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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    ST 120 and the 5ar4 tube.

    Gabriel
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    Gabriel


    Posts : 38
    Join date : 2022-01-23
    • Post n°1

    ST 120 and the 5ar4 tube. Empty ST 120 and the 5ar4 tube.

    Post by Gabriel Sat Jan 21, 2023 9:45 pm



    Hi

    I have the ST120, without the TDR or triode mode switch.
    I installed the GL KT88.
    The sound is great. Unfortunately, the amp keeps blowing the 5ar4 rectifier tubes. Not immediately, but eventually.
    I currently use the 5U4Gb which is OK.
    I prefer the 5ar4.
    Is there any modifications I can do to avoid this situation.
    Thanks
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    Bob Latino
    Bob Latino
    Bob Latino
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    Posts : 3263
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    ST 120 and the 5ar4 tube. Empty Re: ST 120 and the 5ar4 tube.

    Post by Bob Latino Sun Jan 22, 2023 10:36 am

    Gabriel wrote:

    Hi

    I have the ST120, without the TDR or triode mode switch.
    I installed the GL KT88.
    The sound is great. Unfortunately, the amp keeps blowing the 5ar4 rectifier tubes. Not immediately, but eventually.
    I currently use the 5U4Gb which is OK.
    I prefer the 5ar4.
    Is there any modifications I can do to avoid this situation.
    Thanks

    1. Check your line voltage .. Anything in the 122 VAC (or higher) range is too high for the amp. In some places in the USA you can see up to 126 VAC ! A higher than normal line voltage will raise the B+ DC voltage and put extra stress on all of the components in the amp. The B+ high voltage for the VTA ST-120 as measure off pin #8 of the rectifier tube should be in the 490 - 510 VDC range. Higher line voltages can push this value up into the 525 - 550 VDC range. (TOO HIGH !)

    2. The use of a 5U4 in place of a 5AR4/GZ34 rectifier will drop the B+ maybe 35 volts so if your line voltage is high, the use of the 5U4 is helpful. You can still get USA made 5U4's (RCA, GE, Sylvania) on Ebay at reasonable prices.

    3. You can also use a variac to turn high line voltage down a bit. The 5 amp unit at the link below will work fine for your VTA ST-120.

    Variac's at Circuit Specialists

    4. Use a good 5AR4 > I never had good luck with either the JJ or the Sovtek 5AR4. The Tung-Sol, Genalex and Mullard versions seem to last longer ...

    Bob
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    LeGrace
    LeGrace
    LeGrace


    Posts : 388
    Join date : 2016-08-07
    Location : Ontario, Canada
    • Post n°3

    ST 120 and the 5ar4 tube. Empty Re: ST 120 and the 5ar4 tube.

    Post by LeGrace Sun Jan 22, 2023 4:47 pm

    Google GZ37.
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    Roy Mottram
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1839
    Join date : 2008-11-30
    • Post n°4

    ST 120 and the 5ar4 tube. Empty Re: ST 120 and the 5ar4 tube.

    Post by Roy Mottram Tue Jan 24, 2023 4:28 pm

    and don't forget to add the "yellow sheet mod" if you haven't already done so.
    It's one of the "stickeys' on the main menu.
    5AR4s are running at their maximum rating so will wear out fast.
    5U4GB are rated at 10% more current and will probably last twice as long.
    If you can find and can afford a GZ37 they will last for years.

    LeGrace likes this post

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    • Post n°5

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