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Thread: 5U4 or GZ34 in 1970 Super Reverb (460V vs. 495V)

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    5U4 or GZ34 in 1970 Super Reverb (460V vs. 495V)

    I know the original was a 5U4 but many people put a GZ34 and rebias, any prob doing this ? What is the effect of higher plate voltage, a fatter cleaner sound right ? And what do people hate about 5u4, I read they sag more at high volumes, but I read that stuff from not so technical forums so I wanted to check.. thanks Don't worry I have a bias probe a voltmeter and a decent IQ :-P

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    The power transformer is wound for a 5U4GB rectifier which means it can handle the extra filament current of the 5U4GB.
    The PT is wound to compensate for the additional voltage drop across the 5U4GB rectifier tube by having a higher HI-V AC potential from the start.

    I don't know where these people are who write/say that they hate the 5U4GB and if there are any, they're stupid IMHO.
    It is a very good rectifier tube and can handle twice the B+ current of any GZ34 and a higher AC voltage too.
    I suspect that most amp "gurus" who might say they hate the 5U4GB don't or didn't know how to fix/mod their silverface Fender amp's bias circuit and then bias their amp's power tubes correctly.

    Here's my take, if it what you think you've heard about the 5U4GB is remotely real,
    ...because many silver face amps are biased cold from the factory and most of them do not have a real adjustable bias supply, when using the GZ34 and not altering the bias supply in any way, the B+ rises to a higher level, taking idle current up with it, creating a fatter tubey sound.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    But in my case my amp has the correct BF style bias circuit and I tried both biased correctly and it really comes down to having 460V or 495V on the plates. All the electrolitic caps are new so there's no prob. I think I'll just use the 5U4 but I wasn't shure after reading it would sound better with the GZ34. And you're absolutely certain that the 5U4 deos not sag the sound more at high volume ? Cause I think that's one of the major things people hate about the SF Super Reverbs.

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Iplayloud
    But in my case my amp has the correct BF style bias circuit and I tried both biased correctly and it really comes down to having 460V or 495V on the plates. All the electrolitic caps are new so there's no prob. I think I'll just use the 5U4 but I wasn't shure after reading it would sound better with the GZ34. And you're absolutely certain that the 5U4 deos not sag the sound more at high volume ? Cause I think that's one of the major things people hate about the SF Super Reverbs.
    No, I didn't mean to say or imply the 5U4GB doesn't sag more then a GZ34, or that you should swap it back.
    I also have heard and worked on many Super Reverbs and I've never run into anyone who hated one because of the 5U4GB.
    The 5U4GBs internal impedance is higher then the GZ34/5AR4 and there will be more voltge dropped/loss across it, relative to the same current through the tube.
    Basic electronics stuff.
    My point is that the 5U4GB can let a power stage have a nice tubey sound and the Supers are so loud that it still sounds good to me, and many others, having that little softer B+ supply with a bit more bounce to it.
    Yes, you are right in yoru assertion that you will make more clean output power and headroom with the GZ34 then a GZ34/5AR4 but the reason is mostly because the B+ rail is 30v to 40v higher now and you have adjusted the bias supply compensate for the higher B+ thus increasing the power out put.
    Not because there is anything wrong with the 5U4GB.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    I prefer 5U4s to GZ34s in 40-50 watt amps, I guess because they sag more. I can't really hear a difference between a GZ34 and a solid-state plug-in rectifer in a 40-50-watt amp, but I can hear the difference between either one of them and a 5U4.

    I've seen some irrational anti-5U4 rhetoric among the Marshall corksniffer crowd, most of whom don't have a lot of technical knowledge but who all have adamant, inflexible opinions that the components that were used in plexi Marshalls are vastly superior to any other component. They'll fill up pages on other fora talking about how much better their amps sound with Mullard GZ34s than any other makes of GZ34, and in one of those pages I've seen one or two comments disparaging 5U4s. I take anything that crowd has to say with a grain of salt.

    Shea

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Shea
    I prefer 5U4s to GZ34s in 40-50 watt amps, I guess because they sag more. I can't really hear a difference between a GZ34 and a solid-state plug-in rectifer in a 40-50-watt amp, but I can hear the difference between either one of them and a 5U4.

    I've seen some irrational anti-5U4 rhetoric among the Marshall corksniffer crowd, most of whom don't have a lot of technical knowledge but who all have adamant, inflexible opinions that the components that were used in plexi Marshalls are vastly superior to any other component. They'll fill up pages on other fora talking about how much better their amps sound with Mullard GZ34s than any other makes of GZ34, and in one of those pages I've seen one or two comments disparaging 5U4s. I take anything that crowd has to say with a grain of salt.

    Shea
    *********************
    Yes, and thank god... different strokes for different folks.

    There is only about a 15-18v difference between a good GZ34/5AR4 and solid state diodes.
    For some tonal amp applications I wouldn't even think of using a GZ34, but between SS and a 5U4GB it could be 45-55 volt difference! ha ha ha...


    Bruce
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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