The Dynaco Tube Audio Forum

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


    BIAS problem in a vintage st-70

    Share

    zekaaudio

    Posts : 3
    Join date : 2012-03-14

    BIAS problem in a vintage st-70

    Post by zekaaudio on Wed Mar 14, 2012 12:53 pm

    Hello everybody, I am alberto from Spain, new in this forum.
    I have 2 dynacos, VT70 tubes4hifi kt77 gl and a vintage st70.
    The problem if it is, is in my vintage one. Running the bias on 1.35 everything is ok, but biasing 1.57 the bias begin to go up slowly. Half an hour more or less, the bias go to 1,70 ore more and the tubes (SVETLANA WINGED C) are so red and there is a terrible noise that invite you inmediatly to disconect the amplifier. Is there a typical problem?. I dont mind to go at 1.35 but i am afraid that in the future will have the same problem.
    Thank you, please to meet you all and Sorry for my horrible English, I am Bask in the north of Spain.

    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: BIAS problem in a vintage st-70

    Post by GP49 on Wed Mar 14, 2012 1:14 pm

    BOTH channels, or just one?

    If it's just one channel, you may have a bad tube that is going into runaway as it gets hot.

    If it's both, I would repair the bias circuit as needed. If you still have the original selenium rectifier, get rid of it and replace with a modern silicon diode. Don't even take a chance with one of those old rectifiers. If you are lazy (and I admit to that!), you can solder a new diode across the existing selenium rectifier, as its normal failure mode is open or high-resistance...which is what you might be seeing in your amp.

    Do that first, then reset the bias and check again. If the problem still exists, you may need to replace the resistors downstream from the rectifier as they could drift with temperature. But DO THE BIAS RECTIFIER FIRST, if it has not been done already, even if it is not the actual cause of your immediate problem. Due to its tendency to fail open or high-resistance, it's a tube-killer when it goes out.

    zekaaudio

    Posts : 3
    Join date : 2012-03-14

    Re: BIAS problem in a vintage st-70

    Post by zekaaudio on Wed Mar 14, 2012 6:03 pm

    I think both chanels but one tube in the left channel more than the rest. I had changed all the tubes and the problem subsist. Then I ve changed the tubes the left channel to the right and again the bías go up more in the left channel. I take note of your indications and i ll give them to my technician (he is 65 years old but he Worked in Canada so his English is better than mine).
    Thank you very much GP49, I ll tell you the results.

    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: BIAS problem in a vintage st-70

    Post by GP49 on Wed Mar 14, 2012 6:14 pm

    That being the case, have him check for a leaky coupling capacitor in the left channel, to the grid of the SOCKET where the bias "runs away." Better, have him change ALL FOUR of those coupling capacitors.

    If that selenium rectifier hasn't been replaced yet, STILL have him do that. In my shop I could have done the four capacitors and the rectifier in less than a half hour as long as I didn't spill any coffee while I was working; and I am almost as old as your technician! Rolling Eyes

    zekaaudio

    Posts : 3
    Join date : 2012-03-14

    Re: BIAS problem in a vintage st-70

    Post by zekaaudio on Wed Mar 14, 2012 6:26 pm

    65 years old is today a young person but here most people 50 plus (me incluided) are not very computer users and in this case i need to search solutions in the internet for them. Anyway it is true that he told me something about the selenium rectifier would be the problem.
    Thank you for all. Tomorrow i show him your post reply.

    tubes4hifi
    Admin

    Posts : 1282
    Join date : 2008-11-30

    Re: BIAS problem in a vintage st-70

    Post by tubes4hifi on Wed Mar 14, 2012 6:56 pm

    GP49 good recommendation on the coupling caps, not a common problem but certainly a possibility.
    also many tubes do go into creeping mode or runaway when they run hot.
    I have a pair of new repro Mullards that do this if I set the bias higher than 35ma's, that's fairly low.
    Not a cure, but it does help a little, is to lower the "pulldown" resistor just after the output coupling cap.
    On a stock ST70 and on older VTA70 mods, it is a 270K, but changing it to 220K can help.
    With KT88s it needs to go down to 150K.
    Biasing at 1.35v on a stock ST70 is fine, that's still a good 43ma, which will run the tube cooler and longer life
    with very little noticeable effect on the sound quality.
    Also, if your tubes aren't well matched, they won't share the current equally, so one could be running at 60ma and the other at 40ma.

    Sponsored content

    Re: BIAS problem in a vintage st-70

    Post by Sponsored content Today at 5:37 am


      Current date/time is Sat Dec 03, 2016 5:37 am