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


    VTA St-70 won't hold steady bias

    Share

    panaseef

    Posts : 11
    Join date : 2012-04-22

    VTA St-70 won't hold steady bias

    Post by panaseef on Sun Jan 11, 2015 2:25 pm

    I popped the cage off my ST-70 for a routine bias check - it's been over a year since I checked it.  The left side required some adjustment, but holds fairly steady - my digital multimeter jumps between .39 - .41.  I recall being able to get it rock-solid at .40 last time.  The right side is far worse.  The reading jumps all over between .3 and .8; won't hold steady at all.  I'm using JJ EL34 tubes that are a few years old - probably have 1 = 2 thousand hours on them.  So, does this just men that it's time to replace the EL34s, or is there something else I should check?

    I'm checking it with everything unhooked and the amp just warmed up for a few minutes.

    Guest
    Guest

    Re: VTA St-70 won't hold steady bias

    Post by Guest on Sun Jan 11, 2015 3:20 pm

    panaseef wrote:I popped the cage off my ST-70 for a routine bias check - it's been over a year since I checked it.  The left side required some adjustment, but holds fairly steady - my digital multimeter jumps between .39 - .41.  I recall being able to get it rock-solid at .40 last time.  The right side is far worse.  The reading jumps all over between .3 and .8; won't hold steady at all.  I'm using JJ EL34 tubes that are a few years old - probably have 1 = 2 thousand hours on them.  So, does this just men that it's time to replace the EL34s, or is there something else I should check?

    I'm checking it with everything unhooked and the amp just warmed up for a few minutes.

    are your speakers connected?...they MUST be connected to load up the output stage!
    BIAS will vary also with mains voltage variations, but that would affect both channels. Could be the tubes, they are getting 'there'.....but I would also check the BIAS resistors. Turn the amp off, let the capacitors discharge, remove the tubes and put your multi meter across the BAIS resistors and see what you get.

    panaseef

    Posts : 11
    Join date : 2012-04-22

    Re: VTA St-70 won't hold steady bias

    Post by panaseef on Sun Jan 11, 2015 4:10 pm

    Thanks - speakers were unhooked (duh). hooked them back up and I can dial in the bias no problem. Checked the bias resistors while I was under the hood and they all check out.

    Guest
    Guest

    Re: VTA St-70 won't hold steady bias

    Post by Guest on Sun Jan 11, 2015 4:29 pm

    panaseef wrote:Thanks - speakers were unhooked (duh).  hooked them back up and I can dial in the bias no problem.  Checked the bias resistors while I was under the hood and they all check out.

    excellent!...good to hear all is well...I'd still keep an eye on those tubes over the coming months.... Smile

    Sponsored content

    Re: VTA St-70 won't hold steady bias

    Post by Sponsored content Today at 6:12 pm


      Current date/time is Wed Dec 07, 2016 6:12 pm