Official C12 Clone - Build and Support Thread

[Matador]

B+ should not collapse like that:  there is a large current draining to ground which is why there is so much drop.

Until you find it replacing the resistors won't be of much help.  Remove C1 through C5:  you should be able to see a high B+ with no test load.  If B+ is still ~10V with no caps then we can debug further.

 

[teacat]

B+ should not collapse like that:  there is a large current draining to ground which is why there is so much drop.

Until you find it replacing the resistors won't be of much help.  Remove C1 through C5:  you should be able to see a high B+ with no test load.  If B+ is still ~10V with no caps then we can debug further.

Thanks for your input, Matador. C1-C5 removed, B+ now reads 6.5v, but doesn't rapidly drop down from 20v like it did before.

 

[teacat]

Could R4 be the culprit? I get 7v on two of the legs and 6.5 on the third. On the other two trimmers I get a voltage on two legs and ground on the third.

 

[Melodeath00]

Matador,

In Bi-Directional/Figure-8 mode...

If I notice that the front of my C12 is louder and more hi-fi sounding, and when I turn the mic around and speak into the rear side of the capsule, the sound is softer and there's less high end, is there anything to check inside the mic or PSU that might be amiss? Or do you think this is just a result of the variance on the different sides of the capsule? My only thought is possibly the rear diaphragm is not being polarized enough in bi-directional, but it seems unlikely. Other than this, the mic is working exactly as expected. The difference between the front and rear is not HUGE, but it is noticeable.

I am not using headphones, I am listening to clips after the fact. I know talking into the rear of a mic with headphones on can sound weird.

Much appreciated!

 

[Matador]

B+ should not collapse like that:  there is a large current draining to ground which is why there is so much drop.

Until you find it replacing the resistors won't be of much help.  Remove C1 through C5:  you should be able to see a high B+ with no test load.  If B+ is still ~10V with no caps then we can debug further.

Thanks for your input, Matador. C1-C5 removed, B+ now reads 6.5v, but doesn't rapidly drop down from 20v like it did before.

Hrm -  There only two other sources of B+ current:

1) Remove the three wires from the polarization switch - it's the leftmost three-pin green screw header.  Confirm B+ still low.
2) Confirm the value of R6 - tell me the marking
3) If R6 is indeed 470K, remove it and remeasure

As to R4, it is not ground referenced like the other pots.  It cannot drain current to ground no matter what it's set to.

 

[Matador]

Matador,

In Bi-Directional/Figure-8 mode...

If I notice that the front of my C12 is louder and more hi-fi sounding, and when I turn the mic around and speak into the rear side of the capsule, the sound is softer and there's less high end, is there anything to check inside the mic or PSU that might be amiss? Or do you think this is just a result of the variance on the different sides of the capsule? My only thought is possibly the rear diaphragm is not being polarized enough in bi-directional, but it seems unlikely. Other than this, the mic is working exactly as expected. The difference between the front and rear is not HUGE, but it is noticeable.

I am not using headphones, I am listening to clips after the fact. I know talking into the rear of a mic with headphones on can sound weird.

Much appreciated!

Expected.  No capsule sounds identical on both sides.  You should face whatever side sounds better towards the front so you can benefit from it in cardioid mode.

However comparing figure-8 to cardioid can be misleading (polarization isn't the same and the capsule response can/will change):  I would swap the FC and RC wires and flip the capsule 180 around to be sure you have the "best" sounding side (to your ears).

 

[gyraf]

yep, remember that back membrane is out-of-phase when set to figure-8 (!!)

Very common error when evaluating multipattern mics while wearing headphones

Jakob E.

 

[chrispsound]

I noticed in chungers PSU build pictures there is no connection from the HT ground (at first filter capacitor) to safety earth (by the PT) and that the only ground connection to chassis is through the XLR.  Is this normal?  Is it safer and maybe quieter to have HT ground at safety earth?  ChrisP

 

[Matador]

I noticed in chungers PSU build pictures there is no connection from the HT ground (at first filter capacitor) to safety earth (by the PT) and that the only ground connection to chassis is through the XLR.  Is this normal?  Is it safer and maybe quieter to have HT ground at safety earth?  ChrisP

A couple things:

1) The first filter capacitor (C1) is not at ground potential and thus cannot be strapped to ground and/or chassis
2) The circuit "ground" node is already strapped to the chassis via the 7-pin XLR via the chassis terminal
3) The "safety" requirement is met already by strapping the mains earth directly to the chassis where it enters the PSU.  This was already done by the factory via the yellow/green wire directly to the chassis (seen in the build pics)

There would be no advantage making another connection from circuit ground to chassis, as this connection already exists at the XLR.  There should be no appreciable current flow between these nodes (unless there is a fault or something else horribly broken), at which point the safety earth bond takes over.

 

[teacat]

B+ should not collapse like that:  there is a large current draining to ground which is why there is so much drop.

Until you find it replacing the resistors won't be of much help.  Remove C1 through C5:  you should be able to see a high B+ with no test load.  If B+ is still ~10V with no caps then we can debug further.

Thanks for your input, Matador. C1-C5 removed, B+ now reads 6.5v, but doesn't rapidly drop down from 20v like it did before.

