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    ST-120 start up!

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    Maintarget

    Posts : 209
    Join date : 2013-02-10

    ST-120 start up!

    Post by Maintarget on Wed Sep 18, 2013 8:09 pm

    I just finished my ST-120 amp and followed the start up procedure, Made sure 5 amp slo-blo fuse in place, install three #5814a driver tubes, power up and all light up
    I then went to the next step Shut off power and installed #5AR4 Rectifier tube and two KT-88 output tubes on left side only, when I attempt to adjust bias on the front left output tube it raises in value initially but then gradually reduces back to about 1.9 volts no matter were I adjust the pot, I tried turning the pot up all the way (cw) to see if that would stabilize the bias and noticed the tube started to emit a lot of heat and noticed the tube was starting to get much brighter/glowing so I shut off and checked the wiring but found no issues.
    I'm not sure what to do at this point.
    Any help would be greatly appreciated.

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 start up!

    Post by Bob Latino on Wed Sep 18, 2013 9:12 pm

    First test the bias system to make sure the bias system is working. Center the four bias pots. Pull ALL the tubes out and turn the amp on. Measure the negative DC voltage on both pins 5 and 6 on all output tubes. See if you get about -45 to -60 VDC on all output tubes. If you do, then the bias sytem is working OK.

    If the bias system passes the test above - then try that channel test again with the OTHER TWO output tubes. It could be just a bad tube ?

    Bob

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: ST-120 start up!

    Post by Maintarget on Wed Sep 18, 2013 10:05 pm

    All four sets of bias pins 5 & 6 measure -57.16 vdc to -57.96 vdc tried other two output tubes and measure 1.9 volts

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: ST-120 start up!

    Post by Sal on Wed Sep 18, 2013 10:24 pm

    Can you take a clear photo of the under chassis wiring so we can see if we can see if we notice something wrong.

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: ST-120 start up!

    Post by Maintarget on Wed Sep 18, 2013 10:46 pm

    http://s1325.photobucket.com/user/RoscoeAJones/media/photo_zpsfa051e3e.jpg.html?filters[user]=135085427&filters[recent]=1&sort=1&o=0

    Sorry i had to refresh my memory on how to attach a photo I think this will work.

    ruffian

    Posts : 43
    Join date : 2010-02-26

    Re: ST-120 start up!

    Post by ruffian on Wed Sep 18, 2013 11:14 pm

    I must compliment you on your work, that is one very neat and orderly job.  I'm not the guy to see a fault but if there is I'm sure it will be easy to find and fix.

    Bob Latino
    Admin

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    Re: ST-120 start up!

    Post by Bob Latino on Thu Sep 19, 2013 9:00 am

    Maintarget wrote:All four sets of bias pins 5 & 6 measure -57.16 vdc to -57.96 vdc tried other two output tubes and measure 1.9 volts
    Hi,

    Your did an excellent job on your wiring ..

    Check the value of R29, R30, R31 and R32. These are four bias circuit resistors on the driver board just outboard of each bias pot. You can't really get at them from the TOP of the amp because they are under the lip of the chassis. You CAN get at them from the bottom of the chassis. You may have to bend each Russian PIO a little to see the solder terminals for these four resistors. They should all measure about 150K (150,000 ohms). See if they do ..

    Bob

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: ST-120 start up!

    Post by Maintarget on Thu Sep 19, 2013 8:58 pm

    R29=150.1
    R30=149.7
    R31=150.3
    R32=150.2
    I'm guessing that is well within the tolerance.
    Next

    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    Re: ST-120 start up!

