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Substitute GZ34 for 5U4GB?

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  • Substitute GZ34 for 5U4GB?

    Hi,

    This is my first post, so hello and thanks in advance for any help. I have Mesa Tremoverb. It is running Mesa 6L6 STR-440s (Chinese tubes). It also has the EH 5U4GB rectifier tubes. The sag is a little too mushy, so i thought putting in a pair of JJ GZ34s will tighten it up a bit. I know this will increase the voltage, but how will it change the bias to the tubes? As you know, the amp has a fixed bias. Right now, they are all dead even at 30mA, and the amp sounds very good. Will the switch and increase in voltage raise, lower, or not affect the current draw of the tubes? I don't mind if it goes higher since that makes the amp sound better, but I don't want it to go any lower. Also, since this amp has a selector switch for rectifier tubes or silicon diodes, will changing to the GZ34 only raise the voltage when rectifier tubes are selected? Thanks a bunch in advance.

  • #2
    Hi cscotto and welcome to the forum

    A GZ34 will raise the B+ by 30V or so and will definitely stiffen up the sound a bit, and so you may want to re-bias the 6L6s to run a bit cooler. Also GZ34 like up to 60uF reservoir cap (compared to the 5U4G @ 40uF) to get the maximum benefit out of them. But just try changing the rectifiers first and see how you like it. 2CW
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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    • #3
      Shouldn't be a problem in this amp -- GZ34/5AR4 will split the difference between the 5U4 and solid-state rectifier nicely.

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      • #4
        The problem is not the use of 5U4. Itīs the absence of bias readjust when the rectifier tubes are activated.
        With diodes you will read 445 volts approximately and with 5U4, about 400 (it happens in the versions for Europe with 230V). Itīs a good relation (-10% aprox.)
        If you have an adjustment of 36mA per tube with diodes, on having activated the 5U4 the bias adjustment will decrease approximately 30% (25mA per tube). 25mA x 400 volts is a very cold bias adjustment. The amp sounds bad.
        If you manage to automate a good adjustment in both modes it will be able to obtain the best response.
        Regards

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