Tube microphone with ef80 tube and oep transformer.

[peterbach]

The 100 K.ohm potentiometer adjusts the polarisation voltage between 84 and 115 V.
IMHO that is a pretty high value, a bit too high if you would ask me!

Thank you sir

 

[RuudNL]

Although some capsules can withstand a polarisation voltage higher than 60 V, I usually don't go higher.
In some cases even a bit lower...
The sound of the capsule will change depending on the polarisation voltage.
With a higher voltage, the membrane will be more attracted by the backplate and this will influence the tensioning.
With a very high voltage, the membrane will be sucked to the backplate and in extreme case will even arc.

 

[abbey road d enfer]

Also, the bottom side of R6 should go to ground instead of the cathode.

 

[peterbach]

Although some capsules can withstand a polarisation voltage higher than 60 V, I usually don't go higher.
In some cases even a bit lower...
The sound of the capsule will change depending on the polarisation voltage.
With a higher voltage, the membrane will be more attracted by the backplate and this will influence the tensioning.
With a very high voltage, the membrane will be sucked to the backplate and in extreme case will even arc.

I dont know what is the polarisation voltage of this microphone

 

[peterbach]

Also, the bottom side of R6 should go to ground instead of the cathode.

Ok

 

[abbey road d enfer]

I dont know what is the polarisation voltage of this microphone

Understood, but you should know what voltage is required for your capsule.

 

[peterbach]

Understood, but you should know what voltage is required for your capsule.

My capsule is wgtcenter 47 style capsule

 

[peterbach]

I found the answer i think

 

[abbey road d enfer]

As  a Typical LDC, 60V seems to be the optimum value.
You don't need the 100k potentiometer. There's no point in adjusting precisely the bias voltage. Just make R3=120k and connect R3 to R4 directly. You may need to alter the value of R3 or R4 if your B+ is different than 200V.

 

[peterbach]

As  a Typical LDC, 60V seems to be the optimum value.
You don't need the 100k potentiometer. There's no point in adjusting precisely the bias voltage. Just make R3=120k and connect R3 to R4 directly. You may need to alter the value of R3 or R4 if your B+ is different than 200V.

Thank you for helping me sir this place is like a heaven for me
i will share the making procces soon .

 

[peterbach]

I found this schematic and I think it's easy to find the tube and make it but the question . Can I add a polar pattern selector for this circuit ??
Thank's

 

[abbey road d enfer]

I found this schematic and I think it's easy to find the tube and make it

I seriously doubt the LF performance of the OEP transformer driven by such a high impedance.

Can I add a polar pattern selector for this circuit ??

Yes you can; now depending on what choice you want to have, it may be simple or a little complicated. The easiest is card/omni, which just involves connecting the two sides of the capsule together to get omni.
Getting figure-8 involves creating a half voltage bias connected to the backplates, the diaphragms being connected, one to ground, the other to the full bias voltage. A switch that connects the two diaphragms via a capacitor sets the fig-8 pattern.
You should look at the U47 and U48 schematics.

 

[peterbach]

Thank you sir 

 

[mhelin]

There is another OEP transformer which is 1:6 mic input transformer with following inductance measurements from another thread:

OEP Mic input 1:6 model Z21808
Using handheld meter: Primaries 5.83H
Using HP 4262A meter:
Primaries at 120Hz - 10.5H, at 1K - 6.9H
Secondaries at 120Hz - 318H

Haven't used that tx but EF80 with 6:1 should be ok if you find proper operating point/bias.

 

[peterbach]

There is another OEP transformer which is 1:6 mic input transformer with following inductance measurements from another thread:

OEP Mic input 1:6 model Z21808
Using handheld meter: Primaries 5.83H
Using HP 4262A meter:
Primaries at 120Hz - 10.5H, at 1K - 6.9H
Secondaries at 120Hz - 318H

Haven't used that tx but EF80 with 6:1 should be ok if you find proper operating point/bias.

Thank you sir
The frequency response of this transformer are great but I don't understand the pinout of it

 

[mhelin]

Datasheet: https://docs-emea.rs-online.com/webdocs/0d72/0900766b80d72828.pdf

Pin view = bottom view

When wired as 6:1 pins 6 and 9 are your input pins, and pins 2 and 4 your output pins (pin 3 is missing) . I guess at least pin 5 should be grounded also. The odd 1:1.8 output pin is propably there for current mode applications when used as line input transformer for feedback.

This transformer is available from RS at least, google for Z21808E part number (then one without mumetal shield) or Z21808C if you want one with the shield.  In addition to RS this link might work:

https://www.enrgtech.co.uk/buy/search?search_by=keyword&terms=Z21808E

(the images are not correct, E-model doesn' have shield)

It's still only 5k4:150 ohms transformer by design  and doesn't have too much headroom. Have to be seen how it performs in real life.

 

[peterbach]

I have converted the Oliver Ami u47 schematic to pcb and I am started making it but the result is noise only I am not sure if it’s tube noise without signal coming from capsule or something else wrong .
Please help

 

[peterbach]

Pcb silk

 

[peterbach]

Schematic

 

[peterbach]

My build