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Ampeg B15N - 5AR4 vs 5U4GB

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  • Ampeg B15N - 5AR4 vs 5U4GB

    With early (cathode biased) B15N that calls for 5U4GB I have 412V on the plates (filament & high voltages are right), so a bit on lower side. Schematic calls for 450V...

    Later models had 5AR4.

    I know that 5U4GB drops more than 5AR4 and are interchangeable in most cases, so I've tried the 5AR4 and I'm getting now around 421-422VDC on the plates. Would it be 100% safe to use 5AR4 here? With the 220ohm cathode resistor (schem. calls for 250) I'm getting 31V across it, so around 95% dissipation which is okay I would say.

  • #2
    Not quite a direct replacement but the heater draw is only 1.9A in the 5AR4, 3A in the 5U4GB.
    The 5U4GB has directly heated cathodes and the 5AR4 is indirect heating.
    Other than the heater current, a good choice if the extra HT is not an issue.
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    • #3
      If you're looking to raise the B+ a bit, I'd bias it cooler. Just me, though.

      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -


      • #4
        The good thing is the 5AR4 takes a little time to get voltage up, acting as a standby.