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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


5 posters

    Driver tube replacement

    Sfguitarworks
    Sfguitarworks


    Posts : 16
    Join date : 2018-10-31

    Driver tube replacement Empty Driver tube replacement

    Post by Sfguitarworks Tue Jul 27, 2021 6:33 pm

    Hello,

    I rewired my driver board to use the 6GH8A tubes. It was fine, but one became microphonic. I ordered 2, and one of the new ones was microphonic. I've read that they are just kind of crappy tubes. Is there a way to rewire for a more commonly used tube, like a 12au7, to avoid the mega expensive 7199's? I know I can just upgrade the board, but it sounds fantastic as is. Just looking for a DIY solution.

    Thanks!

    Geoff
    peterh
    peterh


    Posts : 1599
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Driver tube replacement Empty Re: Driver tube replacement

    Post by peterh Wed Jul 28, 2021 1:03 am

    Sfguitarworks wrote:Hello,

    I rewired my driver board to use the 6GH8A tubes. It was fine, but one became microphonic. I ordered 2, and one of the new ones was microphonic. I've read that they are just kind of crappy tubes. Is there a way to rewire for a more commonly used tube, like a 12au7, to avoid the mega expensive 7199's? I know I can just upgrade the board, but it sounds fantastic as is. Just looking for a DIY solution.

    Thanks!

    Geoff
    If the board is wired for 6gh8 then you can use any of :
    6U8 ECF80 ECF82 6BL8 and my favorite GB1252 ( a 6U8 for vehicle use)
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1713
    Join date : 2008-11-30

    Driver tube replacement Empty Re: Driver tube replacement

    Post by tubes4hifi Thu Aug 05, 2021 7:14 pm

    not the greatest tube, a simply mod and you can use modern 12AU7s.
    MOST 6GH8 tubes will require some minor changes in the surrounding component values to compensate.
    There are dozens of articles posted about this on the internet, use google to find some,
    It's pretty amazing what you can learn if you just look.
    Sfguitarworks
    Sfguitarworks


    Posts : 16
    Join date : 2018-10-31

    Driver tube replacement Empty Re: Driver tube replacement

    Post by Sfguitarworks Thu Aug 05, 2021 8:02 pm

    I am familiar with internet searches, but was unable to find information directly pertaining to my issue. Would you happen to have a link to one or two of the articles, as I have been unable to find one? Maybe you could briefly explain the simple mod to use a 12au7, or ax7, in place of my 6gh8a? Thanks! Geoff
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1713
    Join date : 2008-11-30

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    Post by tubes4hifi Thu Aug 05, 2021 8:07 pm

    first question, what amp are you asking about??
    Sfguitarworks
    Sfguitarworks


    Posts : 16
    Join date : 2018-10-31

    Driver tube replacement Empty Re: Driver tube replacement

    Post by Sfguitarworks Thu Aug 05, 2021 8:25 pm

    Stock ST70, except for rewiring the driver sockets for the 6GH8 tubes.
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1713
    Join date : 2008-11-30

    Driver tube replacement Empty Re: Driver tube replacement

    Post by tubes4hifi Thu Aug 05, 2021 9:04 pm

    OK, another 5 second google search and you get this page
    https://www.google.com/search?client=firefox-b-1-d&q=using+6GH8+tube+in+ST-70
    you can buy adaptors on ebay . . . https://www.ebay.com/itm/193480210651?
    you can watch this video on YouTube
    you will find this thread on this forum . . . https://dynacotubeaudio.forumotion.com/t1822-stock-dynaco-st-70-using-7199-6gh8a-and-6u8-tubes-video
    you can find this post on AudioKarma . . . . https://audiokarma.org/forums/index.php?threads/dynaco-st-70-pc-board-modification-7199-to-6gh8a-how-to.723611/
    you can buy a new replacement board to use 6GH8 on ebay . . . . https://www.ebay.com/itm/372633540485?hash=item56c2b09785:g:JMYAAOSwVYZflcPi
    the very best solution, you can buy a new VTA70 replacement board from me at Tubes4HiFi.com/ST70.htm which uses two 12AU7 tubes and sounds 10X better
    google searches are really very simple . . . .
    I used to sell a 6GH8 board, but they only save you about $20 compared to my very best VTA70 board, so why bother?
    I do have a PDF file of my older board with a 6GH8 if you want . . .
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1713
    Join date : 2008-11-30

