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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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    Autobias board question

    jsl1234
    jsl1234

    Posts : 68
    Join date : 2017-08-28

    Autobias board question Empty Autobias board question

    Post by jsl1234 Sun Sep 12, 2021 2:43 pm

    I was following the auto-bias board thread(s) of late and even though I have not had any issues with mine, I thought I'd reach out to Pavel and ask a few questions. He responded to my satisfaction and asked if I'd be willing to share the content with the forum members. Below is a edited version of that discussion (edited for clarity..)

    MODERATOR--I hope this is OK, but if not understand. Thanks in advance.

    "Hi Pavel,

    Quick question. I had the auto bias board installed for me by one of Bob's east coast technicians in 2018.

    At that time, I also had him move the blue bias lights onto my driver board so they are visible, along with encasing the unit into a wood frame, add new power switches and speaker posts.  The work is now +2 years old and I'm pretty sure the auto-bias board is 1st Gen (i.e., before the boards had the modifications to include an isolation transformer for the 6.3VAC supply).

    Everything works fine as far as I can tell, however I've never checked the bias since the board was installed (the amp runs fine and sounds great with KT88s).

    If I do have a problem however, how would it manifest itself? Also, I doubt I could undo the board and return the unit to manual bias as Bob recommends, so if it does fail do you have a recommendation with respect to whom I could sent the whole unit to for service (someone with knowledge and experience with the AB board that works with you..)

    Thanks in advance,

    Jay Levine"


    Hi Jay, Thanks for the message.
    If you do not set the BIAS over and over again, nothing will go wrong and the amplifier will continue to work without problems.
    I don't know if you have varistor protection connected in the amplifier, which protects AB autobias in case of short circuit of the KT88 end tube:

    https://www.audioamp.eu/metaloxid-varistor-littelfuse-v8za1p-8v-100a-901899175/

    If there is a problem with autobias in the amplifier, you have two options.

    1) send the board with driver & AB or the whole amplifier to my Holger Schaarschmidt representative
    email: erhardaudio@gmail.com
    www.erhard-audio.com

    2) or me - driver+AB autobias :
    Pavel Kotras
    Velke Popovice- Lojovice 62
    25169
    Prague - East
    Czech Republic

    Send the board assembled together with the signal diodes.
    You don't have to worry about getting repairs. Hundreds of boards have been working reliably around the world for five years.

    Can you please share our interview with others?
    Thank you

    Pavel"
    pavlikkkk
    pavlikkkk

    Posts : 52
    Join date : 2018-03-02
    Age : 74
    Location : Czech Republic

    Autobias board question Empty Re: Autobias board question

    Post by pavlikkkk Sun Sep 12, 2021 3:46 pm

    jsl1234 wrote:I was following the auto-bias board thread(s) of late and even though I have not had any issues with mine, I thought I'd reach out to Pavel and ask a few questions. He responded to my satisfaction and asked if I'd be willing to share the content with the forum members. Below is a edited version of that discussion (edited for clarity..)

    MODERATOR--I hope this is OK, but if not understand. Thanks in advance.

    "Hi Pavel,

    Quick question. I had the auto bias board installed for me by one of Bob's east coast technicians in 2018.

    At that time, I also had him move the blue bias lights onto my driver board so they are visible, along with encasing the unit into a wood frame, add new power switches and speaker posts.  The work is now +2 years old and I'm pretty sure the auto-bias board is 1st Gen (i.e., before the boards had the modifications to include an isolation transformer for the 6.3VAC supply).

    Everything works fine as far as I can tell, however I've never checked the bias since the board was installed (the amp runs fine and sounds great with KT88s).

    If I do have a problem however, how would it manifest itself? Also, I doubt I could undo the board and return the unit to manual bias as Bob recommends, so if it does fail do you have a recommendation with respect to whom I could sent the whole unit to for service (someone with knowledge and experience with the AB board that works with you..)

    Thanks in advance,

    Jay Levine"


    Hi Jay, Thanks for the message.
    If you do not set the BIAS over and over again, nothing will go wrong and the amplifier will continue to work without problems.
    I don't know if you have varistor protection connected in the amplifier, which protects AB autobias in case of short circuit of the KT88 end tube:

    https://www.audioamp.eu/metaloxid-varistor-littelfuse-v8za1p-8v-100a-901899175/

    If there is a problem with autobias in the amplifier, you have two options.

    1) send the board with driver & AB or the whole amplifier to my Holger Schaarschmidt representative
    email: erhardaudio@gmail.com
    www.erhard-audio.com

    2) or me - driver+AB autobias :
    Pavel Kotras
    Velke Popovice- Lojovice 62
    25169
    Prague - East
    Czech Republic

    Send the board assembled together with the signal diodes.
    You don't have to worry about getting repairs. Hundreds of boards have been working reliably around the world for five years.

    Can you please share our interview with others?
    Thank you

    Pavel"

    Of course, varistor protection is excellent not only for the protection of autobias boards, but also in general for all components connected in the end tube circuits. Together with the quick-release fuse, it also protects the OT from overload in the event of a tube short circuit, which often leads to the destruction of the OT.
    The connection is simple (you wrote it in the link). The varistor is connected to the cathode pin against the ground as close as possible to the tube. If an autobias module is used, the cathode is connected to the PIN CATH on the AB via a fuse (ideally a fast 200 mA).
    If a cathode resistor is used (not an AB module) then a fuse is connected to ground through this cathode resistor. Attention! if in this case the resistance value of the cathode resistor is low (less than 5 Ohm) it must be taken into account when calculating the total resistance in the cathode of the tube.
    The resistance value of the fuse is about 1.5 to 3 Ohms depending on the type.

    Pavel

      Current date/time is Thu Sep 16, 2021 7:55 pm