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
Before you do send the amp back try these tricks:

1: Do you have any video components tied into the system if you have a cable box they can have different grounds and can require a ground breaker

2: Did you check your dimmers for lighting these can sometimes cause a ground hum.

3: How did you test floating the ground? At an power conditioner do you use one?

4: Have you methodically tried floating the grounds one at a time, on different components?

5: Did you test without any components connected to the amp? 

6: Have you re-introduced cables one at a time? Sometimes you can have a bad ground on an RCA cable
Hi Michael,

Have you tried changing the 6SN7s? See this thread from 2012, in which you participated. It seems that the VAC Renaissance amps can hum if heater-to-cathode leakage is excessive in the 6SN7s.

Also, I would suspect that you have two problems present, one internal to the amp that is causing the hum with the volume control at min, and a different problem which adds to the hum when the volume control is raised. If you’ve found that lifting the AC safety ground on the preamp or the amp does not affect either problem, the cause of the second problem figures to be somewhere upstream of the amp.

Also, DC offset by definition is zero Hz, and so it would not cause a hum.  And as you realize it is very unlikely to be present in the case of an amp having transformer-coupled outputs.

Best regards,
-- Al

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@audiotroy 
1.  Mondial magic isolator already installed. 
2.  Will have to check dimmers but usually lights off. 
3.  Floated ground at wall and at power conditioner. 
4.  Hum is present regardless of whether other components are on or off. 
5.  Will try, thx. 
6.  I/Cs have no impact.

@almarg- @almarg- forgot about that; will swap out 6SN7s one at a time. Thx. 
 Have not tried lifting ground on preamp but the passive is a 2 prong plug so methinks that may not be it.