Official C12 Clone - Build and Support Thread

[Haberdasher]

I somehow neglected to order 30M resistors (I used 100M for R15, 16).  Looks like mouser is out of stock.  I do have metal film 33M here for R11 Vishay VR37000003305JA100.  Is that going to be ok?

Thanks

 

[rmburrow]

33 megohms within 10 percent of 30 megohms, should work...time constant of capsule and polarizing resistor(s) would increase very slightly..

 

[doggy6812]

Hey guys. I have built two of these and have problems with the pattern switch. It seems the mic wants to both sides of the capsule on. I can turn the switch to Cardiff and the rear capsule will go off for just a second and then comes back on. It also does the same thing In Omni. They both do this

 

[geekmacdaddy]

Can you summarize your troubleshooting? Tube: burn-in and swapping, hiss with capsule and without, etc. It makes it easier for someone quickly browsing the forums to help.

Did you try a new tube? Can you grab one out of a guitar amp? Even if the noise increases with a 12AY7 from a guitar amp it may help you decide the tube is the issue.

When does your 6072a arrive? Is it a Five Star from a reputable source?

All symptoms happened with capsule in. Tried 3 different tubes. 1 (6072) crackled like an egg frying, I know that's bad. 2nd (12AY7) just had the hiss and low vol ground hum, as did the 2nd (the NOS 6072 which arrived), I'm thinking the latter 2 tubes are ok. Disconnected the capsule. Used 2 47pf's to connect the diaphram's front and back capsule. Then used an 80pf to just connect the front capsule, and same reaction from both of the latter tubes. Without the entire mic body assembled, large ground hum. With body screwed in proper, the hiss (it does sound like the pre was turned up too loud. That much hiss should be POPPING volume, but it isn't). I believe the 2 tubes to be sound.
So now the mic has the 80pf connected, Diaphram disconnected.

 

[Matador]

Try to narrow down where the noise is coming from.  I posted this several times in the thread, but the basic steps are:

1) First make sure you have a low impedance ground:  with everything powered off, check the resistance between ground on the PSU PCB and all ground points in the mike body, including all parts of the outside shell.  You should have less than 0.1 ohms everywhere in the mike
2)  Make a small length of wire with two small alligator clips, one on each end
3) Clip one end to ground on the PCB
4) Starting at the 'output' hard ground different points in the circuit:  then measure noise
a) the output of the coupling cap (where it enters the transformer) - it should be dead silent
b) the grid of the tube - this should only output the self noise of the tube
c) the capsule connection - this should only be the noise of the tube + HiZ components

You should be able to see the noise contribution of each stage this way.

 

[Tim Campbell]

If you are sure the transformer is wired right then perhaps it is a wrong value resistor.

 

[geekmacdaddy]

Will do, Matador. Thank you.

 

[geekmacdaddy]

If you are sure the transformer is wired right then perhaps it is a wrong value resistor.

All components measured before going in, triple (OCD) checked afterwards, Tim.

 

[resilient]

Where is everyone getting mic bodies at this point?

 

[Delta Sigma]

Where is everyone getting mic bodies at this point?

I ordered from Chunger. Took quite a while and I could not get the C12 styled head basket. He's been having trouble with his suppliers.

He's not always the best with communication but he's still reliable.

 

[resilient]

I ordered from Chunger. Took quite a while and I could not get the C12 styled head basket. He's been having trouble with his suppliers.

He's not always the best with communication but he's still reliable.

I found some Apex 460b's for $249 shipped.  That was good enough for me for now. 

I also ordered the PCBs, transformers, and parts kit from chunger.  Hopefully he ships soon.  Haven't heard anything yet and it's been several days.

Brandon

 

[earlfsmith]

crazy to read that there are quite a few people with bad T14 ami transformers. All these years i thought i was dealing with bad cap or tube but now seeing it been the transformer!  :-\

 

[Bonnie1]

crazy to read that there are quite a few people with bad T14 ami transformers. All these years i thought i was dealing with bad cap or tube but now seeing it been the transformer!  :-\

Hello earlfsmith,
When did you purchase the T14, is there a sn# on the label, How have you determined it's broken?  It's best to email AMI directly to receive proper customer service.  [email protected]

 

[earlfsmith]

wow Bonnie1, I didn't expect a reply here. Yes i purchased the T14 from you guys some years ago. There were intermittent issues but i just thought i was something i did wrong as at the time i was not extremely experienced and used my u47 mostly. I have been able to get the transformer to work right by repositioning the transformer but when i turn on the next day i have to do again. It's clear that there is a short in one of the cables. Last night i used a utc transformer and it worked great so that was made me come to the conclusion that it is the transformer. I purchased it during the same time all these other work having the same issue. I am planning on building a second c12 soon. If AMI is willing to send a replacement for this broken one i would be glad to purchase second one to come with the replacement. Next month i will be building 3 m49 and planning on purchasing 3 transformers for them.

 

[doggy6812]

I have built two C12s. They both have the same issue. It seems that the mic stays in figure eight. I can turn the selector and it changes for a split second and then returns to figure eight. Any thoughts?

 

[Delta Sigma]

I have built two C12s. They both have the same issue. It seems that the mic stays in figure eight. I can turn the selector and it changes for a split second and then returns to figure eight. Any thoughts?

Measure your voltages on your Polar pattern pin. Should be pin 4 inside the PSU and pin 3 inside the mic. Measure in both places. Measure in Omni, cardioid and fig 8. and post the voltages here.

 

[doggy6812]

Ok. I’ll be back at the studio day after tOmorrow.

 

[earlfsmith]

I'm in shock just how quickly AMI got back to me. From a response on here and then a call the next morning after emailing them late the night before. I spoke with Dennis for quite some time. We had a great conversation and he acknowledged the issue with my transformer and identified that mine was in a bulk of bad ones from years ago. Without hesitation he mailed me a new one. I purchased a second one as im building a second c12. I'm looking forward to the 3 m49's i'll be building next month. I'm definitely putting ami trans in them

 

[earlfsmith]

Is Tim Campbell still around? Im in need of two ck12 capsules. Would prefer to buy from him vs anyone else

 

[Delta Sigma]

Is Tim Campbell still around? Im in need of two ck12 capsules. Would prefer to buy from him vs anyone else

He is, he's quite busy. Send him an email and get on his waiting list. I got a pair slightly faster than expected. His email address is on his website. Google Campbell Transmitter.