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

[gyraf]

from the description, I am pretty sure that you have a ground missing to the chassis/shield/mesh of the mic. Try beeping from ground (XLR3Pin1) to all parts of the mic to find what is not electrically connected to the rest..

Jakob E.

 

[Banzai]

A loose bottom bell can also cause shielding problems. Just tighten a bit more to make sure.

 

[Gustav]

Went through all the ground point nodes on the mic PCB, -P had some gunk on it (had already cleaned up on the p2p section).

All good now, thanks for the help!

Just building these as a favour for friend who has had the parts for ages, so it would have sucked to give him back one not working

edit:this was fun, by the way!!!

Gustav

 

[Gustav]

The pair...

Noting left but tidying up the cabling on the PSUs.

Gustav

 

[Banzai]

If you haven't already, make sure to remove all paint from the top circle of the body sleeve where it connects with the headbasket. Same for the little cutout where the capsule plate tab sits in the body sleeve. These areas need clean contact for the body to shield properly.

It's why the bell can cause problems. When loose, the sleeve and capsule plate tab/headbasket might no longer be properly connected.

 

[PG_Sax]

Has anyone experimented with changing the plate voltage up and down? Thinking 55v-75v?

 

[axlreddy]

Just finished mine!
The problem was sloved!
There are one jumper that I miss between R4(T2) and relay.

Thanks GroupDIY

 

[PG_Sax]

Just wondering whether anyone has any insight to this scenario I'm in,

Was having trouble getting the capsule polarisation to 55v even with the B+ trim pot set to minimum resistance.

When B+ was 120v polarisation voltage was 44v-45v.

So I increased the resistor to the plate from 100k to 125k. This allowed me to get 55v at capsule and the plate voltage was close to 62v.

Would it be better to change the voltage divider (to capsule) at R3 and R5 dropping R3 to increase voltage to capsule?

Any help much appreciated.

 

[Delta Sigma]

How are you measuring your capsule voltage? Where exactly are you putting your VM?

You should be able to read your capsule votage at the R3/R5/R6 junction. To be sure your VM isn't loading the circuit, measure across R3 then subtract that from your measured B+. Result should be the same as what you measured across R5. Don't measure to the grid (R4). That should be 0V as virtually no current flows so assume it is and only reference ground.

The front backplate (plus rear backplate in omni & rear diaphragm in fig-8) sees 0V from the junction of the R4 & the grid. The front diaphragm (plus rear diaphragm in omni & rear backplate in fig-8) sees half of your B+ from the junction of R3 & R5. No current flows through R6 so if R3 & R5 are the same value, you get half of your B+. If you want to change the capsule voltage while keeping your B+ at 120V, change the R3/R5 voltage divider.

If you want 60V on your capsule and a B+ of 120, it's probably safe to assume you have 60V on the capsule if you have all the right component values (matching R3/R5). I think you'd hear some audible issues if you had current flowing where it shouldn't in your hi-z section.

 

[PG_Sax]

How are you measuring your capsule voltage? Where exactly are you putting your VM?

You should be able to read your capsule votage at the R3/R5/R6 junction. To be sure your VM isn't loading the circuit, measure across R3 then subtract that from your measured B+. Result should be the same as what you measured across R5. Don't measure to the grid (R4). That should be 0V as virtually no current flows so assume it is and only reference ground.

The front backplate (plus rear backplate in omni & rear diaphragm in fig-8) sees 0V from the junction of the R4 & the grid. The front diaphragm (plus rear diaphragm in omni & rear backplate in fig-8) sees half of your B+ from the junction of R3 & R5. No current flows through R6 so if R3 & R5 are the same value, you get half of your B+. If you want to change the capsule voltage while keeping your B+ at 120V, change the R3/R5 voltage divider.

If you want 60V on your capsule and a B+ of 120, it's probably safe to assume you have 60V on the capsule if you have all the right component values (matching R3/R5). I think you'd hear some audible issues if you had current flowing where it shouldn't in your hi-z section.

Thanks for your reply.

Yup thats where I'm measuring from. I can double check across R3 but have measured at R3 and R6.

The meter is loading the circuit slightly but then I would expect to see the same thing at R2 which I don't.

