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 4 responses by marakanetz

marakanetz's avatar
marakanetz
5,473 posts
 
actually vtls do have a margin down to 25ma so it realy won't hurt.
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.
marakanetz's avatar
marakanetz
5,473 posts
 
Newbee,

In general I wouldn't say that this is an imagination but closer to truth but not to all tubes and tube components. In general I can state that higher bias current almost identically to transistors bring tubes to more stable and distortionless responce with proper tonal balance as well.

After all in our case we're discussing a device with output transformer that introduces one more variable of load complexity of primary circuit matching of which to the output tubes defines the largest effect compared to the setup of the bias current in tubes.
marakanetz's avatar
marakanetz
5,473 posts
 
Rrick,

Higher biasing implies to lower gain. Lower gain implies to less distortions follow?

On the other hand higher biasing implies to higher stress and higher temperatures and lower tube life.

In tube circuitry biasing is limited with load of the primary circuit transformer and input stage that also have bias and plate voltage limits. A simple turn of a screwdriver to increase the bias can bring you very unwanted results of experimentation. Don't play with fire lad!
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