Official C12 Clone - Build and Support Thread

[DuBmastah]

I know that was quite a lot of comments with informations splitted everywhere, so this is one where most informations are reported in case you join the party

SYMPTOMS :
The microphone does not produce sound except for some hum when opened.
CONFIGURATION :
- Apex 460 body
- Apex PSU (with Matador mod)
- Peluso CEK-12 capsule
- Microphone Parts 6.5/1 output transformer (to be upgraded for T14 later)
- Matador PCB rev 1.4 (2021)
- GE6072 five stars tube, NOS
ALSO TESTED WITH :
- Apex 460 original capsule
- EH 6072 tube, good condition
- Mic and Mod "vintage look" PSU
TUBE VOLTAGES :
- Pin 1 (P1) : 110V (B+ was 120, unloaded at the PSU)
- Pin 2 (hole) : -67mV (bias was -1V at the PSU, unloaded)
- Pin 3 (K1) : grounded (Pin 9 is also grounded)
- Pin 4 : 6.30V
CAPSULE VOLTAGES :
- Front : 0V
- Rear : 7.54V (41.5V before R16, cardioid position)
- Backplates : 1.14V (54.2V before R14)
PCB INPUT VOLTAGES :
- P3 (polar) : 41.5V (carioid position)
- P4 (bias) : -1.7V
- P5 (B+) : 112.3V (get 122V out of PSU, unloaded)
- P8 (heater) : 6.3V

 

[DuBmastah]

Hope someone will find something that is not in the normal range there

 

[Mikael-ange]

B+ need to be at 120v, checked at the psu loaded as I said before (so adjust it with the mic connected).
Same for bias.

 

[Delta Sigma]

I know that was quite a lot of comments with informations splitted everywhere, so this is one where most informations are reported in case you join the party

SYMPTOMS :
The microphone does not produce sound except for some hum when opened.
CONFIGURATION :
- Apex 460 body
- Apex PSU (with Matador mod)
- Peluso CEK-12 capsule
- Microphone Parts 6.5/1 output transformer (to be upgraded for T14 later)
- Matador PCB rev 1.4 (2021)
- GE6072 five stars tube, NOS
ALSO TESTED WITH :
- Apex 460 original capsule
- EH 6072 tube, good condition
- Mic and Mod "vintage look" PSU
TUBE VOLTAGES :
- Pin 1 (P1) : 110V (B+ was 120, unloaded at the PSU)
- Pin 2 (hole) : -67mV (bias was -1V at the PSU, unloaded)
- Pin 3 (K1) : grounded (Pin 9 is also grounded)
- Pin 4 : 6.30V
CAPSULE VOLTAGES :
- Front : 0V
- Rear : 7.54V (41.5V before R16, cardioid position)
- Backplates : 1.14V (54.2V before R14)
PCB INPUT VOLTAGES :
- P3 (polar) : 41.5V (carioid position)
- P4 (bias) : -1.7V
- P5 (B+) : 112.3V (get 122V out of PSU, unloaded)
- P8 (heater) : 6.3V

Bias should be -1.1. Measure at the PSU side of R11, you can't measure it at the tube grid.

Which triode do you have wired up? Pin 1 is the 2nd triode. If it is in use, you should be seeing a much lower voltage than B+. Maybe around 70V, I can't remember what it should be on a C12, but it will change when you correct your bias.

Is your backplate tied to both C13 and R14? When the mic is off and unplugged from the PSU, confirm continuity (your DVM should have continuity / beep mode) between the front diaphragm and 0V, then the front backplate and R14 AND C13. The backplates need to connect to both.

B+ need to be at 120v, checked at the psu loaded as I said before (so adjust it with the mic connected).
Same for bias.

B+ at 112V is a very negligible change in audio level. Some capsules sound better with 55V instead of 60V. I run my C12s with a B+ of 110V. This isn't the reason there is no audio.

 

[Mikael-ange]

B+ at 112V is a very negligible change in audio level. Some capsules sound better with 55V instead of 60V. I run my C12s with a B+ of 110V. This isn't the reason there is no audio.

I agree with you and also run mine at 55v.
I pointed this out for the sake of keeping know value for debuging (since the op keep mentionning is b+ voltage unloaded, and who know maybe loaded it sit at 110v, or 100, or 90...etc).
You can see the point

 

[DuBmastah]

Which triode do you have wired up? Pin 1 is the 2nd triode. If it is in use, you should be seeing a much lower voltage than B+. Maybe around 70V, I can't remember what it should be on a C12, but it will change when you correct your bias.

