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


3 posters

    Original MK3 and ST70 Bias settings, Why?

    avatar
    jwb474

    Posts : 34
    Join date : 2014-07-13

    Original MK3 and ST70 Bias settings, Why? Empty Original MK3 and ST70 Bias settings, Why?

    Post by jwb474 Mon Oct 11, 2021 6:18 pm

    After going thru all of the search results for bias setting, I did not find (or just overlooked) the answer this question:

    Why did the original Dynaco Engineers set the MK3 bias to 70ma per tube and the ST70 to 50ma per tube?
    We all set the bias a lot lower these days but they must have had a good reason.
    One thing I was thinking it kept both tubes conducting for longer periods as power draw is increased.
    Bob Latino
    Bob Latino
    Admin

    Posts : 3080
    Join date : 2008-11-26
    Location : Massachusetts

    Original MK3 and ST70 Bias settings, Why? Empty Re: Original MK3 and ST70 Bias settings, Why?

    Post by Bob Latino Mon Oct 11, 2021 6:50 pm

    jwb474 wrote:After going thru all of the search results for bias setting, I did not find (or just overlooked) the answer this question:

    Why did the original Dynaco Engineers set the MK3 bias to 70ma per tube and the ST70 to 50ma per tube?
    We all set the bias a lot lower these days but they must have had a good reason.
    One thing I was thinking it kept both tubes conducting for longer periods as power draw is increased.

    Yes - The higher bias point keeps the amp in class A longer as you turn the volume up and then at about 1/3 power the amp will switch over class AB1.  The downside is that a higher bias point is harder on the output tubes and will cause those tubes to have a shorter life span. The higher bias point was maybe not a problem with the Mullard output tubes (made in Great Britain) used in the original ST-70's and the 6550/KT88's used in Mark III's. Modern output tubes made in China, Russia and the Slovak Republic are not made with the same care that those Mullard output tubes were made. They also use cheaper alloys for the metal parts inside the tubes. IMHO modern EL34's, as found in the ST-70, should now be biased at 40 milliamps. Modern KT88's or 6550's should now be biased at 50 milliamps. If you try to bias a modern KT88 or 6550 at 70 milliamps, some of these modern tubes will "red plate".

    Bob
    peterh
    peterh

    Posts : 1589
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Original MK3 and ST70 Bias settings, Why? Empty Re: Original MK3 and ST70 Bias settings, Why?

    Post by peterh Tue Oct 12, 2021 1:18 am

    jwb474 wrote:After going thru all of the search results for bias setting, I did not find (or just overlooked) the answer this question:

    Why did the original Dynaco Engineers set the MK3 bias to 70ma per tube and the ST70 to 50ma per tube?
    We all set the bias a lot lower these days but they must have had a good reason.
    One thing I was thinking it kept both tubes conducting for longer periods as power draw is increased.
    The reason was lowest distortion. I have confirmed this in a MkIII using JJ 6550 and old GE 6550.
    Bob Latino
    Bob Latino
    Admin

    Posts : 3080
    Join date : 2008-11-26
    Location : Massachusetts

    Original MK3 and ST70 Bias settings, Why? Empty Re: Original MK3 and ST70 Bias settings, Why?

    Post by Bob Latino Tue Oct 12, 2021 7:22 am

    peterh wrote:
    The reason was lowest distortion. I have confirmed this in a MkIII using JJ 6550 and old GE 6550.  

    This may be true but you can't really hear the difference between say .30 and .50 THD. Running a modern KT88 or 6550 at 70 milliamps per each output tube will cause a shorter tube life on that tube. The old GE 6550 that you have may be able to take 70 milliamps in stride but the JJ 6550 will have significantly shorter tube life if run at 70 milliamps. Maybe you can get away with this in a Mark III which has only two output tubes but in a four output tube amp like the VTA ST-120, that would be an extra 20 milliamps per each output tube or an extra 80 milliamps total. IMHO > not a good idea ..

    Bob

    yel17 likes this post


    Sponsored content

    Original MK3 and ST70 Bias settings, Why? Empty Re: Original MK3 and ST70 Bias settings, Why?

    Post by Sponsored content


      Current date/time is Tue Oct 19, 2021 7:48 am