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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
swampwalker's avatar
swampwalker OP
5,138 posts
 
@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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almarg's avatar
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almarg
9,670 posts
 
@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

atmasphere's avatar
atmasphere
11,841 posts
 
The problem is not the feedback switch. The hum changes because with increased feedback, some of the hum is wiped out.

The hum is independent of tubes, so right now its sounding like you have filter capacitors in the power supply that are failing. It could also be a bad ground somewhere that relies on a mechanical connection, but at any rate it sounds like you're going to have to send it out for repairs.
blindjim
3,092 posts
 

@Atmosphere > “…. capacitors in the power supply that are failing. It could also be a bad ground somewhere that relies on a mechanical connection, but at any rate it sounds like you're going to have to send it out for repairs.”

Blindjim > As I alluded to these two items earlier(caps & grounds), and you have been more specific as to which caps, wouldn’t those devices which have physical movement associated with their functionality be more suspect for abnormal operation of the amp? Eg., relays, power sw, etc., generally speaking?

Just wondering out loud. Thanks.

@swampwalker
I reckon its just a matter of time and well, you know, now. My deepest condolences and best wishes.
I dig a thorough systematic eval to ensure the or those, physical components which have failed, and or are failing to be found out and replaced is in order.


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