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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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misterc57
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    Dynaco Stereo 70 Noise Issue

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    • Post n°1

    Dynaco Stereo 70 Noise Issue Empty Dynaco Stereo 70 Noise Issue

    Post by misterc57 Mon Feb 06, 2023 9:06 am

    I have a ST-70 making awful crackle noises in the left channel only.  All tests done with no source connected.

    I can easily recreate it by touching and slightly moving the 7199 tube.  Seems like the tube socket could be the culprit.  

    Tube feels loose.  I re-tensioned each socket pin.  Still noise.
    I reflowed the 9 pins on the bottom of the board.  Still noise.
    I swapped left and right 7199 tubes.  Still noise in left channel only.

    It can get so bad that just touching the chassis can generate the noise.  

    I have chopstick all components in that area.  Really seems to be a fatigued tube socket.  

    My questions for now are;  

    anything else to check?
    recommended sources for a replacement socket?

    Thank you!  MC

    Dynaco Stereo 70 Noise Issue Img_2010

    Dynaco Stereo 70 Noise Issue Img_2011
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    Post by peterh Mon Feb 06, 2023 10:04 am

    I would do this :
    1/ clean the board , alcohol and a brush could be all that is needed

    2/ reflow the solders on THE SUSPECT SIDE
    See if problem goes away.
    Otherwise get a new board fro dynakitparts https://www.dynakitparts.com/product-category/dynakit-products/pc-boards/
    PC3A is for 7199 tube; you can reuse your 7199 tubes OR
    PC3B is for 6U8 type tube, this will clear the dependency of 7199 tubes and enable a much
    larger supply of tubes ( 6GH8, 6BL8 6U8 ) that will all work fine. I guess dynakitparts can
    supply these too.

    Changning the board is easy, it will fit exactly as the old board.
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    Bob Latino
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    • Post n°3

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    Post by Bob Latino Mon Feb 06, 2023 12:00 pm

    Hi,

    Your amp was a "kit wired" amp. Understand that the Dynaco ST-70 came out in 1959 and many of these amps are now over 60 years old. About the only thing done to your amp is a diode has been put in place of the original selenium rectifier.

    What I would do (if you want to use this amp as your "daily driver") is to completely rebuild the amp. Take everything off.

    1. Sand all the rust off the chassis and paint the chassis. Be careful on the inside of the chassis because some parts have to be grounded to the metal.
    2. Sand and repaint the transformers
    3. Install a new VTA driver board
    4. Install all new tube sockets
    5. New quad cap, other capacitors and new resistors
    6. New stainless steel hardware
    7. Rewire the amp

    You will have in effect an all new and a much more reliable amp.

    Check out how to rebuild to an original Dynaco ST-70 at our forum link below ...

    Rebuild your Dynaco ST-70 videos link

    Bob

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    Post by misterc57 Mon Feb 06, 2023 4:11 pm

    Total rebuild sounds intriguing but this is not my amp.  

    I did a but more testing.  If I hold down the left 7199 at a certain angle there is no noise even with me using the chopstick all over the place.

    So I am pretty convinced it needs a new tube socket.  I am interested in opinions on replacing the entire driver board with a new pre-built unit.  

    I would also like to here opinions on the 7199 vs 6GH8A assembled boards.  Is there a difference in sound between the two versions?

    Why is there a huge price gap between both assembled boards on https://www.dynakitparts.com/product-category/dynakit-products/pc-boards/

    the assembled 7199 board PC-3A ST-70, is $72
    the assembled 6GH8A board PC-3B ST-70, is $120
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    Post by Bob Latino Mon Feb 06, 2023 4:19 pm

    Taking in consideration of the age of the board you now have in there now, tell the owner of the amp that the whole board should be replaced. The VTA driver boards IMHO are the best boards out there. Check the link below for either a parts kit or an assembled board.

    VTA ST-70 parts and boards

    Bob

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    Post by peterh Mon Feb 06, 2023 5:03 pm

    misterc57 wrote:Total rebuild sounds intriguing but this is not my amp.  

    I did a but more testing.  If I hold down the left 7199 at a certain angle there is no noise even with me using the chopstick all over the place.

    So I am pretty convinced it needs a new tube socket.  I am interested in opinions on replacing the entire driver board with a new pre-built unit.  

    I would also like to here opinions on the 7199 vs 6GH8A assembled boards.  Is there a difference in sound between the two versions?

    Why is there a huge price gap between both assembled boards on https://www.dynakitparts.com/product-category/dynakit-products/pc-boards/

    the assembled 7199 board PC-3A ST-70, is $72
    the assembled 6GH8A board PC-3B ST-70, is $120

    I guess it's market talking. Very few aims to keep the 7199 due to the scaricity of these tubes,
    using alternatives that allows plenty of cheap equally good tubes keeps demand higher for
    these boards.

