Teladi K120 tube mic on the bench

[zebra50]

Another small tube mic for dissection and discussion...

This Teladi K120 looks very gefell-like to me. The diaphragm seems to be Ni coated rather than gold, and is rather smaller than most large diaphragm condensers, (but larger than small ones!)

I've traced out the circuit as far as I can. Sadly the PSU is no longer around.

I can't read all of the resistors without unsoldering - that will be the next step.

I thought the capsule polarisation and cathode bias looked interesting - not quite standard.

 

[zebra50]

Hi,

I've had this wee beasty up and running now. It actually sounds very good, but very noisy, so I'm taking the trouble to strip it down and rebuild. Modern caps are smaller so there should be enough room to squeeze in a small cinemag output tranny and run it low-Z, balanced.

Some of the resitors are hard to read so I've pulled them and measured. The diagram above has been updated.

Can someone please have a quick look at this for me? Having a 1 meg plate resistor on the first tube stage seems high to me, but that's how it measures. The cathode resistor is also high (20K + 50K), so I'm reading this as simply running at high impedence.

Thanks!

 

[gyraf]

A 1M Anode resistor for a ECC82 working off some +250V seems very wrong to me - and this is probably the cause of the mic being noisy.

Some of the old carb-resistors would go open over time - this is probably what happened here.

As the cathode resistor for the first stage is rated 70V - and voltage-rating never was cheap in those days - you should expect that the circuit idles somewhere close to this voltage.

70V across the 70KOhm total cathode resistance makes sense because this would draw 1mA through the tube - which is within decent range for a ECC82.

Aim for 60V at the cathode - to prevent overvoltage at the decoupling capacitor: This gives (assuming +250V supply) - 190V to be dropped over anode resistor and tube - try something like 100K to 220K and measure result..

Are you sure that the ECC82 is the original tube? It seems like an odd choice for a mic - a ECC81/12AT7 or a 12AY7 would probably make much more sense here (my guess). Maybe somebody put the '82 in there for an easy and quick get-it-running-fix - which inotially may have worked, but drew too much current and burned the anode resistor over time..?

Jakob E.

 

[zebra50]

Thanks Jakob,

The cathode resistor does look like it has been running hot. From the coloured stripes it looks like one-zero-something, but the next stripe is burnt off or or faded. (Could be green, or yellow ?). Perhaps it was a 100K that is failed-open? But it still works, despite this, which puzzles me.

60V on the cathode would suggest something like 40V for capsule polarisation, which is ball-park OK.

No way to tell if it's the original tube - it is an RFT tube, which looks right for the era, but that tells us little.

 

[rodabod]

I'd go with Jakob and personally guess for 100K judging by the numbers you can read.

I've had carbon comps like that go open before (although they went above 1M - does seem a coincidence if it were dead-on 1M though).

 

[ioaudio]

do you have more pics from the guts?

it seems that the teladi K120 is a figure 8 mic - and this one is the K125.
and it seems ecc82 was indeed the tube used here.
teladi is still around, maybe ask them for help: teladi.de

-max

 

[zebra50]

Interesting.... Where did you find your information? Radiomuseum? This one says K120 on the side, and it is definitely cardioid!

I didn't take many photos but I put a couple of higher res shots up:

http://www.xaudia.com/omnip/Mics/Teladi_K120/Teladi5.jpg
http://www.xaudia.com/omnip/Mics/Teladi_K120/Teladi6.jpg

Thanks!

:thumb:

 

[elektrovolt]

Zebra, I know this topic is already 5 years old.
Still I would like to know if you have made progress with this microphone.

I recently bought a Teladi K125 which is a bit different:
there is no switch, only a small adjustable wedge underneath the capsule that can push the bottom of the capsule.
This can be done with a small screwdriver. I believe this could be some sort of pattern selection system..

Did you get rid of the noise? My K125 does some hiss and hum, plus some random popping noise every 15 - 20 secs..

I'm glad that I do have the original power supply and cable.

I'll post some pics soon.


Jimi

 

[Moskito67]

Hi zebra50,
hi elektrovolt,

I own a Teladi K100/12, which is almost the same as the K120, but also without switch.
I unfortunately don't have the PSU (and also no cable and connector).
I plan to build a new PSU, but don't find any Teladi PSU original scheme for this mic.
Does anybody have any information which could help me ?

Thanks,

 

[elektrovolt]

Hi Fred,

the only schematic of the psu I have is this crappy one I did myself.
see link:

https://dl.dropboxusercontent.com/u/23753033/teladi%20k125%20schem.pdf

 

[rock soderstrom]

@zoubroff

Welcome Arnaud, elektrovolts post is from 2014!
Do it like this and he will get a note that someone has reacted to this post.

Hi Fred,

the only schematic of the psu I have is this crappy one I did myself.
see link

You can PM him as well if he doesn't react to this post.

Cheers

 

[zoubroff]

@zoubroff

Welcome Arnaud, elektrovolts post is from 2014!
Do it like this and he will get a note that someone has reacted to this post.

You can PM him as well if he doesn't react to this post.

Cheers

Hi rock,
yes I saw it's from 2014 !! I'll follow your advice. thanks a lot !
Arno

 

[zoubroff]

Hi Fred,

the only schematic of the psu I have is this crappy one I did myself.
see link:

https://dl.dropboxusercontent.com/u/23753033/teladi k125 schem.pdf

Hi elektrovolt
I see you made a schematic for the teladi PSU, long time ago.
any chances you still have it?
I found a teladi 250 microphone, but no PSU or Cable.
thanks by advance,
Arno

 

[mjrippe]

Here is the file he is referring to...

 

[zoubroff]

Here is the file he is referring to...

thanks a lot, I guess this will be helpfull ! cheers Arno

 

[FrX_74]

I have Teladi K120 complete with PSU. After 68 years I just clean a little bit some contacts and the capacitors inside the PSU are still good. Of course I measure first the ESR and the Vloss (ESR was 0,92ohms and 0,68ohms and Vloss was 3,2% and 4,3% ).
I give a main power to PSU first with Variac and I starting from 50Vac to 220Vac. With 220Vac the PSU exit was 270Vcc and 50Vcc.
Attached some PSU photos.