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


    Low output one channel

    dalemurray
    dalemurray

    Posts : 54
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Low output one channel Empty Low output one channel

    Post by dalemurray on Fri Oct 23, 2020 1:32 am

    I had a tube redplate a couple weeks back, I am finally getting around to working on the amp.

    I replace both tubes on the channel that redplated.
    I replaces all four 10ohm resistors on the output tubes.
    I cleaned up the wiring a bit.

    Powered it up and there is definitely something awry.

    Swapped signal source from side to side - issue persists
    Swapped output tubes from side to side - issue persists.
    Swapped center drive tube with one to the left - issue persists.
    I thought it could be C1/C2 - swapped them from side to side - issue persists.

    At this point I am not sure what I should check.

    So far I have limited my measurements to the driver board starting with Pins 2 and 7 of V2.

    I apologize for the size of these images, it is late and I do not have time to resize.
    Low output one channel Table

    Yellow trace is left channel, white trace is right

    Scope01
    V2 Pin2 & Pin7
    Low output one channel Scope01

    Scope02
    V2 Pin1 & Pin6
    Low output one channel Scope02

    Scope03
    This is the signal after C1 & C2
    Low output one channel Scope03

    Scope04
    V1 Pin2 & V3 Pin2
    Low output one channel Scope04

    Scop05
    V1 Pin1 & V3 Pin1
    Low output one channel Scope05

    Scope06
    V1 Pin7 & V3 Pin7
    Low output one channel Scope06

    Scope07
    V1 Pin6 & V3 Pin6
    Low output one channel Scope07

    Scope08
    NFB Left & NFB Right
    Low output one channel Scope08
    peterh
    peterh

    Posts : 1457
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Low output one channel Empty Re: Low output one channel

    Post by peterh on Fri Oct 23, 2020 10:26 am

    A first step could be to disconnect the NFB and repeat. If NFB is removed any change
    in signal voltage indicates where is happens.
    avatar
    nmchiefsfan

    Posts : 62
    Join date : 2012-03-21

    Low output one channel Empty Re: Low output one channel

    Post by nmchiefsfan on Fri Oct 23, 2020 12:53 pm

    Does this board have the CCS? I have the old, high gain board on my amp but it seems like we have seen several posts with this same issue and a number of times it was the CCS.
    dalemurray
    dalemurray

    Posts : 54
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Low output one channel Empty Re: Low output one channel

    Post by dalemurray on Fri Oct 23, 2020 2:22 pm

    peterh wrote:A first step could be to disconnect the NFB and repeat. If NFB is removed any change
    in signal voltage indicates where is happens.

    Removed L&R NFB.
    This did resolve the voltage differences between the drive tubes.
    Volume issue persists.

    nmchiefsfan wrote:Does this board have the CCS?
    I do not know what the CSS is.

    I checked all the voltages per the build manual:


    Power Transformer
    White500VDC
    Red Power 415VAC
    Green Power4.55VDC
    Brown Power 2.785VDC
    Brown/Yellow2.8VDC
    Green/Yellow 4.56VDC
    Red/Black (B+) 55.7VAC


    Quad Cap
    80 493
    40 499
    30 383
    20 493


    Output Tubes
    Pin V2 V3 V6 V7
    1 .513 .519 .511 .520
    2-76.88VAC 6.85VAC 6.87VAC 6.84VAC
    3 491 492 491 492
    4 492 492 491 492
    5 -63.6 -63.2 -58.9 -56.6
    8 .514 .515 .515 .521



    Driver Tube Heater Voltage
    TubePins 4&5
    V16.82
    V26.8
    V36.8
    peterh
    peterh

    Posts : 1457
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Low output one channel Empty Re: Low output one channel

    Post by peterh on Fri Oct 23, 2020 2:51 pm

    The voltages makes no sense.

    What board is this ? Schematics please.
    dalemurray
    dalemurray

    Posts : 54
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Low output one channel Empty Re: Low output one channel

    Post by dalemurray on Fri Oct 23, 2020 2:56 pm

    peterh wrote:The voltages makes no sense.

    What board is this ? Schematics please.

    It is a VTA ST120 kit from Tubes4hifi. Standard stuff.

    What voltages specifically.
    peterh
    peterh

    Posts : 1457
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Low output one channel Empty Re: Low output one channel

    Post by peterh on Fri Oct 23, 2020 4:15 pm

    dalemurray wrote:
    peterh wrote:The voltages makes no sense.

    What board is this ? Schematics please.

