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?

One more thing to try, although it may be grasping at straws:  With the amp turned off, rotate the feedback control through all of its positions a few times, to wipe off or at least reduce oxidation that may be on its contacts.

The condition of the various contacts on that switch are one thing that comes to mind that could affect both channels in a similar manner.  And the age of the amp makes their condition somewhat suspect.

Good luck.  Best,
-- Al
Jim- yep I had everything off and double checked after reading your post

Al - will try that but if that’s what it is I am going to strangle my tech. The feedback control was kind of funky in its operation and the main reason I brought it in. The switch operates better know mechanically and he SAID he fixed it and also spent 3-4 hours giving it a complete going over. 

Jetter- I guess I’ll have to dig up those old 300 bs. 

1. Hum not present when Atma S30 amp in system.
For me, interchanging in and out amps would be enough for me to point a finger right at where the issue resides. All else being equal and it appears so.

That move alone should resolve any suspicion of wires or speakers, or upstream gear, or even household effects as culpable items of interest.

My long ago EXP in fixing electronics points me towards the practical common sense sied of things as the approach I used back in the stone age, as a rule.

Guessing out loud as its all I can do here….. I’d say whatever is wrong is wrong in the VAC amp and quite immediately. ‘why’ it becomes more and more pronounced is due to the amp coming into a more stable operating posture so some devices are in play right off and if failing or failed, one would think the amp would either not work at all or work poorly.
Capacitors have terribly quick rise times individually. A bank of them even in parallel takes a tick longer to come up fully but no more essentially.

Caps don’t always create an open ckt. Preventing current or voltage transference 100% and hence, a lack of system wide operation. Caps don’t usually become the culprits either…. But nothing lasts forever man. Even caps.. a failing cap might be in play here.

I’d have ?? take a closer look at them IMO.

Also, to lessen or possibly prevent shipping it out, in the DIY vein, depending on just how exciting you want this step to become you can either unplug the amp from power, or leave it plugged in.

Tift the cover from the amp and leaving all tubes in place, as systematically as I could use a bottle of compressed air to clean it out. Or a vacuum cleaner with a thin tipped accessory or merely some pretty good small paint brushes to ensure nothing has gotten into the amp causing the issue. Just a thought. Dust + humidity + heat & repeat causes issues indeed.

Beyond a step by step front to back visual inspection of connections, boards, for issues with attention to excessive heat areas, where insulation or insulators may be compromised, or merely loosened, I’d say something is telling you it is sick and about to quit… but wants to surprise you with the timing of its exodus. Kind of a thoughtful thing if you think about it.

Although, an abrupt failure to operate would probably be less frustrating.

Deep regrets. Best of all possible speedy solutions.

@almarg -
Rotating the feedback switch back and forth reduced the volume of the hum as the feedback was increased and then of course an increase as feedback reduced back to zero.  I believe that this is expected but of course I totally lack the knowledge to understand why (something about changing the impedance maybe...of course I can say those words without knowing what that means, either).  Any harm in trying some good old contact cleaner on the switch?  Would I need to remove the bottom plate on the amp to do that?  If so, will that expose me to danger from high voltages? 
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