Official C12 Clone - Build and Support Thread

[trans4funks1]

Cinemag doesn't post a data sheet for the CM13114, but the data sheet they post for the CM-2461NiCo shows Red (+) and Brown (-) on primaries and Yellow (+) and Orange (-) on secondaries.

http://cinemag.biz/mic_output/PDF/CM-2422.pdf

It seems like it would be niceto have data sheet available for the CM13114.

 

[Dennyp]

Thanks trans...That is a good starting point ;D

 

[dmnieto]

If your pre-amp does have a phase invert button you probably don't care too much

 

[Reverend]

I'm getting a lot of hum and noise out of the mic when I do the test with the stock Alcatron capsule. Is this ok? I'm thinking that I've got a ground problem. What should I check first. The power supply, I've gone over and over - I think that's good. I'm a little confused with the wiring of the test capsule and the wiring of the mic's transformer, but I do think I got that right. How should I go about finding the issue with the hum/grounding issue? Help? Thanks.

 

[Reverend]

Hey guys. I'm back. I just went ahead and moved forward and installed a Peluso capsule even though I was getting hum/grounding issues with the stock Alcatron capsule. Peluso told me to wire the two wires coming off of the side of the capsule to the "Grid on the Tube". Where should I solder these two wires? Thanks again.

 

[LHS]

I just finished wiring my power supply and I'm not getting anywhere near desired voltages on B+ and Bias. Heater started around 7.7 and was easily adjusted to 6.3 but after being powered on for a few minutes the power transformer got extremely hot and I smelled faint burning smell so I powered off  :-\

I'm pretty much a noob (although I built max's 47 without a hitch) so I'm not sure where to start troubleshooting.

 

[LHS]

attached is a photo in case you see any glaring problems with my build

 

[chunger]

I just finished wiring my power supply and I'm not getting anywhere near desired voltages on B+ and Bias. Heater started around 7.7 and was easily adjusted to 6.3 but after being powered on for a few minutes the power transformer got extremely hot and I smelled faint burning smell so I powered off  :-\

I'm pretty much a noob (although I built max's 47 without a hitch) so I'm not sure where to start troubleshooting.

Double check all Capacitor values and make sure they all made it to the right spot. . . especially the 3 small ones.  Your problem (no B+, warm/hot transformer, burning smell) sound like a low voltage cap was placed in the B+ portion of the circuit.

 

[Dennyp]

Hi matador & chung
ok, so this is a bit scary
I build up power supply and adjusted the voltages and started to build up the mic.  I was having a really hard time taking the 7 pin XLR connecter out of its casing so i decided to solder it while in still inside. it was a bit difficult but i did it.  turns out, I did it wrong.  when I plugged in the mic, B+ went from 195v with no load to 6.35v.  I trouble shooted and I had soldered the pins in reverse order.  so i rewired everything correctly and my voltages were right on with the mic connected. 

when I plugged it in to do a test, the output was really low and there were microphonics all over the place. (tapping the shell of the mic produced a louder output than speaking into it).  I figured I fried my tube with my wiring fiasco.  I happened to have an older tube so I tried replacing the tube.  when I fired the mic up.  the tube glowed slightly, a loud hum and the whole mic body became electrified.  I actually dropped the mic from the shock (pun intended).  when i picked up the mic (after unplugging everything) the capsule leads were disconnected.  Im not sure if they had come off before I changed the tube or if they came off when i dropped the mic.  (it was about a 1.5 foot drop on a plush carpet)

the tube that i think I fried was the electro harmonix 6072a.  the tube that I replaced it with might have been the stock tube to be honest im not sure.  should I be worried that I fried  my capsule as well?  how would you guys move forward?

shocked and confused,
Dennyp


Edit: since I had some time today to think, I figured the shock happened because something wasn't grounded.  i checked the cables and sure enough, the ground became dislodged.  I re-soldered everything including the capsule. fired up the mic and sure enough pristine sound!!! woohooo!! Ill record some samples and put them on soundcloud.    To quote chung "humans win" although in my case barely =)

 

[stelin]

Dennyp, about how to connect the transformer to the board:
Almost all microphones are wired so that a positive air pressure will give a positive signal.
If the colors of the wires from the transformer are the same for plus and minus, it's hard to know how to connect them "the right way".
But, you can do a simple test: place your C12 beside another microphone and listen to or record both of them (in mono). If the combined sound is very thin, then the microphones are out of phase. If so, simply switch the wires from the transformer to the bord in your new C12.
It's true that most pre-amps have a phase switch, but it's kind of annoying to have one mic that is out of phase when compared to your other microphones.