Hrm -  There only two other sources of B+ current:

1) Remove the three wires from the polarization switch - it's the leftmost three-pin green screw header.  Confirm B+ still low.
2) Confirm the value of R6 - tell me the marking
3) If R6 is indeed 470K, remove it and remeasure

As to R4, it is not ground referenced like the other pots.  It cannot drain current to ground no matter what it's set to.

Polarization switch unhooked, same situation.
R6= 4703F

 

[teacat]

R6 removed, still getting 6.6v at the B+ terminal.

 

[Matador]

Let's review the bidding:

1) C1-C5 removed
2) No polarization switch
3) No R6
4) No test load or mike connected

Yet you still have 260V where R1 connects to the diode bridge, and still ~250V drop across R1 (implying a 2.5mA current to ground).  I'm beginning to suspect a board fault:  there's no other explanation.

Let's try this.  I assume you can measure resistance:  disconnect the transformer secondary from the board so it's not powered.  Place one probe at ground, and measure the various points from R1 along through R4 and tell me the resistances.

With all of those components removed you actually should see infinite resistances between all of those points and ground.

 

[teacat]

1-yep
2-yep
3-yep
4-yep

I didn't actually wait for the meter to climb all the way up to that point, but it certainly seemed like it was on it's way.

R1,R2- nothing
R3- 2k,2k & .2
R4- nothing

(my meter only reads up to 60k though)

 

[Matador]

What is the input impedance of your meter?  I wonder if it's causing this problem.  Maybe post the make/model?

 

[teacat]

What is the input impedance of your meter?  I wonder if it's causing this problem.  Maybe post the make/model?

It's a Fluke 113, 3k input impedance.

 

[Matador]

What is the input impedance of your meter?  I wonder if it's causing this problem.  Maybe post the make/model?

It's a Fluke 113, 3k input impedance.

Oh boy...I thought that might be a typo...

But I can see that is a utility line DVM, meant for probing voltage and current on regular utility power circuits.  That low input impedance will clobber measurements on these tube mike circuits, as the impedances we are measuring are quite a bit higher than  3K.  For shunt current measurements on utility lines though those meters are great.

In short, there's likely nothing wrong with the circuit.  It also explains why the heater supply measures correctly:  the output impedance of the regulator is probably less than 10 ohms, which can be bridged/measured quite nicely with 3K.  However the B+ line is north of 50K output impedance, so a 3K DVM will induce a 95% voltage error in the measurements.

 

[teacat]

What is the input impedance of your meter?  I wonder if it's causing this problem.  Maybe post the make/model?

It's a Fluke 113, 3k input impedance.

Oh boy...I thought that might be a typo...

But I can see that is a utility line DVM, meant for probing voltage and current on regular utility power circuits.  That low input impedance will clobber measurements on these tube mike circuits, as the impedances we are measuring are quite a bit higher than  3K.  For shunt current measurements on utility lines though those meters are great.

In short, there's likely nothing wrong with the circuit.  It also explains why the heater supply measures correctly:  the output impedance of the regulator is probably less than 10 ohms, which can be bridged/measured quite nicely with 3K.  However the B+ line is north of 50K output impedance, so a 3K DVM will induce a 95% voltage error in the measurements.

I need a facepalm emoticon... 
Yeah, it's my meter for work, but all I ever do is check ac line voltages. Damn impedance gremlins! I will build the circuit back properly and fire the mike up. (and test with another meter)
One more question, though- when I built up the mk47, the voltages on that psu all checked out with this meter, but now I'm worried I should re-test with a proper one?

Oh, and Thank-you! for the project and all the support.

 

[Melodeath00]

Last night I decided to change my fixed bias from -1V to -1.1V just for the heck of it and see if I notice a sound difference. Can anyone definitively say what value AKG used? It doesn't seem to say in the schematic what the fixed bias negative voltage is supposed to be, and I haven't found anything definitive online.

I'm not sure about the 6072 tube specifically, but I know with some tubes even a 0.1V change in the grid can have a huge effect on the current draw of the tube. It seems like that would have some effect on sound, but I'm definitely not a tube expert.

Also, if I want to ground the unused triode of the 6072, can I just tie the unused pins to the cathode (which is grounded in this circuit) of the other triode, or should I tie them to a different ground location for best performance/noise? Would I tie all the pins except for pin 5, which is the heater of the unused triode?

Thanks!

 

[teacat]

Measuring with another meter, I dialed in the voltages perfectly. 
But my switch doesn't seem to work after I took it apart and put it back together again(story of my life...)
I can get a 12 pos 1 pole shorting switch locally, but I wanted to check in and see if it will work specs-wise. It is rated for .3a at 125vac/30vdc.

Edit: Nevermind, I methodically went through the switch and got it to function properly. Mic is now happily burning in.

 

[beatcynic]

I'll preface this with a thank you to Chunger and Matador. Awesome hardware and technical support. Great pcb !
The issue I'm having is not being able to get B+ to 120v loaded with a 12ay7. With a 12ax7 there is no problem.
12ay7 - B+ = 113.2v    Plate 51v    PSU 213 unloaded (maxed)
12ax7 - B+ = 120v      Plate 88v    PSU 195 unloaded

I must say the mic sounds great either way. I understand the voltages of the two different
gain tubes will be relatively different , but I should be able to get the 12ay7 B+ up to 120,
right?
Thanks, B