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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

Showing 6 responses by almarg

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almarg
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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

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almarg
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@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

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almarg
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One more thing to try, although it may be grasping at straws:  With the amp turned off, rotate the feedback control through all of its positions a few times, to wipe off or at least reduce oxidation that may be on its contacts.

The condition of the various contacts on that switch are one thing that comes to mind that could affect both channels in a similar manner.  And the age of the amp makes their condition somewhat suspect.

Good luck.  Best,
-- Al
 
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almarg
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And is there any way the speaker cables could be causing the hum? Capacitance? Inductance? I have read where some speaker cable designs can cause problems for a power amp.
Thanks for your comment, Jim. You may be thinking of the fact that a few speaker cable types having ultra-high capacitance have been known to cause some amplifiers to oscillate, with destructive results in some cases. Goertz and the old Polk Cobra cables, for example, at least if they are used without a Zobel network connected at the output of the amp. The reason for the ultra-high capacitance in those cases being that it is a side-effect of their approach to achieving ultra-low inductance.

Such an oscillation would most likely be at a much higher frequency than is the case here, though, and would typically occur with an amp having low output impedance, considerable amounts of feedback, and particularly wide bandwidth. None of those characteristics apply to Michael’s VAC Renaissance 30/30, and in general are most likely to occur with solid state amps.

Best regards,
-- Al

almarg's avatar
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almarg
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Hi Michael,

My 70/70 emits a slight mechanical buzz for a few seconds before the B+ relay clicks on, which sounds like what you are referring to as the "first hum (maybe more of a buzz)." It disappears completely when the relay clicks on, and I suspect is a mechanical phenomenon involving some of the several power transformers that are in the amp. So I don’t think that initial hum/buzz is an issue.

At this point all I can suggest in addition to what you are planning to do is to reiterate the suggestion of seeing if the hum is present with nothing connected to the amp’s inputs, if you haven’t already done so. Or better yet, with shorting plugs connected to its inputs.

Best regards,
-- Al

almarg's avatar
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almarg
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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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