Biasing an amp for the first time. Qs

Discussion in 'Gear & Equipment' started by FrznTek, Jul 31, 2020.

  1. FrznTek

    FrznTek SS.org Regular

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    I have Jet City Amelia. 50w 4 12ax7's 2 EL34's (from what I read the amp is based on a Cornford MK50 MKII )
    I bought a full set of Mullard reissue tubes for it.

    I bought one of these
    https://www.amazon.com/gp/product/B0777F8DBK/?tag=sevenstringorg-20

    I read some ware that it's biased a bit cold from the factory... but I'm getting about 19 mA reading. which seems very low. (shouldn't it be between like 25 - 40 mA?)
    Jet city also says you can swap to 6L6s without re-biasing.... maybe the cold bias is the reason?

    I also only seem to be getting a reading from one socket. Both tubes have power, they both glow. I tried to switch what meter it was on and then switched the probe (to make sure it wasn't a faulty probe), but it follows the socket.
    The amp has 2 master volumes, do they need to be turned up when biasing or should it not matter? (I have one turned all the way down to work as a mute.)
     
  2. Nicki

    Nicki SS.org Regular

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    I know nothing about biasing an amp and I'm only adding a reply to say this:

    For the love of God, please be careful. Even switched off and/or unplugged, touching the wrong thing in an amp can be lethal since capacitors will still hold a charge for a long time. If you know someone who has experience with electronic repair, please consult them or get them to help.
     
    Chebax and XPT707FX like this.
  3. Chebax

    Chebax SS.org Regular

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    Yes, please be careful because this can get very dangerous. This voltages can kill you.

    Assuming your amp is class AB (most are, but make sure of this) you’ll need to measure the amps plate voltage (DC voltage between pins 3 and 8 of your power amp tubes).

    That voltage, and the tube’s plate dissipation (25W for EL34) will be used to calculate the cathode current.

    19mA is too low, yes. The optimal value will depend on the plate voltage, but 19mA is too low for almost any plate voltage.

    You can use an online calculator for this:

    http://www.tedweber.com/webervst/tubes1/calcbias.htm

    And I don’t know why you’re only getting measured from one socket. That’s strange. You should be able to measure the cathode current for all the tubes in the amp. Have you tried swapping tubes?
     
  4. FrznTek

    FrznTek SS.org Regular

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    yeah well aware of the danger and most of the basic safety tips.

    Thanks for the link and tips!
     

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