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    Prefered rectifier tube for VTA ST-120

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    ImFritz

    Posts : 46
    Join date : 2017-01-01
    Age : 59
    Location : Redford, Michigan

    Prefered rectifier tube for VTA ST-120 Empty Prefered rectifier tube for VTA ST-120

    Post by ImFritz on Fri Mar 29, 2019 12:07 pm

    Seeking opinions from the members of this forum. Its no secret, I'm not as well versed and knowledgeable as many of the fine individuals here.
    I purchased the pre-built VTA ST-120 from Bob. (Thanks again Bob) It came with (3) National 5963/12au7 driver tubes, (4) Sovtek 6550 output tubes,
    and the Tung-sol 5ar4/Gz34 rectifier. I did experience the Tung-sol rectifier tube flash and blow the fuse in my variac volt regulator. I do have high voltage
    from my service at 125 volts. Running it at 117v, I replaced the fuse in the variac with a slow-blo. Since then I purchased a Gold lion U77/GZ34 rectifier, a Sovtek 5ar4 rectifier which Im running now. Both of these rectifiers are running fine now. I've also purchased the Mullard GZ-33 Great Britain tube. (Big bottle tube) like the way it looks,
    pricey little sucker! This runs fine as well but it sounds like a baby's rattle when you gently shake it. One additional purchase was a set of Tung-Sol KT-120 output tubes that i'm currently running. Question is this. (sorry about the long winded explanation) One other variable is I'm currently in the process of getting a set of new Magnepan 1.7i's.
    What tube complement would you go with from these choices and why (other than just saying what sounds best cause I kinda know that already).
    Just looking for your opinions.
    Thanks
    Fritz

    Peter W.
    Peter W.

    Posts : 1268
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Prefered rectifier tube for VTA ST-120 Empty Re: Prefered rectifier tube for VTA ST-120

    Post by Peter W. on Fri Mar 29, 2019 1:40 pm

    Opinions only, as my experience is limited to the Brit Bottle GZ tubes and Maggies:

    a) A Brit Bottle is very likely your best bet, for a combination of voltage-drop and overall ruggedness. These are pre-blight tubes, and so far more rugged than present-production.
    b) About any tube amp into Maggies is a risk. In the right room with the right speaker placement and a gentle  hand, the 120 will do very nicely. Otherwise, not so much. This has to do with headroom and transient response, which will be limited. Not a risk to the amp or to the speakers.

    This link is to a rather comprehensive tube rectifier chart. PLEASE NOTE: Bob L. has published this chart a few times in the past - I take no credit.

    https://2img.net/h/s26.postimg.cc/568ayn649/Tube_rectifier_comparison.jpg

    Note that the GZ33 (not listed) is very nearly directly between the 34 & the 37.

    A few things to note:

    a) 'excessive' voltage drop (B+) is neither necessarily a good thing or a bad thing. It is all in accordance with what sounds good to you.
    b) Filament amps "want" to be as low as practical to reduce strain on the 5 winding of the power transformer - much less of an issue with VTA amps than with OEM Dynaco.
    c) PIV wants to be as high as practical BUT - even 1000 V is more than adequate.

    And, bucking the incoming voltage such as you are doing is a good thing.
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    ImFritz

    Posts : 46
    Join date : 2017-01-01
    Age : 59
    Location : Redford, Michigan

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    Post by ImFritz on Fri Mar 29, 2019 2:09 pm

    Thanks Peter

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