# What value for the bias resistor of a SE 6550?

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#### DAK808

For my single ended 6550 amp I am going to use cathode bias of the 6550 power tubes. My Av is 325v and the G2 voltage is 275v. Your input is most appreciated. cheers, 808

#### costis_n

390 Ohms, 5W minimum.

#### 20to20

For my single ended 6550 amp I am going to use cathode bias of the 6550 power tubes. My Av is 325v and the G2 voltage is 275v. Your input is most appreciated. cheers, 808

What do you want your plate Pd to be? 30W? Do you know the formula for finding cathode current?

#### DAK808

390 Ohms, 5W minimum.
OK, I think I have some 390 ohm. I dunno about the wattage . I might have to double around a 800 ohm 2 watt.

#### DAK808

What do you want your plate Pd to be? 30W? Do you know the formula for finding cathode current?
From what I have gleaned about the subject you would want to have the tube dissipate close to max. That is why I picked about 90% of max. I was using v=ir to calculate the current dissipated by the resistor.
My first resistor was 470 ohm but that was way to cold. If I recall that was about 60ma cathode current. I wanted to know what other builders had previously used in successful amps. cheers.

#### 20to20

If you want @30W of plate dissipation you need @100mA cathode current at your plate voltage. Your cathode voltage should be @15v. Somewhere between 150R and 200R should work depending on the condition of the tube. Start high.

#### 20to20

808, Let us know what you get.

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