Rode NTK Capsule Collapsing

[Danielkorn]

Hey together,

I am using my Rode NTK about 8 years. After the first two years Ive replaced the original capsule with the Tim Campbell CT12. Everything worked well. Since about 3 years it happened randomly that the capsule is collapsing. Sometimes after few seconds, sometimes after an hour, sometimes it works for hours. Since now it happens more and more often. Thought it could be a capsule problem. I dont have the CT12 capsule anymore because I wanted to send it to Tim to check it, but the packaging got lost... Anyway I ordered another one from Aliexpress. I think its the manufacturer of the Microphone-Parts RK12, this one I bought: 50.0US $ |Studio 34mm studio mikrofon kapsel große gold membran einseitig Pro DIY Element Ersatz rand beendet draht|Shell & Body Parts| - AliExpress

But the same problem happens. I also have changed different tubes but its not a tube problem. I have read that it could be a problem that too many voltage will cause to that issue. Had someone the same experience and know what need to be done? Seems that it is a more complex problem where parts need to be checked in detail by measuring, where I have no experience with. I just checked every part in the mic and power supply by taking a look but I cant find any issue. If its a too complex thing I would give it to someone who has interest to repair it. Would sell it for about 120€ together with the Rode SMR mount.

Thanks in advance!

 

[Vac11]

Hi Daniel,

it looks like your mic´s polarisation voltage is too high. You should try to set it about 60V (CT12 from Tim dont like more than 60V). Check polarisation voltage with multimeter before high ohm resistors and say us whats youre measurements are.

 

[Danielkorn]

Hey Vac,

thanks for your reply. Yes as I told I have no experience about measuring it. Curious thing is that the Tim Campbell worked for 2 years without any problem. As I wrote I also have no Tim Campbell anymore. DHL delivery problem and the capsule got lost somewhere in Hamburg... Just got 60€ as compensation. So I bought the chinese C12 version. Anyway I can try to fix it but need some time or help. Is there any good tutorials about it? Otherwise I would need to take a deeper look here in some helpful threads! But as I know me I damage things more instead of repair & fix them.

 

[mhelin]

R4 burned or has cold solder joint?

 

[rogs]

With the values shown on mhelin's schematic, it looks as if the capsule is presently looking at a polarisation bias of c. 78V.
That does seem a little high.
Changing the value of R5 from 2M to 3M3 Ohms should reduce that to c. 62.5V -- which seems less likely to cause problems?

 

[RuudNL]

As far as I see, the polarisation voltage is directly coupled to the grid of the tube.
Would changing the polarisation voltage affect the biasing of the tube?

 

[rogs]

I think you're right Ruud -- Looks like my idea would make the grid bias voltage the same as the cathode. Not good.
Back to the drawing board with that one!
I still think 78V maybe a bit high as a polarisation voltage for a 'standard' cheapo Chinese edge terminated capsule though....

 

[mhelin]

It might be also that the power supply is faulty and the supply voltage is more (negative) than the -125 volts.

 

[gyraf]

..note that the CK12 and its variations sounds significantly better at polarization voltages at or below 60V

On that schematic, I totally don't like the 3G Ohm grid bias resistor for a 6922. You need a good deal of luck (=selection) to make this work even remotely well

/Jakob E.

 

[OPR]

The Ck12 will definitely fail (diaphragm collapse) under not much over 60v. It may work initially but as soon as it gets a plosive or a blast of air the diaphragm will stick to the backplate. Also the Ck12 is designed to sound best at 60v much higher and the diaphragm movement is restricted from excessive static charge. As well as making a conventional Ck12
I also make custom CK12 that works under higher polarisation voltages for K2 and NTK mics.

 

[ZHAOMENG]

The spacer of the capsule is too thin, and the design voltage of the capsule is very low

 

[ZHAOMENG]

You have to remove the back part to work, otherwise it's very difficult for the gas to get in, and I don't show you how to disassemble it because that would be a waste of time

 

[RuudNL]

I suppose this might work?
In this situation the backplate of the capsule isn't connected to ground, but lifted about 22 Volts.
This would result in about 56 Volts over the capsule while still keeping about -78 V on the grid of the tube.

 

[LevinGuitar]

As far as I see, the polarisation voltage is directly coupled to the grid of the tube.
Would changing the polarisation voltage affect the biasing of the tube?

Yes, it will, I remember that the cathode voltage was around -75v/-76v or so, so it should be done as you suggest changing the backplate voltage from 0v to the desired one (negative of course).

 

[RuudNL]

I never mentioned a change of the biasing of the tube (as in the quote), because it won't.
The backplate voltage is 'lifted' a bit, so that the voltage over the microphone capsule will become lower.
In this situation you can safely use a different microphone capsule. (Most capsules can withstand 60 V.)