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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?

swampwalker
atmasphere's avatar
atmasphere
12,152 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.
swampwalker's avatar
swampwalker OP
5,138 posts
 
@atmasphere- Thank you for taking the time to think about this and to reply.  It appears that I was a little unclear in describing the problem.  The amp is dead silent (inputs shorted, no other items plugged in to nearby receptacles) until the soft-start relay starts to open.  For a few seconds I get a buzz which I attribute to mechanical noise as the relay opens up.  Almarg confirms that his Ren 70/70 does the same.  When the relay fully opens, the buzz goes away and then there is a hum from the speakers that is constant in volume and does not change over time (or at least not significantly).  When the amp is inserted into the system, the hum remains the same volume from full attenuation, until reaching around 9 or 10 o'clock. 

I tried shutting the amp down and then immediately restarting as you suggested.  The soft-start circuit kicks in, so we are back to square one.  Quiet, a buzz (only as the relay is opening), and then the hum.  
almarg's avatar
almarg
9,670 posts
 
Hi Michael,

To clarify, on my 70/70 there is a slight mechanical buzz (which I suspect to be transformer-related) for a few seconds just **before** the B+ relay closes, about 20 seconds or so after the amp is turned on. (I assume the relay is closing, i.e., completing a circuit path, rather than opening when it presumably applies the high voltage B+ to the tubes at that time). Also, the relay closure itself is essentially instantaneous on my amp, at least since VAC worked the amp over a few years ago when I sent it in for an unrelated repair. Prior to that the relay would clatter for a couple of seconds when closing, which it should not do if in top condition.

Best regards,
-- Al

atmasphere's avatar
atmasphere
12,152 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.
blindjim
3,092 posts
 

Thanks Ralph. Good to know.
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