C12a mod to remove cathode follower?

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this schematic I guess, what kind of hum do you have? 50Hz or 100Hz? Does it change with the microphone position? What happens when you switch off the PSU? Is the hum there from the beginning?
akg c12a schematic.gif
 
Might wanna consider disconnecting the 3pin XLR's pin 1 from the mic psu earth.

Do your mic cables have both the XLR shell and pin 1 connected to the cable shield?
 
ive just changed the transformer in one mic for another i have lying around, its not the correct ratio but now when i plug the mic in the hum goes, so it must be the transformers.
If there was a winding sheet for a similar spec transformer i will need to rewind these transformers i think!!
 
I think Khron is suggesting an earth loop between mic PSU and the mic preamp. I would concur that there is at least a chance of this being the culprit.

Lifting the earth at the XLR-3 is a good way to eliminate this.
 
And it can only possibly be the fault of the type of circuit, not grounding or wiring or power supply filtering, or anything else..
No, it could filtering in filament supply or lack of it, a h-k short in the tube. If the tube is cold and you are getting hum on turn on then it's not the tube. Pull the tube and see what you get.

The follower drives the transformer and keeps the load off the plate.
 
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