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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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    Hum/Hiss/Static out of Left Channel (Speaker)

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    Chipster1969


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    Post by Chipster1969 Sun Mar 28, 2021 9:52 am

    Again, I appreciate the warm welcome on this site.  So a little about my situation- I have an original Dynaco ST-70 and PAS 3X.  Both are in great shape with the typical maintenance/replacement of 2 non-Mullard EL34s (Sovtek and EH) and NOS RCA (Japanese) GZ34 on the ST and replacement of the 12X4 on the PAS.  I have well over several hundred hours on these two components over 10+ years.  I have seen previous posts on trying to remedy via swapping tubes and have tried that without avail.  Thinking about replacing the resistors from peterh post, but was hoping it was as easy as replacing all original 12ax7 tubes. It sounds as if these rarely fail. I note on the PAS manual it could be isolated in the volume section (PC5) section.  Although I have soldering irons, I am concerned about my own ability to do this without cold soldering. Thanks, Chip
    peterh
    peterh


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    Post by peterh Sun Mar 28, 2021 10:09 am

    Chipster1969 wrote:Again, I appreciate the warm welcome on this site.  So a little about my situation- I have an original Dynaco ST-70 and PAS 3X.  Both are in great shape with the typical maintenance/replacement of 2 non-Mullard EL34s (Sovtek and EH) and NOS RCA (Japanese) GZ34 on the ST and replacement of the 12X4 on the PAS.  I have well over several hundred hours on these two components over 10+ years.  I have seen previous posts on trying to remedy via swapping tubes and have tried that without avail.  Thinking about replacing the resistors from peterh post, but was hoping it was as easy as replacing all original 12ax7 tubes. It sounds as if these rarely fail. I note on the PAS manual it could be isolated in the volume section (PC5) section.  Although I have soldering irons, I am concerned about my own ability to do this without cold soldering. Thanks, Chip


    Try to specify the problem. Is it hum ( low freq tone ) or hiss ( like rain ) ?
    Is it only in the st70 ( pas 3 powered off ) or does it come when the pas is active ?
    If it comes with the PAS, is it on all inputs or only on phono ?

    And finally , what resistors ?
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    Chipster1969


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    Post by Chipster1969 Sun Mar 28, 2021 10:27 am

    So there has always been a slight hum from the ST70 without the PAS hooked up and running directly to my CD player. With the PAS hooked up, I get the additional hiss (almost like radio station distortion)and occasional pops. It is present on all inputs and is more pronounced on initial powering up and when increasing volume (gain)? As far as the resistors I am referring to stevethemaestro post from Jul 2020 post- you had recommended replacing two plate resistors on the line board. I appreciate the quick response.
    peterh
    peterh


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    Post by peterh Sun Mar 28, 2021 10:37 am

    Chipster1969 wrote:So there has always been a slight hum from the ST70 without the PAS hooked up and running directly to my CD player.  With the PAS hooked up, I get the additional hiss (almost like radio station distortion)and occasional pops. It is present on all inputs and is more pronounced on initial powering up and when increasing volume (gain)? As far as the resistors I am referring to stevethemaestro post from Jul 2020 post- you had recommended replacing two plate resistors on the line board.  I appreciate the quick response.
    To start with the hiss , yes the plate resistors ( 470k and less probable 100k) on the line board is a good guess.

    As for the hum in the st70, is it present if a pair of shorting plugs are inserted instead of the PAS ?. If unequal right / left, will it move if the 7199 is swapped between the channels ?

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    Chipster1969


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    Post by Chipster1969 Sun Mar 28, 2021 10:45 am

    So with the plate resistors I (or someone) will need to be re-soldered. With that being said, what is a good video to watch on soldering as I have only attempted once.

    I will definitely try the "shorting plugs"; not familiar with them. I will try swapping the 7199s. Thanks peterh
    peterh
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    Post by peterh Sun Mar 28, 2021 11:21 am

    Chipster1969 wrote:So with the plate resistors I (or someone) will need to be re-soldered. With that being said, what is a good video to watch on soldering as I have only attempted once.

