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Vox AC4 EL84 screen voltage

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ECS, Apr 6, 2015.

  1. ECS

    ECS Member

    Sep 29, 2013
    Working on putting together a Vox AC4 clone right now. I've got the amp built up and it's working at the moment, sounds pretty good! I noticed however while taking measurements that the voltage on the screen of the EL84 are much higher than the plate voltage... can't remember the actual numbers now, but it's somewhere like 30-40 volts higher.

    Reading up on some other posts, I've seen a lot of people recommending either moving the B+ supply for the OT up in front of the 1K resistor, or moving the screen supply further down the chain.

    I built this amp using a choke in place of the 1K dropping resistor... would there be an issue with moving the B+ rail in front of the choke, basically to the first filter cap straight off the rectifier tube?

    Would it be better to move the screen supply after the 22K dropping resistor, or would this cause an issue because then it would be shared with the EF86?

    And a secondary question, would it make sense to install a screen grid resistor for this amp? I'm thinking a 1k 2-3 watt resistor would be about right?

    Last question... is there any chance I've done any damage to the EL84 I've been using in this amp by running the screen higher than the plate for around 5-10 minutes?

    Thanks for the help! Schem for reference:


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