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    5U4GB instead of 5AR4 in my ST70


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    Join date : 2009-03-27

    5U4GB instead of 5AR4 in my ST70 Empty 5U4GB instead of 5AR4 in my ST70

    Post by myaddiction on Fri Aug 07, 2009 8:46 pm

    OK, I am aware that the 5AR4 is an indirect heated rectifer tube and has longer heat-up compared to the 5U4GB which is direct heated. I have seen a few ST70s with 5U4GB rectifier tubes. What do I have to do to use a 5U4GB in my ST70 with the hi-gain VTA board?


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    5U4GB instead of 5AR4 in my ST70 Empty Re: 5U4GB instead of 5AR4 in my ST70

    Post by GP49 on Sat Aug 08, 2009 12:46 am

    Luck, and barring that, perhaps enough money to buy a new power transformer.

    The 5U4 filament winding draws more current than does the 5AR4 (3.0 amps vs. 1.9). The stock Stereo 70 power transformer is marginal to begin with, and hanging more filament draw on it can't help its longevity. Also, do the filaments of the tubes on that aftermarket board draw more current than two 7199s? If so, there's another added strain on the already-marginal power transformer even before your rectifier substitution.

    Also the 5U4 "turns on" faster, as you noted; before the filaments of the tubes have warmed up, you'd be throwing B+ into the tubes. There has been controversy about whether this is actually harmful to tubes (some say it causes cathode stripping which shortens tube life, others say it does not) and Dynaco did use the 5U4 in its Mark II power amplifier.

    The 5U4GB also has a higher series resistance than the 5AR4, so the resultant rectified B+ voltage will be lower, and your amplifier's output power will be reduced.

    Now, WHY do you want to substitute a 5U4GB for the 5AR4?
    Bob Latino
    Bob Latino

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    5U4GB instead of 5AR4 in my ST70 Empty Re: 5U4GB instead of 5AR4 in my ST70

    Post by Bob Latino on Sat Aug 08, 2009 9:00 pm

    GP49 has given a good assesment of the situation. You should always run a GZ34 in an ST-70 if possible. A 5U4 makes a good "backup" rectifier if your GZ34 dies in the middle of a listening session.

    The good thing about a 5U4 rectifier is that AS A USED TUBE they are cheaper and more plentiful than a used GZ34. Many old tube televisions of the '50's and '60's used a 5U4 rectifier and your chances of finding a good used USA made RCA, GE or Sylvania 5U4 at a reasonable price are pretty good. Used USA or European (Mullard, Amperex, Phillips etc.) GZ34's command much higher prices. I have a few 5U4's here and have used them as backup rectifiers in a number ST-70's over the years. Yes - they draw more current but I have never seen an original power transformer harmed by the use of a 5U4 in place of a GZ34. Even Dynaco, on page 9 of the ST-70 manual, suggests the use of a 5U4 as an emergency backup rectifier for the ST-70. Yes - It will give a slightly lower B+ high voltage and reduce overall power output of your amp but they do work and will keep the music playing if your one an only GZ34 craps out on you.


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    5U4GB instead of 5AR4 in my ST70 Empty Re: 5U4GB instead of 5AR4 in my ST70

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