The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
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


    ST 70 bias, line voltage, etc.

    Big Hat
    Big Hat

    Posts : 4
    Join date : 2021-06-24
    Location : West of Weird, Texas

    ST 70 bias, line voltage, etc. Empty ST 70 bias, line voltage, etc.

    Post by Big Hat Fri Jun 25, 2021 2:53 pm

    First, I cannot believe my good fortune in finding this forum. I'm not an electronics tech by any means, but I'm learning a lot here nevertheless.

    I have an OLD ST-70 that may have been upgraded some by the guy I bought it from. IIRC he said he upgraded the storage caps, and the six orange caps on the input board look newer than everything else there. I rolled in a matched quad of the Svetlana winged C EL34 tubes maybe as much as 10 or more years ago. They still sound fine.

    I've always biased them at 1.56V because that's what it says on the test points. A few years ago the amp started blowing fuses, so I set it aside. I dug it back out the other day and haven't had any trouble (yet!) and it sounds fine.

    I see here that the VTA ST-70s want to see 0.4V bias. And I think I read that lowering the bias can be advantageous on a stock ST-70 as well. So lately I've been running 1.35 - 1.4V and it sounds great. QUESTION: Should I go lower on the bias? Go back to 1.56V? Or...?

    I have a Genelex GL GZ34 arriving any day just because I thought my rectifier tube might be going soft, thus the fuse blowing issue. Maybe it was just me not keeping close enough tabs on the bias, I dunno.

    My line voltage varies 122 - 123 VAC.

    Thanks, Jon
    peterh
    peterh

    Posts : 1565
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST 70 bias, line voltage, etc. Empty Re: ST 70 bias, line voltage, etc.

    Post by peterh Fri Jun 25, 2021 3:17 pm

    Total dist will increase marginally when lowering bias. Nut 1.4 Volt is probably not detectable with ears.
    But reducing bias will make life much easier for the rectifier and also to less extent the power tubes
    Big Hat
    Big Hat

    Posts : 4
    Join date : 2021-06-24
    Location : West of Weird, Texas

    ST 70 bias, line voltage, etc. Empty Re: ST 70 bias, line voltage, etc.

    Post by Big Hat Fri Jun 25, 2021 6:38 pm

    Thanks, peterh. While I can still hear what sounds good to me in the music I like, "marginal" is sometimes lost on these old ears, so not much lost there. "Marginally longer equip. life"--Win even if only slight.

    Sponsored content

    ST 70 bias, line voltage, etc. Empty Re: ST 70 bias, line voltage, etc.

    Post by Sponsored content


      Current date/time is Wed Jul 28, 2021 4:22 pm