C12 Help: No heat, Bias way high

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

geekmacdaddy

Well-known member
Joined
May 1, 2006
Messages
137
Location
New Yawk
I built Poctop's C12 with Matador's power supply. The power supply pre-calibration is perfect. B+, Bias and Heater voltage perfect. When I plug the mic in, getting 120 B+, and the tube is lighting up, a bit. Ov in the heater's and Bias is about +57v.  Any help is appreciated.

Thank you
 

Attachments

  • C12 Top.jpg
    C12 Top.jpg
    138.8 KB · Views: 21
Have you modified the stock 7-pin cable?  If you are using an Apex 460/Alctron HST-11A, pin 4 is likely connected to pin 7 (ground).  For both the Matador/Chunger C12 and Poctop D-12, pin 4 is plate bias.
 
Khron said:
Have you considered measuring some voltages inside the mic?

Yes.  ;) As of now, I have the B+ dialed in, but the Bias is at +53v...no bueno.
1) Plate 68.9v
2) Grid  2.28v
3) Cathode 0v
4) Heater 0v
5) Heater 0v
6) Plate  0v
7) Grid 0v
8) Cathode 0v
9) Heater 6.23v

doqmemory said:
Have you modified the stock 7-pin cable?  If you are using an Apex 460/Alctron HST-11A, pin 4 is likely connected to pin 7 (ground).  For both the Matador/Chunger C12 and Poctop D-12, pin 4 is plate bias.

I did modify it. I also clipped the connection from ground to the body of the (male if memory serves) Jack. Was that ok?
 
I now have 120v at the test point in the mic. Got it by NOT grounding the body of the jack. Starting to understand why I'm having the problems. It's a wiring difference between Poctop's mic board and Matador/Chunger PS board. To get full heater voltage to the tube pin 4 AND 5 should have +6.3v and pin 9 should be grounded. You could have ground pin 4 AND 5 and put +6.3v on pin 9 . it would work either way but when I checked I had -6.3v on pin 9 and gnd on pin 4, and pin 5 wasnt connected to pin 4.
any help is appreciated :)
 

Latest posts

Back
Top