I just bought a used F1x and I love it very much but I'm having a hum issue.  The DI out hums.  The ground lift switch has no effect.  If I have the full range out and the DI plugged into the same mixer then the hum is gone.  The Full Range out by itself sounds amazing.  I tried cables, power sources etc.  I cleaned the FX. Return jack, exercised the jacks., tried different tubes.  Anyone have any ideas?  I searched this forum and read up.  I am going to have a tech check it out but the insight on this forum is always top notch and would the tech at the very least.  Thanks!!

Have you checked this post and Edwin's advice.  http://club.alembicguitars.net/index.php?topic=22256.0

Thanks--I'll have them checked!  That actually may be related to another thing I noticed.

I don't think it's filter caps if there is a way which you can hook it up without hum. Old filter caps create global hum. This sounds like a bigger problem between stage power and the FOH or perhaps a bad mic cable. As an interim fix, you could always run a DI off the full range out between the preamp and the amp. If you still get hum, then you know that it's not you.

Thanks--I did try it through two different mixers in different locations with different cables and power still have issues.  I also tested my GK head's DI and it was fine in the same situation.  I appreciate the info-this will help me and the tech narrow it down.

Just for posterity-
I finally found that the xlr jack ground tab (the 4th connection on the jack) was not actually grounding to the chassis!  I noticed that it was a Switchcraft so maybe it was a replacement?  I put a Neutrik in there and it seems to have solved it!!  There was no metal to metal contact on the old one. I realize I could have just scraped the inside of the screw hole on the jack but I figured a nice jack can't hurt....

Thanks again!