Hmmmm....
Maybe we got something strange here :
I can mesure 110V at pin 1 of the tube.

other than that :
- i have contibuity beetween backplates, C13 and R14
- i have continuity beetween front diaphragm and chassis/ground

 

[DuBmastah]

Any chance R17 could be shorted somehow ?

 

[DuBmastah]

Any chance R17 could be shorted somehow ?

Not at all...
I still have 100K beetween P5 at B+ input, and pin 1 of the triode....
Maybe someone could figure out why there is still 110V there...

 

[JeffKall]

Many thanks Delta Sigma for helping me figuring out what the problem might be.
As for the points you enumerate, everything is OK.
(and for the capsules, the peluso uses 2 cables for the backplate wich are linked, and the RK12 from MicParts, there's only one, but the backplates are linked internaly).
At this point i'm checking back all the voltages and will post results in next comment.

I, too, have been down this road in my Newbie DIY journey …

Since your voltages all check out … I agree with Delta that you probably need to look elsewhere …

It happened to me in one of my first mod projects … no audio !

Since I had modded two Apex mics with the Fox Mini Mod … I was able to first confirm that the PSU’s were fine by comparing all the wiring and component connections and then comparing the voltages … first with dummy loads then with my working modded 460 …

The Fox Mini Mod uses most of the original PCB’s and temperamental wiring …so I just traced the audio path from capsule to transformer/7 pin connector … and found a broken wire from moving the PCB around as I was upgrading components …

When you are first starting out down the DIY path … it is a good idea to adapt the carpenters “measure twice … cut once” mentality … which is “check and double check every connection and every component value and orientation (where applicable) as you build”

It is so easy to make a mistake in the complexity of these builds … which … besides enabling recording in stereo … is another good reason for building in pairs …

I just finished a pair of Dan’s (poctop) D-67’s … which are fairly challenging (but well worth it!) … and I had placed one resistor leg in a wrong pad (but I discovered it in my “recheck” BEFORE I powered the mic …

It is a good idea to literally retrace the steps of the build just to confirm connections and component values and orientations BEFORE you connect your mic to the PSU …

Best of luck with your search (it has to be something simple in your audio path) !!!

 

[DuBmastah]

I, too, have been down this road in my Newbie DIY journey …

Since your voltages all check out … I agree with Delta that you probably need to look elsewhere …

It happened to me in one of my first mod projects … no audio !

Since I had modded two Apex mics with the Fox Mini Mod … I was able to first confirm that the PSU’s were fine by comparing all the wiring and component connections and then comparing the voltages … first with dummy loads then with my working modded 460 …

The Fox Mini Mod uses most of the original PCB’s and temperamental wiring …so I just traced the audio path from capsule to transformer/7 pin connector … and found a broken wire from moving the PCB around as I was upgrading components …

When you are first starting out down the DIY path … it is a good idea to adapt the carpenters “measure twice … cut once” mentality … which is “check and double check every connection and every component value and orientation (where applicable) as you build”

It is so easy to make a mistake in the complexity of these builds … which … besides enabling recording in stereo … is another good reason for building in pairs …

I just finished a pair of Dan’s (poctop) D-67’s … which are fairly challenging (but well worth it!) … and I had placed one resistor leg in a wrong pad (but I discovered it in my “recheck” BEFORE I powered the mic …

It is a good idea to literally retrace the steps of the build just to confirm connections and component values and orientations BEFORE you connect your mic to the PSU …

Best of luck with your search (it has to be something simple in your audio path) !!!

Hehehe
I always inspect everything twice, read back the manual, re-inspect again, check all soldier point with magnify tools after decontamination, i just like to do it
Then i learned it is not enough
Not so long ago, i was building 7x the same U47 type clone.
The last one just didn't produce audio.
It took me a little time to see i just inverted some 47K resistor with 470K, but note that i check EVERY resistor below 30Mohm before populating the board. That's just happened because they are located very close and because the color code is pretty near. When you begin to do the same board back again a certain amount of time, you take too much confidence with yourself.
I have quite an amout of things to learn before i can fix myself errors i've done at first place. This issue is a good occasion to learn more, but at this point, fixing this microphone seems to be well over my knowledge.

Right now, i'm just awaiting Delta Sigma, Mickael-ange, Drtechno and possible other good fellows for some comments about having +110V at pin 1 of the tube, wich could lead to some more areas to track down as far as i understand


Btw, if Matador ou Chunger would pass by, i would be very curious and interested to read their comments on the subject ?

