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Thread: NOS Bias (EVH LBX-S)

  1. #1

    NOS Bias (EVH LBX-S)

    So, I received two NOS EL84's. When I took the tubes in my amp out it was set at 28mv AND my biasing screw was at about ten o'clock. I put in the NOS tubes and checked the bias. It was at 10mv. I adjusted the screw up to 28mv BUT now the screw is at three o'clock. I thought not much of it until I smelled resistor then my volume cut in half. The resistor was replaced (solder, traces etc were fine) I put back in the old tubes and the bias was at 60mv and the screw was in the same position, three o'clock. I dialed it back to the original ten o'clock and set it back to 28mv. My volume is still half. Obviously the extreme change of the bias screw from ten o'clock to three o'clock fried the resistor. Nothing else was wrong other than that one resistor. What else could I have fried in there? (On a side note, I did need to replace the 8 pin op amp as well on the removable mute pcb. Again it was all pro soldering clean and tested) yet still, half volume. I've already troubleshat the obvious. Please, can someone lend a thought. A helpful one. Not one showing me how much you know without answering or offering a helpful thought. LoL. Thanks 🤟

  2. #2
    Forum Member
    Join Date
    Aug 2008
    Location
    SW New Mexico, USA
    Posts
    1,674

    Re: NOS Bias (EVH LBX-S)

    Did you check the voltage to calculate the correct ma bias?
    I would bet the two sets of tubes aren't the same, (they usually aren't)!

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