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?

Did the technician put it on the scope?  If not what are the efficiency of his speakers?
Hey Swamp….

Sorry to hear this.

Maybe I’m preaching to the choir but your post did not account for:

You said “… for a while” expressly focusing on the 30/30. What was checked or swapped out upstream to put the finger right on the amp?

Using a diff preamp. Got another preamp to try?

Using a diff amp. Got another amp around there? Does this occur with any amp in the system?

As well, Got another pair of speakers? Hook them up and try it again, using the exact same spkr wires, etc. .

These ought to eliminate one aspect. Hope so, but I’m not all in on that one though it seems an area to check out and its easy enough to do I’d think.

You said your tech looked at it but could not replicate the situation in what I suspect was his shop.

Something loose inside the amp? Moved during travel?

I’d suspect he set It up as I stated above before breaking out the multi meter or scope.

Before Al chimed in I thought ‘volume control’ ckt. Or upstream is developing an issue somewhere.

Thought loose connections on spkr wires too. And if using bare wires, a single strand loose and touching the case.

Have a hard time counting on a ground suddenly arising from an IC, or when one had not been there previously eg., CATV, Dimmers, etc. save for a now failing device..

Re-reading the post, the hum is there from the jump but more noticeable once power ala tubes warm up so I’d agree on swapping out what ever tubes, one by one, could be pretty revelatory.

The mystery for me is the ‘tech’ did his investigations with the same tubes in it, I presume.

Might be as simple as the soft start relay is failing. Moving parts with make & break or vice versa contacts do pose issues with time and use. The coil in the relay could just be ready to quit. Insulation break downs, poor conductivity from pitted or dirty contacts, contacts not fully opening or closing, etc.

Depending on what the tech says elec and visually testing it, Relays often have enough info on them so a replacement can be ordered pretty easily… if you just want to see prior to making any major moves… and provided it ain’t the tubes as was said.

If bad have VAC send a new one. I’d sure look into that before packing it in and sending it to the Sunburn state for a look see.

Not to pile on because I'm not electronically very knowledgeable, but this was my experience in dealing with a phono hum: After much experimentation, I got a tip from the Tweak Shop to wrap my unshielded phono IC with aluminum foil.  Gone!  Obviously, the phono IC was picking up hum from my big Audio Valve Baldur 300s sitting within 2 feet side-by-side of my phono preamp.  Also, one of more tubes went bad in my Kondo KSL DAC a year ago causing the left channel to reduce in output.  So, tubes can cause erratic things to happen.  Good luck in finding the solution.
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