 

[JeffKall]

Hehehe
I always inspect everything twice, read back the manual, re-inspect again, check all soldier point with magnify tools after decontamination, i just like to do it
Then i learned it is not enough
Not so long ago, i was building 7x the same U47 type clone.
The last one just didn't produce audio.
It took me a little time to see i just inverted some 47K resistor with 470K, but note that i check EVERY resistor below 30Mohm before populating the board. That's just happened because they are located very close and because the color code is pretty near. When you begin to do the same board back again a certain amount of time, you take too much confidence with yourself.
I have quite an amout of things to learn before i can fix myself errors i've done at first place. This issue is a good occasion to learn more, but at this point, fixing this microphone seems to be well over my knowledge.

Right now, i'm just awaiting Delta Sigma, Mickael-ange, Drtechno and possible other good fellows for some comments about having +110V at pin 1 of the tube, wich could lead to some more areas to track down as far as i understand


Btw, if Matador ou Chunger would pass by, i would be very curious and interested to read their comments on the subject ?

We have clearly traveled that same cautious Newbie path !

I am grateful for everyone on GroupDIY … that have the knowledge and patience to come to our rescue when we are STUCK !

Matador tends to be very responsive … especially if you message him directly …

But just about any of the well known members here should be able to help you sort this out …

It’s what makes this community indispensable !!!

Good luck !

 

[Delta Sigma]

Hmmmm....
Maybe we got something strange here :
I can mesure 110V at pin 1 of the tube.

other than that :
- i have contibuity beetween backplates, C13 and R14
- i have continuity beetween front diaphragm and chassis/ground

But which triode are you using? Pin 1 is the plate of triode #2.

Edit: I see your photo shows you are connected to what the tube datasheet calls triode 2, but (confusing for this discussion) the PCB labels it triode 1.

If this is the case, your tube may not be conducting. I assumed, incorrectly, that you had measured that 110V at the unused triode.

With the power supply unplugged from the wall, measure for continuity between pin 4 of the tube socket, and H+ inside the power supply. Then measure between pin 9 of the tube socket and 0V inside the power supply. Then check continuity between pin 3 of the tube socket and 0V.

Remember that measuring voltages in hi-z sections if difficult because your DVM loads the circuit. Best to check voltage before the Hi-z section, then print out the schematic and check each node for continuity between all places the schematic shows a connection (with the power off).

Once the tube is conducting, you should be able to measure your plate current by measuring the voltage drop across your load (plate) resistor, then use ohms law. It's generally less than 1mA with common mic tubes in common mic circuits. Maybe 0.6mA-0.8mA.

 

[DuBmastah]

Thanks.
I'll make sure i understand eveything that youwrote and come back with the mesurements

 

[DuBmastah]

Once the tube is conducting, you should be able to measure your plate current by measuring the voltage drop across your load (plate) resistor, then use ohms law. It's generally less than 1mA with common mic tubes in common mic circuits. Maybe 0.6mA-0.8mA.

I don't get it. what's my "load (plate) resistor" ?
If i understood properly, my plate is supposed to be pin 1 at the tube socket, right ?
Then the resistor you're talking about would be R17 ?
I got 120V at the PSU on B+ (unloaded)
Then i got 110V all over this line straight to pin 1 of the tube when loaded.
Do you mean the R17 (100K) is responsible for this 10V drop ?
I'm pretty confused as i mesured also 110V at P5, wich is before R17.

This is surely a dumb question that shows i'm a perfect newb if anyone has doubpt on the subject :-D
Else : i've checked the continuity's (P4->H+ and Pin's 9/3->0V), everything was tested OK.

 

[DuBmastah]

Maybe this voltage could have drop because a little part of the current is sent to ground via R12 and R13 ?
I feel far from even understanding the basics....

 

[DuBmastah]

From what i think i know about the Ohm law, i could imagine :
U = 110V
R = 100.000Ω
then...
I = 1.1mA
One thing i don't understand is why intensity could change (depending on circuits)...
R won't change as it's fixed, right ? So to get an intensity drop down, voltage should change, right ?
Maybe i could unsolder R17 to get my multimeter inserted there and mesure intensity...

 

[Mikael-ange]

What I don't understand is why you don't get half your b+ voltage value after R17.
That the only thing between your b+ and your plate.

 

[DuBmastah]

What I don't understand is why you don't get half your b+ voltage value after R17.
That the only thing between your b+ and your plate.

I don't understand either.
R17 mesure 100K as expected on my multimeter

 

[Delta Sigma]

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.

 

[DuBmastah]

Maybe the plate drains the 110V from another source also ?
But if that the case, i just can't figure out how it does that.