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.
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.
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