 

[LHS]

I just finished wiring my power supply and I'm not getting anywhere near desired voltages on B+ and Bias. Heater started around 7.7 and was easily adjusted to 6.3 but after being powered on for a few minutes the power transformer got extremely hot and I smelled faint burning smell so I powered off  :-\

I'm pretty much a noob (although I built max's 47 without a hitch) so I'm not sure where to start troubleshooting.

Double check all Capacitor values and make sure they all made it to the right spot. . . especially the 3 small ones.  Your problem (no B+, warm/hot transformer, burning smell) sound like a low voltage cap was placed in the B+ portion of the circuit.

Double checked all caps and they are correct. Still not getting B+

 

[Matador]

I just finished wiring my power supply and I'm not getting anywhere near desired voltages on B+ and Bias. Heater started around 7.7 and was easily adjusted to 6.3 but after being powered on for a few minutes the power transformer got extremely hot and I smelled faint burning smell so I powered off  :-\

I'm pretty much a noob (although I built max's 47 without a hitch) so I'm not sure where to start troubleshooting.

Double check all Capacitor values and make sure they all made it to the right spot. . . especially the 3 small ones.  Your problem (no B+, warm/hot transformer, burning smell) sound like a low voltage cap was placed in the B+ portion of the circuit.

Double checked all caps and they are correct. Still not getting B+

Your C1 looks like a 25V part (just checking at the "2" on the top).

Can you take a picture not directly over the top, but at an angle so we can better see the part values on all the caps (just move your camera like 6 inches to the left and snap the same picture).

The 100uF caps and 4700uF caps are the exact same size, just different capacitance and voltage ratings.

 

[LHS]

Double checked all caps and they are correct. Still not getting B+
[/quote]

Your C1 looks like a 25V part (just checking at the "2" on the top).

Can you take a picture not directly over the top, but at an angle so we can better see the part values on all the caps (just move your camera like 6 inches to the left and snap the same picture).

The 100uF caps and 4700uF caps are the exact same size, just different capacitance and voltage ratings.
[/quote]

Here is a photo of all the big caps. I triple checked the small caps as well.

 

[dmnieto]

One question...

I went for a shootout today with the C12 and it appears to be much darker and quieter than a Mojave MA-200, actually very surprising, it is using a RK12 capsule while I wait for the Beesneez CK12 capsule, but I am wondering if there could be something wrong in my build that causes that. The tube is a GE 6072A from the 60s and the transformer the AMI T14

I am using the 1uF output capacitor though...

----

I tested it after coming back home, this is what I found.

- Gain appears to be ok, but it seems to distort a bit. It clips on the possitive edges at about ~3V input (Vpp)
- Injecting signal on the grid of the tube, the frequency response is flat till a resonance point at ~70Khz  if measuring after the transformer.
- If measuring at XP1 (before the transformer) there is a high frequency rolloff with 5 dB down at 20KHz (possibly because of the capacitance of the probe?).

Also measuring, B+ voltage is not stable... it decreases steadily at a very low rate. (Could it be also related with the multimeter?). It shows this behavior even with multiple tubes

When comparing with a matched capsule on an APEX460 circuit (with the 0.1uF removed to remove the resonance at 25Khz) the circuit does not show the same frequency peaks at ~10KHz and 15KHz that the APEX 460 has...

 

[dmnieto]

Actually, by reviewing the data I realize there is something funky...

To test the pre-amp circuit I disconnected the capsule from the circuit and applied the voltage directly to the grid terminal.

- B+ went from 120V down to around 99V.
- Output voltage before the transformer does not seem right, it seems to be clipping at about 3VRMS...
- I am unsure of the gain either seems ballpark around ~20 something dB.

How *the heck* can B+ drop that much when connecting the generator? The power supply is able to deliver up to 125mA on B+, so it has to be that:
1) C13 is leaking DC and somehow some current is being dropped to ground through the generator... but it has to drain trough the 250MOHM resitor plus wathever leakage C13 is letting pass... I dont believe that could be the cause.
2) The tube is iddling at a higher current than I though, but again only when the generator is connected.


