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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
9 responses Add your response
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
4,767 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.
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.
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.
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!
marktaylor22fb
8 posts
 
Most likely the tubes you have obtained do not quite fall into the required bias to plate curve, the total ma for the circuit is 120 ma with the 4 EL-34's which as I remember from reading Manleys Book the plate voltage is around 500V on that amp and 25 ma per tube is a more reasonable idle current draw for that amp, my Quick Silver M-80's are between 120-160 ma for the quad how ever I have never pushed the amp that far into class A since to me there is no sonic benifit and rapid ageing to the out put tubes will take place, you will find the next batch of tubes you use will bias up differant again, may be less current or more depending on the batch, and match quads do not hold matching evenly anyway it just gives the tube a starting set point and we hope that each will fall some where in the guide lines and does not become a current hog down the road.
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