    My advice is to get PC-3B including tubes and sell your 7199 on ebay for a substantional sum.
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    lorne
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    Post by lorne Thu Feb 09, 2023 1:35 pm

    Over the years, I have read that the original board material itself can cause problems. It was alright back in the Cretaceous era, but it may have cooked itself, absorbed moisture and cycled until it is conductive somehow.

    Years ago, I replaced mine with a glass board that retains the original architecture — because I had a supply of NOS 7199's.

    Of course, Bob's suggestion is the best solution. But if that is not an optimal way for you to go — considering that the amp isn't yours — try to source a new stock board. Just my two bits, cuz wiggling tubes and guessing must be pretty frustrating.
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    Post by misterc57 Fri Feb 10, 2023 8:50 am

    I am waiting on the assembled VTA-70 board to arrive! Will let you know how it goes.

    Thank you all.
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    Post by lorne Fri Feb 10, 2023 9:16 am

    Congrats ... you can't go wrong there!

    And guard those 7199 tubes. Your friend can sell them. They are rare birds these days, and someone like me who still runs a stock board will definitely want them.
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    Post by lorne Fri Feb 10, 2023 9:24 am

    I'll be waiting to see the installation Smile
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    Post by misterc57 Mon Feb 13, 2023 11:13 am

    Board is installed and wired.  I am not getting past the bias adjust on the left channel.  Bias should be 0.50 V but is exceeding that value quickly so I power down.

    Instructions say bad wiring or bad tubes.  I know the tubes are good and I think I got the wiring correct.  I have attached pics.  Anyone see anything?

    Thank you, MC

    Dynaco Stereo 70 Noise Issue Img_2014


    Dynaco Stereo 70 Noise Issue Img_2012


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    Post by misterc57 Mon Feb 13, 2023 1:26 pm

    I have no bias VDC at pins 5 and 6 of each output tube.

    With no output tubes installed, I have - 74 VDC at the bias anode on the new board.
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    Post by peterh Mon Feb 13, 2023 1:45 pm

    misterc57 wrote:I have no bias VDC at pins 5 and 6 of each output tube.

    With no output tubes installed, I have - 74 VDC at the bias anode on the new board.

    With no output tubes you should have - 20-50 (adjustable ) on pin 5 och each EL34 socket
    ( pin 6 should not be connected )

    Have you removed the old bias circuit ?
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    Post by misterc57 Mon Feb 13, 2023 2:31 pm

    peterh wrote:
    misterc57 wrote:I have no bias VDC at pins 5 and 6 of each output tube.

    With no output tubes installed, I have - 74 VDC at the bias anode on the new board.

    With no output tubes you should have - 20-50 (adjustable ) on pin 5 och each EL34 socket
    ( pin 6 should not be connected )

    Have you removed the old bias circuit ?

    Old bias circuit is still there but disconnected.  

    The 4 white wires from the new board for the bias is connected at pin 6, where there is a 1K resistor connected to pin 5.

    There is no VDC on any of the white wires.

    I have attached the wiring diagram and a new pic.

    Thank you

    Dynaco Stereo 70 Noise Issue Img_2016

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    Post by peterh Mon Feb 13, 2023 3:13 pm

    Seems to be a problem on the new board. Double check the board, follow the tap from the
    transformer. One problem might be that the caps in the bias circuit is mounted in wrong polarity
    or that the diode is mounted in wrong direction.

    Send roy a mail .
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    Post by Roy Mottram Wed Feb 15, 2023 2:35 pm

    seems this thread has been going in parallel with my personal email correspondence. Thanks to PeterH to have him email Roy @ tubes4hifi
    Sharp lookers will notice that R39 was not installed. My fault. That corrected the bias problem.
    Also the builder did not follow instructions with the resistors on the quad cap. (remove 6.8K and 22K and add 2.2K 3w).
    That's all been taken care of now.

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    Post by misterc57 Wed Feb 15, 2023 4:01 pm

    Roy has been a lot of help! Thank you!

    Almost done. Will post pics on completion.

    MC
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    Post by MechEngVic Fri Feb 17, 2023 6:40 pm

    From what I remember, the 7199 and 6gh8a are the exact same tube spec-wise, the only difference is the pin arraignment.
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    Post by misterc57 Wed Feb 22, 2023 12:04 pm

    Final pics.  Thank you everyone for your help.  Starting another ST70 driver board upgrade this week.

    Dynaco Stereo 70 Noise Issue Img_2018

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