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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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    ST70 bias is .56 instead of 1.56

    CAEMI
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    CAEMI


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    Join date : 2023-08-15
    • Post n°1

    ST70 bias is .56 instead of 1.56 Empty ST70 bias is .56 instead of 1.56

    Post by CAEMI Wed Aug 16, 2023 9:08 am

    I have owned an ST70 for many years, inherited from my brother in law who built it in the early 60's (I think).

    I know nothing about electronics, nothing about tube amps except I'm a guitarist (ex-pro) and have never owned or used anything but tube amps. I've never really had one fail me so I don't have much experience with technicians, nor do I know one that I can turn to. I do own several tube amps that get used only in my home.

    I have used it periodically and sometimes used the Dynaco extensively. Recently, I decided to put it back in the stereo system and check the bias. I found it measured .56 on each side.

    The chassis says the bias should be 1.56.

    Can someone please advise me on this? I don't want to use the amp if it could be harmed and will have it looked at by a tech if need be.

    Thank you,

    Craig
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°2

    ST70 bias is .56 instead of 1.56 Empty Re: ST70 bias is .56 instead of 1.56

    Post by Bob Latino Wed Aug 16, 2023 5:48 pm

    CAEMI wrote:I have owned an ST70 for many years, inherited from my brother in law who built it in the early 60's (I think).

    I know nothing about electronics, nothing about tube amps except I'm a guitarist (ex-pro) and have never owned or used anything but tube amps. I've never really had one fail me so I don't have much experience with technicians, nor do I know one that I can turn to. I do own several tube amps that get used only in my home.

    I have used it periodically and sometimes used the Dynaco extensively. Recently, I decided to put it back in the stereo system and check the bias. I found it measured .56 on each side.

    The chassis says the bias should be 1.56.

    Can someone please advise me on this? I don't want to use the amp if it could be harmed and will have it looked at by a tech if need be.

    Thank you,

    Craig

    Yes - The original Dynaco ST-70 came out in 1959 and used a bias point of 1.56 volts for each channel. They used one 15.6 bias resistor for the two tubes on each channel. This gave a bias current of 100 milliamps for the two tubes for that channel OR 50 milliamps for each of the two tubes. This bias voltage for each tube is a little high by today's standards. Dynaco sent very durable Mullard output tubes with each kit and these Mullard tubes took the higher bias point in stride. Back then tubes were made in the USA, England and Germany. Now all tubes are made in China, Russia or the Slovak Republic. These modern tubes are not made to the same standards as the older tubes and the bias now should probably be dropped to about 40 milliamps per each output tube. A bias setting of 1.25 volts on the older ST-70's will give about 40 milliamps of current for each output tube and is probably a better idea when using modern output tubes with an original Dynaco ST-70. You won't hear any difference with the sound and your output tubes will last longer.

    Bob

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    CAEMI
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    • Post n°3

    ST70 bias is .56 instead of 1.56 Empty Re: ST70 bias is .56 instead of 1.56

    Post by CAEMI Wed Aug 16, 2023 10:22 pm

    Bob,

    Thank you for the thorough explanation. Does the .56 reading that I'm seeing (I tried adjusting the bias up to 1.56 but it would not make it past .900) mean that my output tubes are shot? The amp still sounds pretty good to my aged ears.
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    CAEMI
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    • Post n°4

    ST70 bias is .56 instead of 1.56 Empty Re: ST70 bias is .56 instead of 1.56

    Post by CAEMI Wed Aug 16, 2023 10:24 pm

    Also, if me tubes are kaput, what brand would you recommend to replace them? The amp still has the original tubes. I have a retired man's budget...I'm not cheap but neither am I flush.
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    peterh
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    • Post n°5

    ST70 bias is .56 instead of 1.56 Empty Re: ST70 bias is .56 instead of 1.56

    Post by peterh Thu Aug 17, 2023 1:48 am

    CAEMI wrote:Bob,

    Thank you for the thorough explanation. Does the .56 reading that I'm seeing (I tried adjusting the bias up to 1.56 but it would not make it past .900) mean that my output tubes are shot? The amp still sounds pretty good to my aged ears.

    Do diagnose the issue measure the B+ .
    The first cap after the rectifier , if it's 435 (or higer) your EL34 are gone.
    If the voltage at this point are significatly lower either the can cap or rectifier needs replacement.

    As for EL34 tubes, a quad of JJ EL34 from eurotubes.com would fit fine but before installing
    new tubes turn the bias adjustment fully down ( anti-clockwise )

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    Bob Latino
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    • Post n°6

    ST70 bias is .56 instead of 1.56 Empty Re: ST70 bias is .56 instead of 1.56

    Post by Bob Latino Thu Aug 17, 2023 7:36 am

    CAEMI wrote:Bob,

    Thank you for the thorough explanation. Does the .56 reading that I'm seeing (I tried adjusting the bias up to 1.56 but it would not make it past .900) mean that my output tubes are shot? The amp still sounds pretty good to my aged ears.

    I would say to post a photo of the inside of your amp here but you have to be a member for a week to post photos.

    1. If your amp is an original Dynaco ST-70, you should replace the original selenium rectifier with a diode. A diode is inexpensive and much more reliable. The common 1N4007 diode will work fine here. Read more on how to do this at the forum link below.

    replace the selenium rectifier

    2. You should also replace the quad cap if your amp still has the original Dynaco quad cap. After time (40 - 60 years) most of these original quad caps will slowly go bad. The one from Dynakitparts at the link below works fine. The original Dynaco quad cap was a 30, 20, 20, 20 cap. The one that Dynakitparts carries is an 80, 40, 30, 20. When you install this cap have the 80 section face the rear of the amp. This is the best way to install this cap on an original Dynaco ST-70.

    Bob

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    Buck_R0gers
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    • Post n°7

    ST70 bias is .56 instead of 1.56 Empty Re: ST70 bias is .56 instead of 1.56

    Post by Buck_R0gers Thu Aug 17, 2023 8:09 am

    Check for the low ohm resistor on pin #8 of the output tubes, There is only one per side.

    Some people change it to a 10 ohm which will change your bias voltage down.

    Measure the resistance from the bias test point to chassis ground to confirm size.

    But, You need a digital or accurate meter to test low ohms.

    Also, When tubes get tired they will draw less current, Hence the adjustment.

    Nothing is damaged and it would be hard to hear less performance from a lower bias point.

    Good luck.

    Buck

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    • Post n°8

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