Official C12 Clone - Build and Support Thread

[Delta Sigma]

Does your tube glow when it's warmed up? It should glow faintly.

 

[Delta Sigma]

Do you measure a small resistance between pins 4 and 9 on the tube when it's pulled form the socket? Same when it's installed? Could your socket be bad? If any one of pins 1, 3, 4, or 9 are not making contact, the tube won't conduct.

 

[DuBmastah]

R17 is your load resistor.

If you are not seeing any voltage drop across R17, then the tube is not conducting. There is no current through the tube. This is your issue.

If you have 120V DC on one side, then 55V on the the other side, then you'd have 65V dropped across the resistor and 55V dropped across the tube. R17 being 100k, you would have 0.65mA of quiescent (idle) current running through the resistor. There is no other path to ground for the current, so that tell you that you have 0.65mA of DC current from plate to cathode. As the input to the grid changes, the amount of voltage dropped across the tube changes. This changing voltage is the ac signal that represents the audio.

Load resistor is the resistor from plate to (audio) ground. B+ and ground are the same to audio. In other words, it's a 100k load from plate to ground which is also in parallel with your transformer secondary.

Wooaaa, pretty impressive.
I didn't really understood the concept and i know i have to dig far more in theorics before it could make sens to me.
But this would lead to a possible problem with the tube itself then ?

 

[DuBmastah]

Does your tube glow when it's warmed up? It should glow faintly.

No, it doesn't.
When the GE was in place i could see something shining in a yellow or orange color at the top of a plate on an area like 1mm large, but there was also no audio.

 

[DuBmastah]

Do you measure a small resistance between pins 4 and 9 on the tube when it's pulled form the socket? Same when it's installed? Could your socket be bad? If any one of pins 1, 3, 4, or 9 are not making contact, the tube won't conduct.

Will check that right now....

 

[Delta Sigma]

If the pins are too loose, you can stick a small screwdriver or dentist pick type tool in there to push the contacts back together.

I think a tube heating element should be in the 30ohm range. That's what you should measure between pins 4 and 9

 

[DuBmastah]

Delta Sigma, i think you're in the right path now.
let's see what i've got :

At first i had :
- 17,5Ω beetween pins 4 & 9 of the tube (pulled out)
- infinity at the socket pins (tube in place)
Then, i tightened the pin receptor n°4 on the socket and got :
- 17,5Ω beetween pins 4 & 9 of the tube (pulled out)
- 17,5Ω at the socket pins (tube in place)

Sounds pretty cool, right ?

Next, i could mesure changes in the voltages :
- I read 102,8V at the P5 position (B+ from the PSU)
- this drops down to 88.7V after R17

So i checked for audio, but...
1) the Hum had considerably increased
2) i could hear loud cracks manipulating the microphone, cables, and anything lying on the table around.

(See attached video for details)
View attachment C12 cracks.mp4

 

[DuBmastah]

The sound is not very loud on the video because it comes from a distance ;-)

 

[DuBmastah]

So, that sounds pretty much like a bad contact, i'm going to check other pins for loose connections beetween the socket and the tube.

 

[DuBmastah]

POPOPOOOOOO, THAT WAS IT !!!!!!!!
I HAVE SOUND !!!!

 

[DuBmastah]

Delta Sigma, YOU'RE A BOSS!
I have no words to say how much i thank you

 

[DuBmastah]

I use a pen to gently loosen up the tube sockets because I could not insert the tube into the socket otherwise without risking bending the solder leads. Do not loosen them too much. Just enough to allow insertion and removal of the tube but retain a solid grip and contact.

Sorry Chunger, but this part of the manual is a bit tricky.
I used the same "pen" technique as you described and i think this is where the problem came from at first place.
You probably may advice newbs that this could lead to problems later on and then invite builders to check continuity in those connections if they experiment issues like i did.
I was convinced my problem was something weird as the built itself looked clean, but i was not searching something dumb as this from a new, well soldered socket.
I know this is a problem of lack of experience from my side, but maybe this also could fool other inexperienced builders

 

[Delta Sigma]

Good to hear!

