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Old 8th March 2009, 10:49 PM   #11
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If I place a 1 ohm resistor between my cathode and ground, this is parallelling the cathode bias resistor, which is NOT what I want to do. What am I missing here ?

I don't want to run A/B. I was under the impression that when running a single cathode bias resistor without a bypass cap (differential) that you MUST run class A , or that the amp will have major distortion when it goes into B.

Would it help to see a schematic ?

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Old 8th March 2009, 11:10 PM   #12
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If you have cathode bias, no need to add a resistor - just measure across the one that's there. I = V/R
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Old 8th March 2009, 11:38 PM   #13
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OK, that's what I previously thought. Sometimes I get a little turned around with all the different options. . . .

So I have 60Ma across my Cathode resistor, according to Ohms law. So that is 30 Ma per tube, or 11.25W give or take.

For a triode tied 6P6S , this is about 85% of Max power at the Anode (13.2W).

For my situation using differential bias on the outputs , should I bias it up higher, closer to the theoretical max ?

What is the practical upper limit , 90% , 95% of Anode Max ?

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Old 9th March 2009, 06:37 PM   #14
Hi_Q is offline Hi_Q  England
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I made my own unit-

Although intended to check out my Yaqin EL34 amplifer, it has been very useful for matching up tubes in guitar amplifiers such as Marshall's.

I have been experimenting, with some success, a 100/120Hz low pass filter for feeding an oscilloscope. It enables one to crank up the scopes gain without noise issues and observe the ripple on the amplifiers output. After setting the correct bias on both PP output valves, although the ripple is so low, the filter enables one to fine tweak the bias on one tube for minimum ripple.
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