    Post by Analog Man on Thu Sep 19, 2013 9:07 pm

    Maintarget wrote:I just finished my ST-120 amp and followed the start up procedure, Made sure 5 amp slo-blo fuse in place, install three #5814a driver tubes, power up and all light up
    I then went to the next step Shut off power and installed #5AR4 Rectifier tube and two KT-88 output tubes on left side only, when I attempt to adjust bias on the front left output tube it raises in value initially but then gradually reduces back to about 1.9 volts no matter were I adjust the pot, I tried turning the pot up all the way (cw) to see if that would stabilize the bias and noticed the tube started to emit a lot of heat and noticed the tube was starting to get much brighter/glowing so I shut off and checked the wiring but found no issues.
    I'm not sure what to do at this point.
    Any help would be greatly appreciated.
    Re check everything! Especially solder connections. Pull and wiggle on the wires and also do continuity checks on all connections. Use a magnifying glass on all of the driver board connections, look for solder blobs and reduce the size of them with solder wick which you can get from Radio/Cell phone shack.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 start up!

    Post by Bob Latino on Thu Sep 19, 2013 9:18 pm

    Maintarget wrote:R29=150.1
    R30=149.7
    R31=150.3
    R32=150.2
    I'm guessing that is well within the tolerance.
    Next
    The bias system is fine .. Have you tried the other channel with different output tubes ? See if the OTHER channel will bias up alone with two different output tubes ..

    Bob

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: ST-120 start up!

    Post by Maintarget on Thu Sep 19, 2013 9:30 pm

    Analog Man
    I traced all the wiring out and verified connection however I did not re flow any connections or check continuity.
    I actually checked my solder connection as I progressed with the build with a magnifying glass.
    I will check continuity and see what I find
    Thanks

    Bob
    I did try switching tubes and no difference still "One point nine" volts not the optimal bias setting of "Point five fifty hundred" Volts
    Next:shock: 
    Thanks


    Last edited by Maintarget on Thu Sep 19, 2013 9:31 pm; edited 1 time in total (Reason for editing : added verbage)

    Bob Latino
    Admin

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    Join date : 2008-11-26
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    Re: ST-120 start up!

    Post by Bob Latino on Thu Sep 19, 2013 9:47 pm

    Are you saying that ALL the bias pots won't go below 1.9 VDC bias voltage with ANY output tube?

    Bob

    Maintarget

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    Join date : 2013-02-10

    Re: ST-120 start up!

    Post by Maintarget on Thu Sep 19, 2013 9:48 pm

    Yes sir.

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: ST-120 start up!

    Post by anbitet66 on Thu Sep 19, 2013 10:06 pm

    Maintarget,

    This may be a stretch, but can you verify your meter is accurate?  Test line voltage for AC, and maybe try testing a 9V battery to test the DC function to see if the meter is correct.  I did have one that was wildly off years ago.  Gotta remember not to buy a mulitmeter from a guy in a trench coat in a dark alley...tongue 

    Tony

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: ST-120 start up!

    Post by Maintarget on Thu Sep 19, 2013 10:59 pm

    I'm using a Fluke 87 I think I may have had the meter on the wrong current AC No , I gave it another try (On DC) and was able to bias the tubes fairly close to .550 VDC and while listening to my first song heard what sounded like a dud fire cracker that kind of fizzles out and saw sparks/bright glow from the rectifier tube, I shut off immediately but not quick enough blew the fuse.
    Replaced fuse checked bias again and found all low while attempting to re bias same issue in the rectifier tube.
    I'm guessing I may have smoked the rectifier tube from my own ignorance, any thoughts or suggestions?
    Signed feeling really dumb.

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: ST-120 start up!

    Post by anbitet66 on Thu Sep 19, 2013 11:15 pm

    Now would be the time to buy some rectifiers.  I like the Sino brand from Amplified Parts.  They are cheap enough that if one dies it isn't a big deal.  Or you can go with a copper cap.  Good luck with the amp.  Tony

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 start up!

    Post by sKiZo on Thu Sep 19, 2013 11:24 pm

    I'm taking notes here ... just in case I ever get around to finishing mine ...




    skriefal

    Posts : 118
    Join date : 2011-09-20
    Location : Utah, USA

    Re: ST-120 start up!

    Post by skriefal on Fri Sep 20, 2013 12:01 am

    sKiZo wrote:I'm taking notes here ... just in case I ever get around to finishing mine ...

    ... need to get that ST-120 up and running, and enjoy some sweet tunes. Smile 

    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    Re: ST-120 start up!

