Keith G wrote:

OK, OK, I get it - 'neither one, go and fix the valve
mic', eh?

Replaced this (probably perfectly OK) JAN Philips with a
nice little 'Chelmer' badged short plate (for, er, low
microphony) Sovtek 12AX7 and thought I had at least
significantly reduced the hum for a while, but it's back
now!!

http://www.moirac.adsl24.co.uk/showntell/AKG-1.jpg


(Guess I now know why the mic was on eBay!! :-)

Here is a shot of the orange LED that makes the valve
'light up':


Hum?

Power supply should supply regulated DC for heaters and HT.
Does yours have a "ground lift" switch?

AKG mics seem to be well supported with info such as he

http://www.akg.com/mediendatenbank2/...5d251a2cd0.pdf

Ian