Basically there are 10v going walkabout after the voltage divider between R3 & R5. (ie voltage in 110v voltage out 45v)

Mic works great but just bugging me a little why I'm seeing this drop in voltage for the capsule.

With B+ at 110v I see 55v at plate as I should but 45v at the R3 and R6.

I have checked R3 and R5 and they are the same value. Its a weird anomaly.

Interestingly I'm running the plate voltage higher at the moment and the most I can get at R6 and R3 is 54v. I should say that I have now returned R2 back to its original value of 100k did have it at 125k to balance the voltage differential.

Just measured across R3 and R5 I see 54v across both of them which is the same voltage I see at the capsule. At the moment the B+ is at 134v so that would mean 80v at capsule! Which I think would mean no more capsule surely? As I said mic has been working great just recorded on it all day.

Can't see why my meter would load down 13v each side but maybe I'm wrong? Input impedance is 1Mohm on meter.

 

[Avgatzeblouz]

Just finished mine!
The problem was sloved!
There are one jumper that I miss between R4(T2) and relay.

Thanks GroupDIY

Hey, this body looks nice, which one is it ?

 

[PG_Sax]

Anyone with deeper knowledge know,

Is there any disadvantage to using the a 1/4w grid to ground to resistor at R4. I see most people including Dany put a 1/4w in there so I'm guessing not as can't see current / voltage being an issue. Also its 1/4w on the BOM, but on all Schematics its 1/2w.

Cheers

 

[Delta Sigma]

Nothing wrong with 1/4W.  Telefunken probably could only source 1/2W high value resistors.

Unless there is something wrong with your mic wiring, half of your B+ is at your capsule. 1M VM input impedance is quite low and will load the 500k resistors. Most modern meters will have an input impedance of 10M or greater in voltage mode.

You may want to find a voltmeter with a 10M input impedance , even for measuring your B+. Set your B+ for 120V if that's what it's supposed to be. If you want 55V at your capsule, change R3 & R5 for the correct voltage divider.

 

[Delta Sigma]

Hey, this body looks nice, which one is it ?

I'm curious about this as well. Did you powder coat the mic & PS? Looks very nice!

 

[PG_Sax]

Nothing wrong with 1/4W.  Telefunken probably could only source 1/2W high value resistors.

Unless there is something wrong with your mic wiring, half of your B+ is at your capsule. 1M VM input impedance is quite low and will load the 500k resistors. Most modern meters will have an input impedance of 10M or greater in voltage mode.

You may want to find a voltmeter with a 10M input impedance , even for measuring your B+. Set your B+ for 120V if that's what it's supposed to be. If you want 55V at your capsule, change R3 & R5 for the correct voltage divider.

Cheers Delta!

I'm going to test with a different meter tomorrow that has a higher Input impedance (10M). I may build another circuit as well.

The B+ voltage was higher as I was testing the effect of more voltage to the tube. Have had the B+ at a lot of different levels.

I still don't understand why I would see 54v with the meter connected at R3 and ground? It's not in parallel to the 500k resistor then? So meter loading shouldn't be an issue should it?

 

[Delta Sigma]

By measuring at the junction of R3 to ground you are in parallel with R5.

 

[PG_Sax]

Sweet! Thats excellent Delta, nice one!

Will confirm tomorrow with the other meter, fingers crossed!

By measuring at the junction of R3 to ground you are in parallel with R5.

 

[100WChris]

Hi,
i´m planning to build a ELA 251 based von the pcb-set from vintagemicrophonepcbkit.
Can somebody give me a hint, where i could buy a prpoer donor-mic in europa?!
Thanks
Chris

 

[TillM]

you can take this
https://www.musicstore.de/de_DE/EUR/Fame-Audio-Pro-Series-VT-12/art-REC0009822-000

or look at Thomann. There is a T-Bone with the same mic body, you just had to drill the holes in the mic PSU for the capsule orientation switch.

 

[100WChris]

Thanks TillM,
i tried the tbone SCT-800, but both pcbs (mic, psu) do not fit!
I searched all over the www, but i could not find any of those Alctron HT-11-Mics in Europa
The Fame-Mic seems to be the only one, but it´s expensive… i could probably order a donor mic from Studio393 for less…
 
Is there no source for a HT-11-Style-Mic in Europa (below 200€…) ?!?
kind regards
Chris