (membrane connected to ground via 100M resister)?

I am not sure if there is any other compelling reason for choosing to connect backplate or membrane to grid, other than the desired polarity patterns (with the second side of the capsule active).
 

vmanj

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It turns out that in the ELA M 250 version, the membrane will collect more dust .. But maybe this somehow affects the sound the same way?
 

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gearfreq

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Hello guys! I've finally had a chance to build the D-251 and had an issue that I cant wrap my head around. I looked through 41 pages of this thread without definitive answer. I've seen various completed D-251 projects pictures with the PSU wired one way and the transfunk schematic seems to be wired differently. I attached below a rough drawing of both wiring configs in hopes that someone can clarify which setup is correct before I blow myself or the unit up hahah. Any help would be greatly appreciated.
 

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gearfreq

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Hey guys! HELP!! I could sure use some of your expertise with a problem I having. So I was able to complete the wiring and assembly of both the mic and PSU but I am not getting any output or signal what so ever. Attached is the schematic with all the voltage readings and info.

AMI T14-- GE 12AY7 tube - Dachman C12 (for now to test)

- Checked all voltages and continuity for both the mic and psu (attached below)
-Checked XLR7 and XLR3 cables continuity- good
-tested 3 different GE 12AY7s

Any help would be greatly appreciated as I have been struggling with this for some time now.
 

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gearfreq

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Perhaps a capsule with three wires and not with four...
View attachment 97094 View attachment 97095
I spoke with Dachman Audio and they stated that this capsule is indeed dual isolated backplate capsule. This capsule worked great in a prior modded mic in 3 wire configuration. I have tried to test the cardioid pattern (2 wire FC and FB) as well as your suggestion with 3 wire just now. It seems it is microphonic when i crank my preamp all the way up and tap lightly on the housing with slight noise but no speaking output. I made sure continuity was good from capsule to terminal and screws were tight on the capsule. I have probably worked on this project for close to 100 hours from start to now (building, research & troubleshooting etc) and getting frustrated and discouraged.
 

Baderjoe

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I spoke with Dachman Audio and they stated that this capsule is indeed dual isolated backplate capsule. This capsule worked great in a prior modded mic in 3 wire configuration. I have tried to test the cardioid pattern (2 wire FC and FB) as well as your suggestion with 3 wire just now. It seems it is microphonic when i crank my preamp all the way up and tap lightly on the housing with slight noise but no speaking output. I made sure continuity was good from capsule to terminal and screws were tight on the capsule. I have probably worked on this project for close to 100 hours from start to now (building, research & troubleshooting etc) and getting frustrated and discouraged.
When I see that kind of effect, where you get microphonics, but no audio signal. The input impedance of your front end is too low and is swamping out the diaphragm to backplate capacitance. Do you have a really high value resistor in series on the input of the amplifier circuit? If you don’t, you need to add one.
 

gearfreq

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When I see that kind of effect, where you get microphonics, but no audio signal. The input impedance of your front end is too low and is swamping out the diaphragm to backplate capacitance. Do you have a really high value resistor in series on the input of the amplifier circuit? If you don’t, you need to add one.
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!!
 

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