Hum problem


Been chasing a hum in my VAC Ren 30/30 for quite a while.  My technician has been all through it and does not find a problem.  Says he cannot hear it in his system, but it's quite apparent on my Horning speakers (94 dB maybe?).  As soon as the soft start relay starts to open, it begins to be audible and when it clicks open it's fully audible, maybe from 5-6' away, with the pre-amp fully attenuated.  Once you advance the attenuator past about 9 or 10 o'clock, it starts to get louder, but not before.  It's not a transformer mechanical hum; no sound at the amp but clearly audible through the mid range of the speakers.  Present w no other components turned on (or any/all turned on).  No change after swapping out power cords, lifting ground, swapping interconnects.  Changing the position of the ground switch on the amp has no impact.  Same w AC straight from the wall or w AC from a Dodd Audio Balanced Power System iso transformer.  Since this is a transformer/tube amp (not an OTL), I assume there can be no DC offset, and cannot really check that because I don't think I can operate it w/o a speaker load and the info I find on the web says it must be checked w/o a load.  

Any ideas before I ship this 85 lb beast back to VAC?

128x128swampwalker
I have  a few questions.

If your amp did not hum at the techs shop then it's not the amp.
1. Is your amp in a rack or free standing on the floor.
    ( if in a rack take it out of the rack  / unplug everything except the speakers )
2. You lifted ground, how did you do that . Switch on the amp or removing the ground leg off of the power cord. You need to lift the electrical ground of the power cable to the amp.Using a 3 to 2 adapter  does the trick put you have to pull off any metal loop that would touch the center screw that holds the wall plate on the wall outlet plug.
Take your power DODD transformer out,plug straight into wall jack.
3. Have you tried other speakers to make sure it's not your speakers crossovers.
If the ground is till there then it's the amp for sure.
 If you are clean then plug in the Preamp  with nothing in the input section,turn it on. ( do not put the frame of the preamp in contact with the amp frame ).
4. LED lights of any kind on the audio AC wall socket all the way to the circuit breakers ( notoriously buzzy and it goes down the ground leg)
Same applies to dimmers of any kind.
5. If you put a meter on your AC wall jack ,what is the reading between the + leg to ground ( should be 120V ac ) then - to ground ( that should be 0 ac volts ).
If you have dimmers on that AC line turn them low to midway and then check your ground voltage again.If it goes higher than previous test with them off you might have found your problem.Even completely off LED light circuits still burn power that is  leaked by the dimmers ( hence LED light glow when in the off position )

It makes no sense that the amp is quiet with another system and installed in your set up it starts buzzing.
Anyhow a lot of good thoughts in the upper threads but there is no mention on how your systems is installed in the room ( rack or no rack ) Components touching other components. I fight those issues daily with amp racks more so in permanent installation steel frames and rails.

Good luck.

I had this with an Audible Illusions modulus 3a pre. The tech fellow I hired could find no explanation. I used a DAC direct to power amp for about 3 Mos. One day I tried the preamp and the hum was gone. My neighbor was big into ham radio. He had moved. So rf? 
Couple of tips:
1) Try using only 1 outlet for all of your components. Obviously using a multiple outlet strip that can accommodate everything and is rated at 20 amps. Insure that the outlet is dedicated if you can.
2) Experiment floating the grounds on the power amp. If no difference, then try the preamplifier.
3) Ultimately it may come down to new shielded Interconnects.
Best of luck!

Addendum to my previous post above.
**Important consideration when using a cheater plug to float the ground on any component.
Many cheater plugs are polarized. You must eliminate the polarized lugs so you can change the orientation of the plug. One way may not change the hum but rotating it (which you would not be able to do prior to eliminating the polarized lugs) may do the trick.
Then using a volt/ohm meter with everything plugged in but off. Measure from the ground lug on the preamplifier to the chassis ground on the amplifier. Obtain the lowest resistance reading by changing the power plug orientation.