Hum problem

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. 

swampwalker

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.

https://www.electronicproducts.com/Passive_Components/Capacitors/Failure_modes_in_capacitors.aspx

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? 

@swampwalker Michael, yes, increasing the amount of feedback lowers the gain of the amp, and therefore a corresponding reduction in the hum level is not surprising.

The one possibility of harm if you were to spray contact cleaner on the switch would be if you were to inadvertently physically damage something during the process, which is a real possibility given the bulkiness and weight of this beast. And yes, you would have to remove the bottom plate, and lethal voltages might be present at various circuit points under the chassis if a considerable amount of time has not elapsed since the amp was turned off. So it’s probably not worth bothering to do that unless your experiments suggest that marginal contact integrity in the switch is contributing to the problem. If wiggling the switch slightly without changing its setting does not affect the hum level it would very likely be an indication that the switch is not contributing to the problem.

Best regards,
-- Al