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Ampeg B18N Plate voltage and bias current too high

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  • #16
    Originally posted by Helmholtz View Post
    Speaking of no-suffix version.

    You wrote "The GZ34 only drops 10volts....a 5AR4 drops 45volts (and 5AR4GYB drops a pleasant 50volts)."
    I have updated my error and included a list.
    Thanks for pointing it out,
    the joys of being dyslexic.
    Support for Fender, Marshall, Mesa, VOX and many more.


    • #17
      Thank you for the info! I greatly appreciate the help and discussion!

      I have ordered a 5R4GY-A-B tube from Tube Depot since I don't have any on hand.


      • #18
        Originally posted by Jon Snell View Post
        My Typo I meant 5R4GYB

        Click image for larger version Name:	Valve rect. voltage drop.png Views:	0 Size:	587.8 KB ID:	930619
        I guess they use different test currents to measure those voltage drops. The chart I have attached shows even bigger drops.
        Attached Files
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


        • #19
          I generally distrust any rectifier table that doesn't specify the load current for the voltage drop. Often they just use he voltage drop at the max. DC current, which doesn't allow to compare different tubes. Also sometimes drops for capacitor and inductor input are mixed up.

          What really matters is the drop at around 100mA for capacitor input.. This can be found from the tube charts.
          Last edited by Helmholtz; 05-03-2021, 08:13 PM.
          - Own Opinions Only -


          • #20
            I received the 5R4GYB tube today and got it installed without issue. Plate voltage is 510, pretty much where it should be and the stock 47k resistor biased in nicely.

            However, a new problem arose when trying to finish up this amp. The customer provided an Eminence Sigma Pro 18A-2 speaker as the cabinet came unloaded. Several of the #8 studs for speaker mounting were bent so it took a while to get them lined up and the speaker installed.

            I played thru it for about 30 minutes, enjoying it, but when I went to put the amp into the speaker cabinet, it was hitting the speaker basket. Talked with the shop and since the customer provided the speaker, we'll see how they want to proceed.