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
Thanks everyone for your help.  Alas, no cigar;-)  BTW, hum is not so bad that I cannot listen, but it is audible from 8-10  away during quite parts of songs.

@fineito- Not piling on at all.  I'll try anything I can do easily this weekend and then call my tech and VAC on Monday. 

@blindjim- upstream checks:
    1.  Hum not present when Atma S30 amp in system.
    2.  Hum present with 2 pre-amps (one active and one passive).
    3.  Hum NOT present on immediate turn on.  I hear what sounds like a gradually increasing hum after 30--40 seconds.  The relay clicks open a few seconds later and then the hum is very apparent in the speakers.  It's possible that the first hum is coming from inside the amp.  I assume that the first hum (maybe more of a buzz) is the sound of the soft start relay starting to open but I have absolutely zero knowledge of whether or not that is a reasonable assumption. 
   4.  Hum present with multiple I/Cs.  Have not checked speaker cables but seems very unlikely that the hum would be identical in both channels if it were hum induced in the speaker cables.

@almarg- I swapped in about 12 different 6SN7s in various combos.  No impact on any aspect of the problem, so I THINK I can rule that out.  Please disabuse me of that if I am wrong. 

To all- I will try swapping out 300bs but it's also hard for me to believe that all 4 were identically bad, resulting in identical hum in both channels.  Again, if I'm off base on that, please let me know.  As I've said, I have ZERO technical knowledge of how these things operate. 
Hi Michael,

My 70/70 emits a slight mechanical buzz for a few seconds before the B+ relay clicks on, which sounds like what you are referring to as the "first hum (maybe more of a buzz)." It disappears completely when the relay clicks on, and I suspect is a mechanical phenomenon involving some of the several power transformers that are in the amp. So I don’t think that initial hum/buzz is an issue.

At this point all I can suggest in addition to what you are planning to do is to reiterate the suggestion of seeing if the hum is present with nothing connected to the amp’s inputs, if you haven’t already done so. Or better yet, with shorting plugs connected to its inputs.

Best regards,
-- Al

At this point all I can suggest in addition to what you are planning to do is to reiterate the suggestion of seeing if the hum is present with nothing connected to the amp’s inputs, if you haven’t already done so. Or better yet, with shorting plugs connected to its inputs.
Al,

I was thinking the same thing. That eliminates everything ahead of the amp. I would probably go so far as to unplugging all other associated equipment from the AC mains wall outlet.

What about the sensitivity of the speakers Michael is using. Not sure which model he has. One review for Horning speakers, I quickly read over, said the sensitivity was rated at 100db.
And is there any way the speaker cables could be causing the hum? Capacitance? Inductance? I have read where some speaker cable designs can cause problems for a power amp.

Jim
Jim- The Hornings are an older model, Perikles Ultimate.  They are spec'd at 96 dB, so I guess I'd call them moderately high efficiency.  I think I've got some shorting plugs here so I'll try that out and report back later this evening. 
And is there any way the speaker cables could be causing the hum? Capacitance? Inductance? I have read where some speaker cable designs can cause problems for a power amp.
Thanks for your comment, Jim. You may be thinking of the fact that a few speaker cable types having ultra-high capacitance have been known to cause some amplifiers to oscillate, with destructive results in some cases. Goertz and the old Polk Cobra cables, for example, at least if they are used without a Zobel network connected at the output of the amp. The reason for the ultra-high capacitance in those cases being that it is a side-effect of their approach to achieving ultra-low inductance.

Such an oscillation would most likely be at a much higher frequency than is the case here, though, and would typically occur with an amp having low output impedance, considerable amounts of feedback, and particularly wide bandwidth. None of those characteristics apply to Michael’s VAC Renaissance 30/30, and in general are most likely to occur with solid state amps.

Best regards,
-- Al