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Tube Amp Hum

I have a new VTL ST-150 for about two weeks now. Its breaking in nicely, but one thing I notice with this amp is a slight hum through the midbass drivers. Not audible from the listening position, but there nonetheless. I have the amp on a dedicated 20 amp circuit which was quiet as a mouse with the SS amp that was on it before this amp, which eliminates it being a ground loop. Also, all tubes are all seated properly.

So my question is, is this a normal operating characteristic for a tube amp? If so, does it have a degrading affect on the sound? I would like to hear just pure air when my equipment is on like I had before if possible.

thx!
jc51373
jc51373 OP
319 posts
 
My refrigerator? Definately not....The system is on a completely dedicated circuit, there is no cable system on this circuit whatsoever. It is not a ground loop.

Newbee, I will try the tube reversal as you mentioned. Sounds like an idea.
jw94055
125 posts
 
I know this sounds left field, but try different cables from your pre to power amp. I have a tube pre and ss power. I bought some high end cables and they produced a buzz. Replacements were tried with the same result. Put in old generic cheapos and the buzz went away. I would make sure the shielding on the interconnects is plenty thick. Again, this is one person's suggestion.
mechans
2,999 posts
 
I echo and emphasize the fact that some tubes have a high noise floor. Try different tubes. Also some trannies, but first and foremost are the tubes. I have an all tube set up.
unclejeff
1,313 posts
 
Are your interconnects/speaker cables anywhere near your power cord? Also, try changing power cords.
jc51373 OP
319 posts
 
Heres a theory...I have Herbies Audio Lab isolation feet, which have a good reputation and I am pleased with them. But...Is is possible that because they are squishy material, the transformer hum noise is being trapped inside the amp and coming through the speakers?

Just a thought, maybe its stupid but thought I would throw it out there.
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