2. tech-talk
  3. 132075-128

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

Showing 3 responses by atmasphere

atmasphere's avatar
atmasphere
11,632 posts
 
Thanks- sounds like the relay is a secondary problem. The hum is nothing to do with the relay though. Its a process of the amplifier's operation. If you've already played with the tubes, then the filter caps are still on the table.
atmasphere's avatar
atmasphere
11,632 posts
 
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?
Not really, and not on a hum problem like this. Initially- the unit is better, gets worse as it warms up. That's not a mechanical issue.

@swampwalker here's a test for you- run the amp with shorted inputs and wait for it to hum. Then turn the unit off for 5 seconds and then back on again. Did the hum die instantaneously with the switch?

If yes that's a power supply problem.

Did the hum not come back as loud when you turned it back on?

This is a bit of a trick- failing filter caps will sometimes 'heal' for a bit with an off/on surge, but the hum will come back. So if the answer is 'yes' its just about certain its a filter cap, but if it was no, filter caps are still at the top of the list regardless.
atmasphere's avatar
atmasphere
11,632 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.
More to discover