akg c28c noisy

Hi

I was at a friends studio yesterday & we were discussing AKG c28's.  He mention that he had a Nuvistor C28c that was noisy.

Does anyone have a circuit diagram for it, & it's PSU ?

Also having never dealt with nuvistors before.  Do they go noisy or is this noise leikely to be from elsewhere in the circuit.

Many thanks

HI Rob,



    This is from Klaus's place. I've had a noisy nuvistor, but also bad resistor and cap, so take your pick!



  Kindest regards,



    ANdyP

I hate those bloody nuvistors. Noisy and microphonic. See if there is any low-level popping or "waterfall" sounds.

My C28a was noisy - but it was the capsule. I'm guessing you've already explored that?

Thanks for the responses so far.

I'm not sure if they tried swapping the capsule, or if they even have one to swap out.

On another note which is the correct PSU for the C28c.    The one they got with it is an N12, which I'm guessing is for the C12 since it has a pattern switch.  Is this properly compatible ?? 

Hi Rob,

Here's some info from the moderator at the AKG forums...

http://www.akg.com/forum/index.php/topic,73.0.html

"Hi, the first series was called C28 or C28A and was introduced in 1955. It had a tube 6072 and was operated by the power supply N12 which was originally designed  for the famous C12. The second version was the C28B, again with the 6072 but with a smaller sized newly designed power supply N12A. The last version was the C28C where the 6072 was replaced by a Nuvistor tube 7586. Power supply for this version was the N28A which is certainly a little confusing because of the suffix letter."

The PSUs are essentially all the same. They all have a wirewound rheostat for trimming the filament supply and can all be adjusted for either 6072 or 7586.

Thanks again guyts, this is all really useful.