2. amps-preamps
  3. 1096239202

Tube brightness and bias

I have a pair of VTL Deluxe Mono 120 and I notice that one of them have its tubes lit up quite a bit brighter than the other although all the tubes have the same brand name. It also comes on a lot faster, about 4 or 5 sec, than the other. If I turn on the preamp first, then turn on both amps at the same time, then music from the amp with bright tubes comes on about 4 or 5 sec before the other.

I don't have much experience with tubes but I wonder if this has anything to do with the tubes being either over or under biased. If I reduce the bias, would the brightness then reduced?
andy2
3 responses Add your response
elevick
2,636 posts
 
First of all, if the tubes are glowing hotter (probably blue), shut it off. This is probably a bias problem. You are killing the tubes if burning too hot.

Even if it is simply a bias problem, you should have a tech do the biasing. His hourly rate is much less than 2 matched quads of tubes.

The problem could be simple as bias or much more. I would get it checked out.
the_smokester
174 posts
 
From your description it seems that the filaments might be powered differently between the two monoblocks.

You might try interchanging the tubes between the two units to see if this effect is associated with the tubes or with the other electronic components.

You should get to the bottom of this, perhaps consulting a technician if necessary.
chriskeating
189 posts
 
I'm not sure if the VTL's are tube rectified, but if so, the rectifiers might be different and could cause the "timing" issue when powering up.
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