Bias Resistors?

kam4ff

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I want to put some in my two Marshall's (jcm 800 and silver jubilee) so I can bias them up myself. Will a 2 watt 1k be alright? Does what it's made out of matter?

Also is there any good article or place to read up on how to calculate the bias correctly?
 

V2

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I assume that you want to add current sense resistors so that you can measure the bias and then adjust the trim pot accordingly. If so, here's what I'd recommend: 1 ohm high precision metal film or wire wound resistors from pin 8 to ground for each EL34. Set your VOM to measure millivolts, and measure the voltage across each of these cathode resistors. Following ohm's law, current = voltage divided by resistance. Since your resistor would be 1 ohm, your bias current would equal the measured voltage (e.g., 35 millivolts / 1 = 35 milliamps).

There is likely free info online; check the valve wizard site. Personally, I'd get a good book or several. The valve wizard (Merlin Blencowe) doesn't have a book on the power amp, so I'll steer you to Kevin O'Connor's TUT Volume 2 book. Come to think of it, get his external bias kit from the london power site:
BMK2 - Bias Mod Kit

It's not super cheap, but it comes with some installation notes (although KOC expects DIYers to have enough background to follow the instructions; if you don't have that background, read TUT2 or equivalent).

The kit would give you parts for external bias jacks along with bias pots for each tube.
 

kam4ff

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I know about the voltage inside an amp and how deadly it can be. Dont worry. Ill be careful.
 

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