2. amps-preamps
  3. 1056672446

Tube Biasing in Amplifiers


I have a VTL ST-85 amplifier which takes 4 EL34 tubes that need to be biased. VTL says each tube should be biased to 30mA at idle (i.e. preamp muted or volume off). I just got some tubes from a "reputable" store that claims they can support only 29mA. I set them up on my system, and sure enough, I can't set the bias as high as VTL suggests. From my estimates, the new tubes bias at around 26mA, instead of the 30mA VTL suggests.

I have several questions. If I use these tubes, I'll have to run them at their top rating all the time. There will be no extra room for bias adjustment in the future. Is this a good idea? I've heard that the life of the tube can be shortened doing this.

I'm actually more interested in the sound that is produced. How will the low bias on these topped-out tubes affect the push-pull nature of the amp. Am I sacrificing some other area? Do I loose bass, mids, or highs. Will I get a muddled sound? (I do plan to test this, but I thought I would also ask.)

I suspect I will loose some maximum wattage, but will the amp still be able to produce that wonderful sound I like so much.
rrick

Showing 1 response by rrick

rrick OP
23 posts
 
Okay, the lower bias level doesn't seem to be a problem as long as I run it within an acceptable range. How the other way, does it make any sense to overbias the tubes? I have other tubes which support a much higher level of biasing.

What about the other issue of biasing the tube to its max current? Will this limit or affect the sound later on when the inputs are cranked up? I suspect not. I believe other parts of the tube will handle that job, but any quick description would help.
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