Help?

 

[Dennyp]

so this is a quick audio sample of my finished build.  I have an electro harmonix 6072a tube in there with an advanced audio k47 capsule.  I guess this isnt really a c12 but more like a hybrid.  sounds buttery to my ears.  The nasally stuff you might hear is a result of a cold Im fighting.  there is no eq or compression or pop filter for that matter,you can hear that in order to get a usable recording, pop filters are a must with such a sensitive mic.  Thank you guys for all your help! you guys are an amazing resource and community.  its nice to ask questions and not be made to feel like a total noob!

Dennyp

https://soundcloud.com/dennyp/mic-test-matador-c12

 

[Matador]

Actually, by reviewing the data I realize there is something funky...

To test the pre-amp circuit I disconnected the capsule from the circuit and applied the voltage directly to the grid terminal.

- B+ went from 120V down to around 99V.
- Output voltage before the transformer does not seem right, it seems to be clipping at about 3VRMS...
- I am unsure of the gain either seems ballpark around ~20 something dB.

How *the heck* can B+ drop that much when connecting the generator? The power supply is able to deliver up to 125mA on B+, so it has to be that:
1) C13 is leaking DC and somehow some current is being dropped to ground through the generator... but it has to drain trough the 250MOHM resitor plus wathever leakage C13 is letting pass... I dont believe that could be the cause.
2) The tube is iddling at a higher current than I though, but again only when the generator is connected.


Help?

I suspect the tube:  remember that the grid is essentially "floating" WRT ground via the large grid resistance.  Your scope probe is probably 10M, which means the grid resistance changes abruptly when the probe is attached.  Some tubes just don't high high impedances on the grid.

In terms of the output:  20dB gain sounds about right, but you should be able to swing something on the order of 60V PTP with this circuit.  That means a 1V PTP should cleanly swing 10V PTP.

Where is the plate idling?  Measure it "before" the output cap.  It should be sitting at around 50V or so.

 

[dmnieto]

I suspect the tube:  remember that the grid is essentially "floating" WRT ground via the large grid resistance.  Your scope probe is probably 10M, which means the grid resistance changes abruptly when the probe is attached.  Some tubes just don't high high impedances on the grid.

In terms of the output:  20dB gain sounds about right, but you should be able to swing something on the order of 60V PTP with this circuit.  That means a 1V PTP should cleanly swing 10V PTP.

Where is the plate idling?  Measure it "before" the output cap.  It should be sitting at around 50V or so.

I suspect the tube too, The measurement device has 1M//100pF, and the output impedance in 50 ohm...

I will measure the plate voltage. BTW what do you think about the high frequency roll-off when measuring at XP1?

 

[Matador]

I suspect the tube:  remember that the grid is essentially "floating" WRT ground via the large grid resistance.  Your scope probe is probably 10M, which means the grid resistance changes abruptly when the probe is attached.  Some tubes just don't high high impedances on the grid.

In terms of the output:  20dB gain sounds about right, but you should be able to swing something on the order of 60V PTP with this circuit.  That means a 1V PTP should cleanly swing 10V PTP.

Where is the plate idling?  Measure it "before" the output cap.  It should be sitting at around 50V or so.

I suspect the tube too, The measurement device has 1M//100pF, and the output impedance in 50 ohm...

I will measure the plate voltage. BTW what do you think about the high frequency roll-off when measuring at XP1?

Was the transformer in-circuit when you measured?  If so, disconnect XP1 and substitute in a 100K resistor for the transformer and recheck for the roll-off (the resistor goes between the XP1 and XP2 nodes, or tack-soldered from the output cap to ground).

 

[Matador]

Here is a photo of all the big caps. I triple checked the small caps as well.

That looks correct.  I'm assuming this isn't working even with no mike installed?

You need to start at the beginning and work forward.  Start by disconnecting the transformer and making sure that you see about 200V AC on the secondary.  Lift one side of R1 (the side that goes to C2), reconnect the transformer, and make sure you get the rectified ~280V on the "left" side of R1 (it's the side facing the rectifier diodes, that's still soldered to the board).  Then reinstall R1, and lift one side of R2, check that node, etc.

You may have a filter cap that failed short, which would collapse B+.  You just need to narrow down exactly which one it is.