    I will definitely try the "shorting plugs"; not familiar with them.  I will try swapping the 7199s. Thanks peterh

    The plate resistors should be replaced ! Use 1/2 w metal or carbon film.
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    Chipster1969


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    Post by Chipster1969 Sun Mar 28, 2021 11:40 am

    Will do- is there a recommended video on soldering for dummies? lol
    peterh
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    Post by peterh Sun Mar 28, 2021 12:57 pm

    Chipster1969 wrote:Will do- is there a recommended video on soldering for dummies? lol

    Try google.  But a very importent detail : use leaded solder ( 60/40 ) or (63/37)

    The old resistors are easises removed by cutting both sides and then gently picking the
    leads out while heating them with the solder iron. Then you need to open the hole with either
    a de-soldering pump or a cupper strip.
    See https://youtu.be/pA6wFV9NLSw  for the usage of a de-soldering pump

    Hmm. this is better : https://youtu.be/kd9FJgZE77I

    Note that keeping the board steady is important , and sucking fast before the solder solidifies.
    For professional work one can use a iron with a hole in the middle connected to a vacuum pump, that begins to be expensive.

    Solder wick is an alternative, it will however not clean withing the hole as well as a pump.



    Last edited by peterh on Sun Mar 28, 2021 1:10 pm; edited 1 time in total (Reason for editing : Added video)
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    Chipster1969


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    Post by Chipster1969 Sun Mar 28, 2021 1:05 pm

    Thank you! Wish me luck
    peterh
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    Post by peterh Sun Mar 28, 2021 3:33 pm

    Chipster1969 wrote:Thank you! Wish me luck
    To add an idea;
    on these types of boards i usually cut the component, heat the solderpad in order to gently remove the component leg and then heat again and use the desolder pump to clean the hole in the board. Best is to such from the component side, that way one can suck at the same time as the iron heats. It also in my opinion is gentler to the board to such thus way.

    These original boards are fragile. (Modern "through the hole plated" fiberglass boards can stand more abuse.) But even if the copper islands loosens one can rebuild the path with pieces of wire soldered on the component leg and the remaining copper.

    Don't use a solder iron that is heated to cope with lead-less solder, they will easily destroy the glue between board and copper.

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    Chipster1969


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    Post by Chipster1969 Wed Jul 28, 2021 2:34 pm

    I appreciate answering my previous question "hiss/hum" on my PAS-3X several months ago.  The situation has been resolved and the sound is fantastic!
    My new question is whether I need to re-set the bias for a "paired quad" set of new Electro Harmonix EL34s for my ST70. Thank you, Chip


    Last edited by Chipster1969 on Wed Jul 28, 2021 2:37 pm; edited 1 time in total (Reason for editing : mistake)
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    Post by peterh Wed Jul 28, 2021 3:39 pm

    Chipster1969 wrote:I appreciate answering my previous question "hiss/hum" on my PAS-3X several months ago.  The situation has been resolved and the sound is fantastic!
    My new question is whether I need to re-set the bias for a "paired quad" set of new Electro Harmonix EL34s for my ST70. Thank you, Chip
    After changing power tubes bias should always be checked / adjusted. Even better is to
    turn bias adjustment pot at the end that has lowest current when new tubes are installed ( highest negative grid voltage), that way there is no risk of current run-away before bias is adjusted.
    Or did a misunderstand the question ?
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    Chipster1969


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    Post by Chipster1969 Thu Jul 29, 2021 11:17 am

    So is there a step by step process to this? What direction am I turning the screws- clockwise or counterclockwise? Thanks
    peterh
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    Post by peterh Thu Jul 29, 2021 1:21 pm

    Chipster1969 wrote:So is there a step by step process to this?  What direction am I turning the screws- clockwise or counterclockwise? Thanks
    start the amp with current tubes, attatch voltmeter at the bias adjustment taps and turn
    the bias adjustment pot. The direction that reduces the voltage is correct.
    Then change tubes, power on and after several minutes start adjusting bias as per the manual.
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    Chipster1969


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    Post by Chipster1969 Thu Jul 29, 2021 2:41 pm

    Thank you again peterh!
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    Chipster1969


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    Post by Chipster1969 Fri Jul 30, 2021 7:28 am

    One final question peterh- does the amp need to be hooked up to the speakers and pre-amp for me to do that, or can I simply plug the unit into the wall and power it up?
    peterh
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    Post by peterh Fri Jul 30, 2021 8:10 am

    Chipster1969 wrote:One final question peterh- does the amp need to be hooked up to the speakers and pre-amp for me to do that, or can I simply plug the unit into the wall and power it up?
    ALWAYS have a load connected to the speaker outputs !! It can be your speakers or it could be a suitable power resistor ( 8 ohm > 10w )
    The inputs might be open or connected, accurate bias adjustment however needs a "no signal" .
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    Chipster1969


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    Post by Chipster1969 Fri Jul 30, 2021 8:28 am

    Thank you.

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