geekmacdaddy

C12 Help: No heat, Bias way high
« on: December 07, 2020, 10:27:51 AM »
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
« Last Edit: December 08, 2020, 12:33:42 PM by geekmacdaddy »


geekmacdaddy

Re: C12 Help
« Reply #1 on: December 07, 2020, 10:28:41 AM »
and

Khron

Re: C12 Help
« Reply #2 on: December 07, 2020, 11:48:36 AM »
Have you considered measuring some voltages inside the mic?
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Rob Flinn

Re: C12 Help
« Reply #3 on: December 07, 2020, 12:02:32 PM »
The valve not lighting up is quite a big clue.
regards Rob

RuudNL

Re: C12 Help
« Reply #4 on: December 07, 2020, 01:01:20 PM »
So, what is the filament voltage on the pins of the tube?
There is a solution for every problem!

http://www.vansteenisaudio.nl

Re: C12 Help
« Reply #5 on: December 07, 2020, 01:56:25 PM »
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.

geekmacdaddy

Re: C12 Help
« Reply #6 on: December 07, 2020, 07:39:19 PM »
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

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?

geekmacdaddy

Re: C12 Help New
« Reply #7 on: December 08, 2020, 12:23:29 PM »
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 :)
« Last Edit: December 08, 2020, 01:00:12 PM by geekmacdaddy »


 

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