Tube Pre-Amp Question

If the hum changes with an increase in volume it is likely that it occurs in a source/pre-amp connection, or a component part or tube in the unit that preceeds the line amplifying section of the pre-amp. I'm not familar with the VTL but if it has one a rectifier tube could be a problem as could a tube in the phono stage (if it has one).

I would suggest that you disconnect all of your sources and see if the hum goes away. If it does then add them back one at a time checking the results.

That will get you started.

I would also try floating the AC ground on your power amps. You might be experiencing a ground loop if the power amps are grounded through the AC cords.

Thanks for the feedback. The "Hum" is actually more like the sound of air/ static...nothing really. Not really a hum, more like nothingness/ sound. If that makes sense.

You are probably hearing tube 'rush' from less than the quietest tubes, or you are hearing the pre-amps' intrinsic noise floor. Do you only hear this with you ear within inches of the tweeter? If so I wouldn't be concerned, its not unusual.

That is right, only when I'm very near...like ear within 6-12 inches away.
Does this effect sound when playing music?
Should I consider rolling or changing the tubes to a different brand?
Also, could this be corrected with better IC's and/ or PC's?

Here is away to check it out....Play some music loud. Don't touch the volume control but turn off the source. How loud is the 'hiss'? If you can hardly hear it don't be concerned its very normal. If it is loud it could effect the 'black' background prized by many.

If it is loud at normal playback level I'd consider getting new low noise tubes, not necessarily a different brand unless you cannot get good low noise tubes of that brand.

PC's and IC shouldn't improve or mask this noise in any meaningful way.

you are probably hearing tube hiss, which is normal for a tube preamp, but it should not go up with volume.

I have the same VTL 2.5 preamp with a SS amp and it's dead quiet even if you put your ears next to the speakers and turn up the volume. The output impedance of the 2.5 preamp is 200 ohms. I've read here on Audiogon that the input impedance of the amp should be at least 10 times this number for a good match, which would mean 2000 ohms or higher. The amp I use has an input impedance of 22,000 ohms. I'm not sure whether this would cause the problem you're describing. If not, then it may be a bad tube(or tubes) or a ground issue.

I've been thinking of changing the tubes anyway. People have recommended Telefunken Tubes, and say it's night and day in performance when you do it.