C12 Clone Tube Mic low output

[C12VR]

I modded an Apex 460 to c12ish specs but the output is extremely low. I checked the backplate screws for continuity and they test good. I quintuple-checked my wiring and everything seems correct. The output is low with no high end; what could be the cause of this?
Any assistance is greatly appreciated.

 

[crossed.out]

If you put a new transformer in, did it test ok with a multimeter before install?

 

[C12VR]

If you put a new transformer in, did it test ok with a multimeter before install?

I used the stock transformer. I assume that it is good, but I haven't tested it.

 

[C12VR]

I used this schematic except for, cx, c6, r6, and r7. I resoldered the transformer secondary leads and the - side of the primary--could a bad joint here be the culprit? Everything looks sound.

 

[C12VR]

bump

 

[TLRT]

Check transformer ratio (you need 12:1) check anode and polarization voltages(you need an electrometer for pol. Voltage check)

 

[C12VR]

Anode is 122v (I removed a bias resistor and put a jumper in its place). I don't have the tool you describe, so the polarization voltage is suspect--I measured 48v with a standard meter. Cathode was 3.5v.

 

[kingkorg]

What bias resistor? Why would you do that anyway? You can check pol.voltage with a regular meter, at R3 R4 junction. 48v pol. voltage is just fine. Is it 122v at anode or before anode resistor (r2)? What did you use as c8? Pio? Have you tested it?

 

[C12VR]

What bias resistor? Why would you do that anyway? You can check pol.voltage with a regular meter, at R3 R4 junction. 48v pol. voltage is just fine. Is it 122v at anode or before anode resistor (r2)? What did you use as c8? Pio? Have you tested it?

Bias resistor r2--voltage read at anode pin. I removed it to increase plate voltage--I may be mistaken but I believe the tube operates cleaner at 120v vs 65v. C8 is a CDE 930c MKP capacitor. I would be surprised if it was dead because I ordered it direct from Mouser.

 

[kingkorg]

Wow, i don't mean to be rude, but i don't think you understand what you are doing. And i wouldn't be playing with circuits like this and voltages, even though i assume there isn't much current there. Put that resistor back there, follow the schematic, and don't remove or change stuff randomly. There's a lot material to educate yourself out there, that is if you don't want to burn your house down first. What's your cathode resistor? Is it bypassed if so what's the cap value?

 

[C12VR]

The cathode resistor is 1.8k and the bypass cap is 47uf. I read 3v on pin 3.

 

[C12VR]

I read the back diaphragm voltage again and it seems pretty low--at supercardiod setting it is about 55v. I get a lot of bass and no high end. Please, there must be a simple solution to this. I am stuck here with a dead project.

 

[C12VR]

bump--switched capsules to see if that was faulty and now I have no sound at all.

 

[rock soderstrom]

Please, there must be a simple solution to this. I am stuck here with a dead project.

Hi C12VR, it is very difficult to help you with your project because it is quite unclear what the current state of the circuit is.

My suggestion is to document the exact construction as it is in a new schematic. Mention all the important information and leave out everything unimportant (like resistor manufacturer etc.). Draw your measured (as far as possible) voltages in this circuit diagram.

Then take some good photos of it and post them here, this will help you much better.

Good luck