Biasing Question

[bsco]

At the risk of sounding stupid in some people's eyes I will ask anyway........ I get confused with this procedure..probably because I don't do it enough.....when calculating the amount of bias, you go by the plate voltage in the particular unit....not the maximum rated plate voltage that is stated in the tube manual.......right........Let's take a KT88......the maximum plate voltage is 800V...the max. plate current is 230mA, and maximum power dissipation is 42Watts...that is what is on the data sheet that I have......So I have a set of KT88's running at 550 plate voltage...with a cathode current of 80mA...in order for me to calculate the proper bias value, I need to stay under 70% of that maximum....42X70% is 29.4Watts......so that is 70%......So....Plate voltage X plate current equals the plate dissipation......550V X 80mA is 44.4 Watts.....that is for two tubes.....so for one tube, provided that they are matched fairly close, should be around 22 Watts......so the 80mA setting is roughly 50%....it could stand to go a tad higher......right......and the actual negative voltage at the wiper arm of the bias pot is about -63V...the schematic states -55V and if you turn it down to -55V then the plate current increases to somewhere around 123mA....550 X 123mA is 67.5 Watts or roughly 33watts for each tube.....that is higher than 70% of the maximum......Don`t forget that there is screen current but for the math I don`t think that would throw the calculations off by too much......so I didn`t include it......so am I on the right track here.......
Cheers......
P.S. I just thought about it some more....Correct me if I am wrong, but when I changed the bias setting from -63 down to -55V, I was supposed to measure the plate voltage and redo the calculations.....but I used the previous measured plate voltage reading of 550.....

[Tom Phillips]

It seems that you are on the right track to me.

The absolute value of the bias voltage is not part of the calculation as you know. The bias voltage value printed on the schematic is just a ballpark reference. In my experience the bias voltage needed to correctly bias current production tubes is substantially different from that required for the original versions of a given tube type. As long as you don't end up with a really high or low value (such as -20V or -75V) you should be OK as long as the measured bias current is correct for the tube type and plate voltage.

Edit: Answer to PS - Yes. Use the plate voltage and current measured under the same conditions to calculate the plate dissipation

[bsco]

Thanks Tom....The maximum bias voltage on the data sheet that I have is -100V of course that would be way out of wack for a guitar amp...I guess it is subjective to the particular tubes used but the procedure in determining the calculations are be the same.....I just wanted to make sure I had the right procedure....
Cheers....

[Tom Phillips]

Yep. The ratings in the data sheets need to be applied with knowledge and caution. As you pointed out the KT-88 listing shows 800V Max plate voltage and 230 mA max plate current. However, you can't let both exist at the same time because that would produce 184 Watts plate dissipation which far exceeds the stated max power dissipation of 42 Watts.