I've never widened my tube socket pins using a pen. I usually put an old tube in and take it out repeatedly until it's loose enough that removing a tube on a finished mic is possible.

 

[DuBmastah]

Now it's time to re-decontaminate everything, check and tune properly all voltages from the PSU, put back the Peluso capsule, put back the GE NOS tube, re-check for audio, close the mic, test the mic in the studio, wait for Moby to send a T14/1 transformer, open the mic, change the transformer, re-decontaminate solder points at xp1 and Xp2, check for voltages and audio, close the mic, test the mic in the studio, and then.....
I would probably have a working nice C12 clone back again

I have a least (or i hope so) a last question for the community.
I'm a bit confused now about voltages tuning.
Seems like everyone is making a bit of his own method for this.
Should i use Matador's protocol to do so ?
I'm not sure now when i must load the mic or not, and where exactly i must put the probes for each mesurements, even not sure anymore about 110V or 120V at B+....What's your opinion on that ?

 

[DuBmastah]

Good to hear!

I've never widened my tube socket pins using a pen. I usually put an old tube in and take it out repeatedly until it's loose enough that removing a tube on a finished mic is possible.

I'll keep that in mind for later.
That was my second "tubed mic" project, as the first one was the same mic, but with the Fox kit and this one keeps the original Apex 460 tube socket untouched.
I've learned good things with you guys, but this one i'm pretty sure i will never forget.

Btw Delta Sigma, i putted my fingers on the tube multiple time at the end. Should i decontaminate the tube also ? and if the answer is yes, would isopropanol fade the inscriptions on the tube wich could lead to an impossibility to identify it in the futur ?

 

[Delta Sigma]

I'll keep that in mind for later.
That was my second "tubed mic" project, as the first one was the same mic, but with the Fox kit and this one keeps the original Apex 460 tube socket untouched.
I've learned good things with you guys, but this one i'm pretty sure i will never forget.

Btw Delta Sigma, i putted my fingers on the tube multiple time at the end. Should i decontaminate the tube also ? and if the answer is yes, would isopropanol fade the inscriptions on the tube wich could lead to an impossibility to identify it in the futur ?

Just clean the pins if you touched them with bare hands. Oil on the grid pin could be an issue. Not as critical as the othe hi-z sections, since the pins really only touch the socket pins.

 

[Mikael-ange]

@DuBmastah glad to hear everything is working now
As far as voltage tuning go, everything is with the mic plug into the psu.
Heater checked at the mic itself (tube socket pin)as you have done a million time now.
B+ and bias at the psu (with the mic pluged).
Now for b+ choise, max is 120v (so 60v polarization) for a c12/m251 (for a ck12 capsule in general).
But a lot of people prefer to run with a lower polarization voltage (55v instead of 60v for exemple, so 110v b+ vs 120v).
Sound a little bit more relaxed imho. But that more a taste thing.
So you can try both and choose for yourself what you like.
Anyway, well done

 

[DuBmastah]

@Mickael-ange : following your advices, i opted for 110V B+ and will give it a try
All voltages checked Ok, decontamination OK, Peluso cap and GE tube are back inside, mic is closed, sounding apparently OK, so far so good...
Will check this all properly in the studio tommorow and will try to figure out what kind of preamp i got does the best job with it and how it compares against other mics i got that uses CK12 type of caps.

Personnal message : Je vais suivre tes posts sur ces forums avec beaucoup d'attention, je suis sur que je vais apprendre plein de choses.

 

[Mikael-ange]

@Mickael-ange : following your advices, i opted for 110V B+ and will give it a try
All voltages checked Ok, decontamination OK, Peluso cap and GE tube are back inside, mic is closed, sounding apparently OK, so far so good...
Will check this all properly in the studio tommorow and will try to figure out what kind of preamp i got does the best job with it and how it compares against other mics i got that uses CK12 type of caps.

Personnal message : Je vais suivre tes posts sur ces forums avec beaucoup d'attention, je suis sur que je vais apprendre plein de choses.

So where are you located in France?

 

[DuBmastah]

So where are you located in France?

Villemoisson sur Orge (Essonne, 91)