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
marakanetz's avatar
marakanetz
5,473 posts
 
actually vtls do have a margin down to 25ma so it realy won't hurt.
aida_w
308 posts
 
Hi,
you wrote: I suspect I will loose some maximum wattage, but will the amp still be able to produce that wonderful sound I like so much?
That's exactly what I think and experienced in my
McIntosh MC 2000. I have my bias set at the lower end of the bias scale with the result that the tubes are running much cooler. I might lose a bit of high power but other than that it's ok.
Good luck!
marakanetz's avatar
marakanetz
5,473 posts
 
Please, be noted folks that if there's too much under-bias the mismatch with transformer primary circuit load may take place that can also affect negatively on tube life and sonics.
aida_w
308 posts
 
True, Marakanetz! But I was saying, that I run my bias AT THE LOWER END OF THE SUGGESTED SCALE! Not below that , of course.
newbee
4,093 posts
 
From purely listening to the differences I find that biasing in the lower range produces not only cooler running tubes, but a "cooler" sound as well. Perhaps its just my imagination.
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