6072A/12AY7 for an AKG C12 Microphone

[Puggie]

I have a faulty AKG C12 here which has a VERY thin sound, no bottom end what so ever. I am presuming the Valve is at fault as the mic contains only a bunch of wire wound resistors, foil caps an electrolytic and a transformer. The electrolytic cap 'looks' ok and the transformer doesn't smell burnt so the GE 6072A is currently public enemy number one! I do not have any spare valves to try but I do have a couple of 12AX7 (ECC83s) to hand, could one of these be substituted just to confirm the valve is at fault, what are the differences between the 12AX7 and 12AY7? I presume gain is different (AX higher) but is there any reason it wouldn't work just for testing to prove the current 12AY7 is faulty, then I'd get a good replacement 12AY7.

I have the idea its not that simple but here is hoping.

Thanks.

[rdf]

Hi Puggie. Is the mic otherwise quiet and outputting normal levels? If so I'ld look at the electrolytics first. (Is it just one? http://www.coutant.org/akgc12/c12schem.jpg) They can dry out over time without any visual indication.

[Puggie]

Yes just the one electrolytic, I bridged another 0.5uF electrolytic cap in parallel with that one and it made no difference (that cap is very hard to replace the Mic is built around it). I'm now suspecting the capsule

Edit, thanks for the schematic link, I have a very bad fax of that, your scan is much easier to read

[EC8010]

One reason for no bass out of an electrostatic microphone is the capsule seeing a low input impedance - they typically need to see 500M and it's easy to degrade such a high value. I think ECC81 is closer to 6072 but I believe that the main problem is that there's not a lot of room inside a C12 and the length of the valve is critical.

[Puggie]

thanks guys, manages to get a replacement valve and rule out the valve as the problem, swapped the capsule with another from an old AKG414 (identical CK12 capsule) still a lack of bass though (although the old capsule had differing responses on both sides so it needed replacing). I have found having a random poke around in the mic gave a good pop and the bass came back, after re-assembling it had gone again, damn thing, all the solder joins look good, I have replaced the wire that made the pop as I touched it and I'll keep you posted.

lovely sounding mic though...


when it works!



edit: the bass just went again, gave it a gentle tap.... thump the bottom end came back.