Official C12 Clone - Build and Support Thread

[Matador]

Yikes! I was actually running to my mic before I realise that if the cathode was not plugged in I would have noticed

Matador, did you have any thoughts on the Bias measurement drift?

What kind of meter are you using?  Do you know what it's input impedance is?

 

[jplebre]

For the fluke 175 what I found on the manual for voltage DC the input impedance is 10Mohm, 100pF

 

[Matador]

- Bias: I'm using a fluke 175 but I'm not sure I'm measuring Bias accurately. Every time I probe P-ground and P-Bias, the voltage shown is lower (-1.3/-1.5v) and creeps up to -1v where I've set it previously. Is this normal? if so, should I aim for the DMM-free voltage or loaded by the DMM?
If it's not normal, any ideas?

How long does it "creep"?  If I understand this correctly, the meter shows -1.5V at first then settles down to -1V after some period of time?

This is not unexpected:  the bias supply is low pass filtered via R5 (1M) and C5 (4.7uF).  The time constant is 4.7 seconds.  So long as it settles at the proper value it should be fine.  The absolute value, so long as it's around -1V it will be fine. 

 

[jplebre]

Yes, indeed, it's about 4/5 seconds, then it stabilizes on the new reading.
It starts at -1.6v ish and then creeps down to -1v.

I was just wandering if I did something wrong

And if that +-0.5v difference would matter.

 

[kcatthedog]

just a status report , i have had my build checked by Brain fox and i had built mike fine but one of my large caps on power supply pcp was not operating properly and I had not done the ground properly: the shunt.

So no wonder I couldn't get proper readings , so mostly right but need to be more careful next time. I remember one cap wasn't seated flush so i redid it: too much apparently.

Anyway, I'll be more careful next time: thx for the thread and parts etc.!

 

[Nagskof]

Hi Metador, hope you are well.
I am down here in South Africa  :'( and I bought the kit from Studio 939 for the C12. WOW, I am impressed!
But the postal service sent the Alctron PSU back to Chunger that he sent me  , so I am stuck. Could you tell me the rating of the secondary windings of the power transformer so that I can get one wound in SA to at least test the mic until I get the real power supply please? I asked Chunger and he said I must talk to you on this technical manner.
I would appreciate your feedback.

 

[Matador]

Hi Metador, hope you are well.
I am down here in South Africa  :'( and I bought the kit from Studio 939 for the C12. WOW, I am impressed!
But the postal service sent the Alctron PSU back to Chunger that he sent me  , so I am stuck. Could you tell me the rating of the secondary windings of the power transformer so that I can get one wound in SA to at least test the mic until I get the real power supply please? I asked Chunger and he said I must talk to you on this technical manner.
I would appreciate your feedback.

I don't know the specs of the Alctron unit, but I can still tell you what you need.

The B+ supply is modest:  200V secondary @ about 5-10mA (which equates to about 5VA) would be sufficient.  No center tap is needed.  For the heater, a 9.5V secondary rated at about 500mA is good:  again, about 5VA.  Again, no center tap needed.

 

[RuudNL]

Just wanted to check: what is the voltage you are measuring at the anode? (At 120 V from the power supply)
Thanks!

 

[Matador]

Just wanted to check: what is the voltage you are measuring at the anode? (At 120 V from the power supply)
Thanks!

Anode?

Those are the transformer secondary ratings.

 

[RuudNL]

Maybe I wasn't completely clear.
The 120 V. is the voltage coming from the power supply, 'on top of' the anode resistor (100 K)
There is a voltage drop across the anode resistor, depending upon the bias voltage.
So: I am interested in the voltage on the anode (referred to ground), to check the bias of the tube!

 

[Matador]

Maybe I wasn't completely clear.
The 120 V. is the voltage coming from the power supply, 'on top of' the anode resistor (100 K)
There is a voltage drop across the anode resistor, depending upon the bias voltage.
So: I am interested in the voltage on the anode (referred to ground), to check the bias of the tube!

Oh sorry.

