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Biasing a
tube is adjusting the amount of current that flows between the anode and cathode of a
tube. Exceeding tube manufacturer specifications at the least will result in shorter tube
life possibly stressing the tube to failure. Like many amps, from the factory the EVJ Hot
Rod exceeds recommended tube manufacturer specifications. On a JJ EL84 spec sheet the specified maximum plate voltage
is 300 volts and the maximum plate dissipation is 12 watts. Manufacturer
specifications may vary for different tubes but most list these values.
Biasing the EVJ Hot Rod YouTube Video |
Through the cathode flows the plate current and the
Screen current (pin9), measure the current that flows through the cathode then subtract
the Screen current.
10.5 volts is measured across the cathode resistor with a value of 225 ohms, Cathode
current = 10.5 volts / 225 ohms = 46.6mA
To measure the Screen current a resistor will need
to be added between the screen (pin9) and the B+.
To insert the resistor between the screen and the B+, the trace could be cut on the
circuit board and a 1k resistor added.
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Instead a pin is cut on a socket saver and the 1k
resistor is connected between the pin on the socket saver and the pin on the tube.
Current = Voltage / Resistance Ohms Law
Calculator
4.4 volts is dropped across this 1k resistor, 4.4 volts / 1000 ohms = Screen
current (.0044 Amps or 4.4mA). Cathode current (46.6mA) - Screen current
(4.4mA) = Plate current (42.2mA)
To find the plate dissipation of the power tube,
multiply the voltage drop between the Plate (pin7) and the Cathode (pin3), by the current
that flows between the plate and cathode. Plate Voltage 312 volts x Plate current .042Amps
= Plate Dissipation (13.1Watts).
Most EL84/ 6BQ5 tube Manufactures Recommend a
Maximum Design centre value (should not be exceeded under normal operating conditions)
300 Plate Volts and 12 Watts Plate dissipation, while allowing for 10% +/- for variation
in supply voltage. |
Some of the Capacitors and resistors have been
replaced to improve the tone and bass response and increase the harmonics.
To Lower the plate voltage the stock 220 ohm
dropper resistor (1) is replaced with a 680 ohm resistor.
To Lower the plate current the stock 220 ohm cathode resistor (2) is replaced with a 270r
ohm resistor.
With these two resistors swapped - Cathode
current = 10.6 volts / 270 ohms = 39.2mA, - Screen current 3.6mA = Plate
current (35.6mA).
Plate Voltage (298 volts) x Plate current (.036Amps) = Plate dissipation
(10.7 Watts).
No two power tubes are exactly the same. Some
may move more current than others. Whenever you change the tubes you should check the
bias.
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