    It is a VTA ST120 kit from Tubes4hifi. Standard stuff.

    What voltages specifically.
    I guess you refer to AC voltages.
    So you say that with NFB disconnected the AC voltages are similar between channels ?
    This indicates something in the powerstage, transformer or broken powertubes or possibly
    an open solder

    What is the AC voltages on the powertubes grid ?
    ( if you have a 500volt probe you could measure AC on the powertube plates)

    dalemurray
    dalemurray

    Posts : 54
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Low output one channel Empty Re: Low output one channel

    Post by dalemurray on Fri Oct 23, 2020 4:28 pm

    Removing NFB I removed the disparities noted here (both channels are even):
    Low output one channel Table

    I've checked continuity on all the P to P wiring.

    All the voltages I listed in my recent reply are DC unless otherwise noted.
    avatar
    nmchiefsfan

    Posts : 62
    Join date : 2012-03-21

    Low output one channel Empty Re: Low output one channel

    Post by nmchiefsfan on Fri Oct 23, 2020 5:07 pm

    CCS is constant current source that the newer VTA boards use. It is my understanding that if your VTA board uses 12AU7s then it is the low gain board and has constant current sources to balance the tubes. I have the high gain board so I am just picking this up from reading the forum.
    peterh
    peterh

    Posts : 1457
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Low output one channel Empty Re: Low output one channel

    Post by peterh on Fri Oct 23, 2020 5:18 pm

    What is the AC voltages at the power tubes ??
    And what is the AC voltages at the ot transformer , say 16 ohm tap ?
    dalemurray
    dalemurray

    Posts : 54
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Low output one channel Empty Re: Low output one channel

    Post by dalemurray on Fri Oct 23, 2020 5:31 pm

    I have the classic driver board, not the recent ccs.
    dalemurray
    dalemurray

    Posts : 54
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Low output one channel Empty Re: Low output one channel

    Post by dalemurray on Sat Oct 24, 2020 3:11 pm

    I've worked on it a bit today.
    I swapped the LM334 with each other, no difference.
    I swapped C1&C2 with each other, no difference.
    I swapped C11&C12 with each other, no difference.
    I swapped output transformers with each other, no difference.
    I swapped leads from my signal generator, no difference.

    I suppose I could try swapping L&R 6550 tubes again.

    Signal strength diverges at V2 on the driver board.

    This shows signal at Pins 2 & 7 of V2 - They are effectively identical.
    Low output one channel Pin2and7V2


    This shows signal at Pins 1 & 6 of V2.
    Pin1 = 1.16 volts (left channel)
    Pin2 = 893.7 millivolts (Right channel)

    The kicker is, the left channel is the side with nearly zero output.
    Low output one channel Pin2and7V2

    I have also taken measurements of everything I could think of within reason using my MM, did not do all of these with a scope.

    Excel file can be downloaded here: http://dale-murray.com/ST120/Measurements.xlsx
    PDF of excel file can be downloaded here: http://dale-murray.com/ST120/Measurements.pdf
    dalemurray
    dalemurray

    Posts : 54
    Join date : 2018-09-25
    Location : Wheaton Illinois

    Low output one channel Empty Re: Low output one channel

    Post by dalemurray on Sat Oct 24, 2020 8:51 pm

    I poked and prodded everything with no result.
    I went so far as to touch each solder point with my iron to be sure everything was good, same result.
    I had already hit each solder point on all the octals so I was quite certain those were good too.
    After all the swapping of parts and soldering, none of this made sense. So I went for broke and replaced the female connections on the left channel octals - I still had the ones that came in the power takeoff.

    I replaced female connections 3-6 on V2 & V3 (left channel octals).
    I replaced the wires from the pin6 connections on the driver board.
    Now everything is working correctly.

    I can only imagine one of those female connectors was hogged out and not making a solid connection with the tube itself. I had already checked, rechecked, and checked again, all the solder points.

    Anyway, its up and running !

    10-E-C and MechEngVic like this post

    fredeb
    fredeb

    Posts : 7
    Join date : 2017-04-22

    Low output one channel Empty Re: Low output one channel

    Post by fredeb on Wed Nov 04, 2020 12:59 pm

    Good instinct , that seems like an almost impossible problem to solve , I'm sure you experience posted here will help the rest of us , should we ever encounter a similar problem . Thank you for posting .


    Sponsored content

    Low output one channel Empty Re: Low output one channel

    Post by Sponsored content


      Current date/time is Sat Dec 05, 2020 4:51 am