I can't answer in this specific case, but the "typical" 6072A/12AY7 tube under these conditions draws 0.7mA, and that across 100K implies a 70V drop from 120V, which is about 50V.  In the three mikes I've built, I've seen anywhere between 50V and 70V right on the tube plate.

 

[scott_humphrey]

From Klaus Heyne on the R/E/P forum: "65-80v."

Mine runs at 68v with a 5 star GE. Of course, it depends on the tube. There is variation from tube to tube (NOS vs new, one brand vs another). Each is going to draw a slightly different amount of current, so the voltage will vary.

 

[RuudNL]

Ok, that's all I wanted to know. Thanks!!!

 

[Aaronrash]

Any news on converting the heater voltage to negative voltage?

 

[chunger]

Any news on converting the heater voltage to negative voltage?

I suspect one of the hard-to-execute steps would be to grind and isolate the "-" terminals on the filter capacitors.  Seems like there will be a lot of hacking to make it happen.  I've asked Matador for a bullet point list of mods that need to happen to convert.  I'm sure he just ran into a busy spell or discovered more hurdles than previously known regarding the mod and is thinking it over.

FWIW, the new fully regulated PSU board will be designed to easily accommodate the negative heater supply.

 

[Category 5]

Completed both PSUs tonight.  Mic PCB's done, but mics themselves not assembled yet.  Stuck a 180K resistor between B+ and Ground and was able to dial in a solid 120V on one PSU, but can't get higher than 92V on the other (R4 screwed in all the way).  Both heater and bias voltages can be dialed in on both PSUs.

Wondering if I need to troubleshoot the second PSU B+ before finishing the mics themselves.  Looked all over both and assembly is perfect, everything is where it should be, solder joints are perfect and output at the transformer looks good.  Without the 180k dummy load the second PSU was up around the 180s (V) on B+.  Bad pot a possibility?  There is plenty of adjustment range, just won't go high enough.

EDIT - Don't know if accidentally shorting B+ to Ground before the caps discharged has anything to do with this, but there was a scary pop and spark between the resistor legs.  Accidentally shorted it with by screwdriver.

Anyway here are the voltages with and without the dummy load

Without dummy load - 275 going into r1, 235 going into r2 and 196.6 coming out of r2.  B+ at the terminal is 169.5

With dummy load - 274 going into r1, 205.2 going into r2 and 137.5 coming out of r2.  B+ at the terminal is 92.2  (was 110 before the accidental short of B+ to ground)

R4 fried?

Also maybe worth noting that after the short the bias voltage went from -1 to -.5 but was adjustable to -1.1 thereafter.  Thought it could help lead to an answer.

 

[Aaronrash]

Any news on converting the heater voltage to negative voltage?

I suspect one of the hard-to-execute steps would be to grind and isolate the "-" terminals on the filter capacitors.  Seems like there will be a lot of hacking to make it happen.  I've asked Matador for a bullet point list of mods that need to happen to convert.  I'm sure he just ran into a busy spell or discovered more hurdles than previously known regarding the mod and is thinking it over.

FWIW, the new fully regulated PSU board will be designed to easily accommodate the negative heater supply.

Cool,

So there's a new power supply board in the works?

 

[Melodeath00]

Wasn't the original C12 PSU non-regulated? Can we still get the old-style board? I'd like to get as close to vintage C12 as possible.

 

[ln76d]

Wasn't the original C12 PSU non-regulated? Can we still get the old-style board? I'd like to get as close to vintage C12 as possible.

If you want the "vintage" close as possible, make it without pcb, use selenium rectifiers etc
Am planning to make the "close" copy Heaters were regulated!

 

[Melodeath00]

Wasn't the original C12 PSU non-regulated? Can we still get the old-style board? I'd like to get as close to vintage C12 as possible.

If you want the "vintage" close as possible, make it without pcb, use selenium rectifiers etc
Am planning to make the "close" copy Heaters were regulated!

Well I am not going for exact clone, as I don't have the know-how or the money to do this. But if the older PCB for the PSU is closer to the original than the new, I'd like to get the older one. My Tim Campbell capsule is on the way, as well as my early 1960s 6072