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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


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    Bias and plate voltage

    mazeeff
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    mazeeff


    Posts : 155
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    Location : Grand Rapids, Michigan, USA
    • Post n°1

    Bias and plate voltage Empty Bias and plate voltage

    Post by mazeeff Mon Feb 24, 2014 8:01 pm

    I was over on a Marshall Amp forum, and they were discussing the importance of measuring plate voltage, in relation to setting bias. They focus on plate power dissipation as a percentage of maximum plate wattage (25W). They measure the plate voltage, and adjust the bias to achieve 65-70% of 25W. They seem to get a lot of variability in the Marshall's B+, and seem to think that this is important. With all the same variability in the ST-70, should we be focused on plate dissipation as well. I have a SS conversion on my ST-70, so my B+ is around 20V higher. At 430V and 50ma, I am running at 86% (21.5W) of rated dissipation, which is much higher than the Marshall folks. Is this something that is important to consider for the ST-70? Thanks,

    Mike
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°2

    Bias and plate voltage Empty Re: Bias and plate voltage

    Post by Bob Latino Mon Feb 24, 2014 9:21 pm

    Hi Mike,

    Guitar tube amps and audio tube amps, although similar, sometimes have different objectives. Guitar amps sometimes run on the ragged edge of clipping (distortion) in order to achieve a certain "sound" from the guitar. Audio tube amps should not be run into heavy clipping. You can take out rectifiers and damage speakers if you run audio tube amps into heavy clipping.

    Your ST-70 running at 430 plate voltage is normal. The 50 ma bias setting is a little on the high side by conventional wisdom for the Dynaco ST-70. 40 ma per each output tube on a Dynaco ST-70 will give longer tube life and no real difference in sound quality. Years ago the original Dynaco ST-70 WAS set up with the 1.56 VDC bias point for each output tube pair or 50 ma per each output tube - BUT - these older original Dynaco ST-70's were supplied with very high quality and durable MULLARD EL34's that could take the higher bias point (50 ma per each output tube) in stride. The EL34 output tubes of today which are made in China, Russia or the Slovak Republic, if used at 50 ma, just won't last as long as if they were biased at 40 milliamps.

    Bob
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    mazeeff
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    mazeeff


    Posts : 155
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    • Post n°3

    Bias and plate voltage Empty Re: Bias and plate voltage

    Post by mazeeff Mon Feb 24, 2014 10:10 pm

    Thanks Bob. I will crank them down to 40ma.
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    mazeeff
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    mazeeff


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    • Post n°4

    Bias and plate voltage Empty Re: Bias and plate voltage

    Post by mazeeff Tue Feb 25, 2014 6:11 am

    Bob. I tried running the bias at 40, 45 & 50. These are new JJ 34L's. ST-70 has diytube board and SDS capboard with SS rectification. 50 sounds best to me. When you mention shortened reliability, what should I expect? If I could get 1,000 hours (1 year) out of them, I would be happy.
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    Bob Latino
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    • Post n°5

    Bias and plate voltage Empty Re: Bias and plate voltage

    Post by Bob Latino Tue Feb 25, 2014 8:47 am

    There is no real way to predict the life span of the output tubes in your amp .. You might get 1000 hours out of them biased at 50 ma but you might get 3000 hours out of them biased at 40 ma.

    As to which bias setting sounds "best" ... the only way you can do a definitive test on which bias setting sounds best is for you is to go out of the room and have another person set the bias and then listen to a track NOT KNOWING what the bias setting is and then giving your opinion on whether it is better, worse or the same as a previous setting (a blind test). When you KNOW what the bias setting is, it will bias (pun intended) your judgement on whether it sounds better, worse or the same as a previous setting.

    There are those that say even blind testing is not valid and that you really need a DOUBLE blind test in which even the person doing the switching doesn't know which component has been changed or "altered". For components an ABX comparator is used in which you try to figure out whenther component "X" is A or whether component "X" is B. This type of test has humbled many a manufacturer. One of the most famous was the Monster cable vs. a coat hanger wire here > Monster Cables vs a coat hanger

    There are some more blind and double blind audio component testing links at the link below ..

    Testing audio gear ...

    Bob
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    mazeeff
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    mazeeff


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    • Post n°6

    Bias and plate voltage Empty Re: Bias and plate voltage

    Post by mazeeff Tue Feb 25, 2014 10:00 am

    I think you are correct. I kept the volume on my preamp at -20db for all three tests. I was likely just responding to the increase in volume, from going from 17.2W (430x40ma) to 21.5W (430x50ma). If I increase the preamp, while at 40ma, it sounds the same as 50ma with lower preamp.
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    • Post n°7

    Bias and plate voltage Empty Re: Bias and plate voltage

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