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    Strange bias readings after VTA ST-70 board install

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    Brent70

    Posts : 9
    Join date : 2009-12-08

    Strange bias readings after VTA ST-70 board install

    Post by Brent70 on Fri Dec 18, 2009 12:23 am

    Got the VTA board installed on the Stereo 70 tonight. I'm getting some wild bias readings. It will read normal one minute (.40v DC) then jump to 1.26v DC then drop to .26v DC. This is happening on every tube randomly. I tried a quad of JJs then Winged Cs and it was the same on both. I have a Fluke 175 meter that is functioning properly. The amp is playing fine. Thoughts?

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Strange bias readings after VTA ST-70 board install

    Post by GP49 on Fri Dec 18, 2009 3:53 am

    Are you doing the bias measurements when the amp is quiescent, not playing anything?

    Brent70

    Posts : 9
    Join date : 2009-12-08

    Re: Strange bias readings after VTA ST-70 board install

    Post by Brent70 on Fri Dec 18, 2009 6:48 am

    Yes. No signal going in when measuring.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Strange bias readings after VTA ST-70 board install

    Post by Bob Latino on Fri Dec 18, 2009 9:49 am

    Brent70 wrote:Yes. No signal going in when measuring.

    Hi Brent,

    Did you have your SPEAKERS connected when you did the bias readings? Sometimes if you have no load on the output transformers you will get osscillating up and down bias readings as you describe.

    If you did have your speakers connected and that happened I would try a new rectifier tube in there and check the bias again.

    Bob

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Strange bias readings after VTA ST-70 board install

    Post by GP49 on Fri Dec 18, 2009 1:18 pm

    Bob Latino wrote:

    Hi Brent,

    Did you have your SPEAKERS connected when you did the bias readings? Sometimes if you have no load on the output transformers you will get osscillating up and down bias readings as you describe.
    Bob

    Should this REALLY happen? One thing I have always prided myself on, when building or rebuilding an amplifier, is that it be as close as I can get it to absolutely stable, from "normal" loads to open-circuited. I've always felt that if an amplifier can go into instability...and varying or oscillating current in the output stage when unloaded IS instability...it will be more likely to exhibit this behavior into the varying loads of loudspeakers.

    Instability under CAPACITIVE loads can occur with almost anything if sufficiently provoked, but open-circuit instability in a tube amplifier is something I would try to avoid.

    Brent70

    Posts : 9
    Join date : 2009-12-08

    Re: Strange bias readings after VTA ST-70 board install

    Post by Brent70 on Sat Dec 19, 2009 1:20 pm

    After taking a day to recheck all connections the bias is stabilized at 1.26v DC per tube with the pot at 12:00. When fully ccw it goes down to about .9v DC. Any suggestions? I do have an email in to Roy.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Strange bias readings after VTA ST-70 board install

    Post by Bob Latino on Sat Dec 19, 2009 3:40 pm

    Brent70 wrote:After taking a day to recheck all connections the bias is stabilized at 1.26v DC per tube with the pot at 12:00. When fully ccw it goes down to about .9v DC. Any suggestions? I do have an email in to Roy.

    Hi Brent,

    On the clockwise - counterclockwise thing ... When you turn those DC (bias) balance pots on the VTA board CLOCKWISE the bias should go DOWN. When you turn them COUNTERCLOCKWISE the bias should go UP. These DC balance pots SHOULD work exactly opposite of a normal volume control like on a radio or a stereo. Turning the control CCW should not turn the bias DOWN as you mentioned above but UP.

    Those values are way too high for an EL34 with a 10 ohm bias resistor. Measure the value of R39 which is the main bias resistor. It should be 10K on the VTA driver board for the ST-70. Double check that it is 10K. If it is 10K - then try strapping on another 10K in parallel with that one on the bottom of the driver board to give you an effective 5K resistor there. With less resistance in there you should be able to get your bias readings down to .40 VDC per each EL34 tube.

    Bob

    Brent70

    Posts : 9
    Join date : 2009-12-08

    Re: Strange bias readings after VTA ST-70 board install

    Post by Brent70 on Sat Dec 19, 2009 3:47 pm

    Biasing via the transformer shunt method I am able to dial in a consistent 40mA per tube. Not sure why I can't get a consistent reading off the 10 ohm resistors on each cathode. It's working now...sounds great...stable bias. All's well that ends well.

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