Tuesday, June 2, 2009

Tube Curves is Your Friend

Common stereotype misconceptions that tube design is for tube gurus with in-depth tube knowledge, Master Yoda-ish character and is one-with-the-force; In reality, anyone who is competent in prevent/avoid electrocution while working with high voltage and still remembers Ohm’s Law is competent enough. All you need is the data sheet. Allow me to demonstrate.

Firstly we get hold of the 6DJ8 data sheet as follows:-

You can start by determine the max and min safe work condition, gain, Rp, rp, plotting the best loadline… etc.

BUT I WON'T!! I will reserve that for your self-amusement during your dreary hours…

Use the Data Sheets recommendations and get straight to it before the irrational impulse of building a tube preamp subsides.

From the #Page 1 of the Data Sheet we have
Filament voltage, Vf = 6.3V,
Filament Current, If = 365mA
From this, we calculate the power required by th filament Pf = Vf x If = 2.3W

#Now write down this on your shopping list, “ 6V, >2x365mA (1A is common),transformer”

At the end of #page 2, "maximum rating, absolute value"
Series grid Resistor = 1 megohm max

you will find "Typical Charateristics" and the followings

Plate Voltage, Vp = 90V, This is the voltage across the anode and cathode
negative grid voltage, Vg = 1.3V, this is be the Bias Voltage
Plate Current, Ip = 15mA, with triode, Ip= Ik, cathode current

Now some maths,

Find Rk, using Ohm's Law,

Rk = Vbias/Ik = 1.3/0.015 =87 ohm #

Determine B+,
We will use Isolation Transformer, In my part of the world, main power is 230V AC,
Using Isolation Transformer with a bridge will yield,

Raw B+= Sqrt(2) x 230 = 325V,
we will filter raw B+ to 300V

VLoad = 300V - Vp - Vbias = 300V - 90V - 1.3V = 208.7V #

RLoad = VLoad/Ip = 208.7V/15mA = 13,913 ohm we can choose 15K resistor.

Well after all this hard work, I need to get my regular coffee fix.... We'll continue on the next installment

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