    Post by Analog Man on Fri Sep 20, 2013 7:31 am

    skriefal wrote:
    sKiZo wrote:I'm taking notes here ... just in case I ever get around to finishing mine ...

    ... need to get that ST-120 up and running, and enjoy some sweet tunes. Smile 
    Also check for wiring shorts caused by a inadvertent bump of the soldering iron. Make sure no wires are melted together on the insulation. I had a couple of these but I caught them before power up.

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 start up!

    Post by Bob Latino on Fri Sep 20, 2013 9:28 am

    Either an output tube took out the rectifier (less likely) or for some reason the rectifier died. (more likely) You live in a large city. Go to a "Guitar store" in your city and pick up another GZ34/5AR4 tube rectifier and replace the original rectifier. In fact get TWO rectifier tubes and keep an extra rectifier handy ... The rectifier tube in these amps is the tube most likely to go bad ..

    I probably should have thought to ask whether you were using the correct scale on your meter to measure bias. In step 3 under "Initial Startup" on page 11 of the VTA ST-120 manual it does say "Get your multitester and set the range for 0 to 2 volts DC."

    Bob


    Last edited by Bob Latino on Fri Sep 20, 2013 1:21 pm; edited 1 time in total

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 start up!

    Post by arledgsc on Fri Sep 20, 2013 10:30 am

    "... sparks/bright glow from the rectifier tube, I shut off immediately but not quick enough blew the fuse."
    Ahhh.. birthing a new amp brings excitement and drama sometimes.  My rectifier blew on first power up so welcome to the club.  The rectifiers are worked hard in this amp so only the strong will survive.  And nice job on wiring!

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: ST-120 start up!

    Post by Maintarget on Fri Sep 20, 2013 12:55 pm

    Thank you all for your help, it was a rookie mistake I'm excited to get the tunes playing!

    ArlanB

    Posts : 52
    Join date : 2011-01-23
    Age : 69
    Location : Santa Cruz, California

    ST-120 start up!

    Post by ArlanB on Fri Sep 20, 2013 6:26 pm

    Maintarget,

    You will enjoy the fruits of your labors.  I have one of the first ST-120's off the line from Bob.  I purchased it over 5 years ago in the standard kit form without the mods.  It sounds so good just as it is, I have been chicken to upgrade to the mods that are offered.   The old adage "leave well enough alone" especially now that the electronics have been working well as a team for this long, I hate to change the lineup.  My amp still has all its original parts that were supplied by Bob and worked from frist smoke test which speaks well of his design and his choice of parts to supply with the kits.  The only thing that routinely fails on the amp are as you are finding out are the tubes.  We all go through the "smoke test blues" at one time or another.  You will get it right and when you do the satisfaction is a huge boost to your feeling of accomplishment.  Many years of enjoyment are ahead.

    Good Luck,
    Arlanb

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 start up!

    Post by Bob Latino on Fri Sep 20, 2013 6:38 pm

    The sad thing is that although the resistors, capacitors, tube sockets, PC boards, chassis etc. are a lot better made than those of the 1950's and 1960's, the tubes of today are NOT as well made. Years ago all the tubes were made in the USA, Germany or Great Britain and were very well made with high quality materials and stringent quality control. This why NOS tubes of yesteryear go for such high prices on Ebay. Today's tubes are made in China, Russia and the Slovak Republic. They tend (for the most part) to use lower quality materials inside the tubes and their quality control is not as good also. Many crooked or sloppily made tubes seem to made it out the door of these tube factories in an effort to maximize profit. Bad and/or short lived tubes are something that tube amp users of today will have to learn to live with.

    Bob

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: ST-120 start up!

    Post by Maintarget on Fri Sep 20, 2013 7:24 pm

    When I decided to go with a tube amp in kit form I was fully ready raise my level of knowledge about them and to face some challenges (Even if they are self inflicted LOL)
    I have a line on some JJ Gz34 tubes for $17.00 a piece locally and should be back in business tomorrow after work.
    Thanks again to all for the kind words and encouragement!

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