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Old 26th December 2009, 03:29 PM   #1
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Default amplifier bias

I have a home built mono block for my home theater. Edcor 50 watt OPT with 6,600 ohms. Have 468 volts at the plates. Running ultralinear with screen hooked to OPT taps. Current output tubes are 7581's but I also have 6550s, KT66's, 6L6GC's, EL34's, and KT88's lying around. Have 10 ohm resistors on the cathodes to measure bias and seperate bias pot for each tube. Not interested in max power from the tubes or getting to a full 50 watts on the edcor. Am currently running 500 mv on each cathode (50 ma idle current per tube). Is this a good value. What would be good values for the above listed tube sets?
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Old 11th January 2010, 06:40 PM   #2
knausz is offline knausz  United States
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Seems pretty hot if only because your burning over 23W just idling (plate+screen). A 6L6GC has a plate dissipation of 30W and screen of 5W. That doesn't leave a lot of power left to burn to drive a signal. Tube life is better at lower currents. Try turning it down and see if it still sounds good, that's a bottom line. If you're not cranking it, the current bias setting is fine.
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Old 11th January 2010, 11:11 PM   #3
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Shouldnt be a problem, 50 ma is fine. Remember, a music signal is more or less symmetric, so after the current rises above 50 ma, it goes below 50 ma. So the average is still 50 ma. For el34 , kt66 and 6l6gc it should be about 40-50 ma. For KT88 and 6550, it can be a little more. Dont know the 7581.

If you increase the idle current, your amplifier gets more class A operation, but there is a catch. Because the bias voltage gets less negative (it gets closer to 0v) when you increase the bias current, you run a bit earlier in class ab2.
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