    Driver tube replacement Empty Re: Driver tube replacement

    Post by tubes4hifi Thu Aug 05, 2021 9:23 pm

    change the original 0.05uF cap to a 0.22uF cap.
    BTW, the Dynaco ST70 series 2 amp used the same circuit with the 6GH8, so if you look at that schematic
    (certainly you can find it on google if you try) you'll see it's near identical, other than that one cap being 0.68uF, but 0.22 works fine.
    If you've got microphonic tubes, just get some others. All those tubes are 60 years old. I've got six brand new ones you can have for $5 each.
    Haven't come across a microphonic one yet. Do you have your NFB connected? (the negative feedback from the speaker outputs).
    Sfguitarworks
    Sfguitarworks


    Posts : 16
    Join date : 2018-10-31

    Driver tube replacement Empty Re: Driver tube replacement

    Post by Sfguitarworks Thu Aug 05, 2021 10:12 pm

    Google searches are very simple. Thanks for reminding me. If I was searching for how to use a 6GH8 in my ST70, I would have found that information easily and on my own. It seems you didn't understand my post. I have already done the mod to use the 6GH8 tube. I asked if anyone knew of an easy way to rewire the sockets to use 12au7 in place of that tube. Looking back, the original post was very clear. I'll just keep enjoying my amp the way it is unless someone chimes in with some applicable information. Thanks!
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1713
    Join date : 2008-11-30

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    Post by tubes4hifi Fri Aug 06, 2021 12:02 am

    changing from a 6GH8 to a 12AU7 on a stock PCB would be a nightmare of cuts and jumpers and changing almost every single component.
    EZer to just pop a new board in.
    Jbacik
    Jbacik


    Posts : 18
    Join date : 2012-07-14
    Location : N.C. USA

    Driver tube replacement Empty 6GH8A tubes.

    Post by Jbacik Sun Aug 08, 2021 3:24 pm

    The 6GH8A tube was a find I did many years ago that I made and sold adapter's for. The 6GH8A tubes that I found made the ST70 sound brilliant! I purchased about 450 of these tubes that were nos in boxes of 100 made in Japan in the mid 80's. These were the last run made on these tubes that I know of. I still have a couple of hundred of these and have not found a bad one yet. No microphonic's or noise what so ever. Though the 6U8 was a closer match to the 7199, the 6GH8A has always been my favorite. No mods are necessary to use these 6GH8A's. If interested, send me a post and I will make them available. Many of my previous customers have reached out to me for extra of the tubes that I have. I also use these in my MK III's with adapters that I made for them. These are available now on fleabay for less than I can make them for. Regards, Jbacik.


    Last edited by Jbacik on Sun Aug 08, 2021 3:26 pm; edited 1 time in total (Reason for editing : Spelling)
    pavlikkkk
    pavlikkkk


    Posts : 55
    Join date : 2018-03-02
    Age : 74
    Location : Czech Republic

    Driver tube replacement Empty Re: Driver tube replacement

    Post by pavlikkkk Mon Aug 09, 2021 1:29 am

    Jbacik wrote:The 6GH8A tube was a find I did many years ago that I made and sold adapter's for. The 6GH8A tubes that I found made the ST70 sound brilliant! I purchased about 450 of these tubes that were nos in boxes of 100 made in Japan in the mid 80's. These were the last run made on these tubes that I know of. I still have a couple of hundred of these and have not found a bad one yet. No microphonic's or noise what so ever. Though the 6U8 was a closer match to the 7199, the 6GH8A has always been my favorite. No mods are necessary to use these 6GH8A's. If interested, send me a post and I will make them available. Many of my previous customers have reached out to me for extra of the tubes that I have. I also use these in my MK III's with adapters that I made for them. These are available now on fleabay for less than I can make them for. Regards, Jbacik.

    We solved the design of this board

    https://www.ebay.com/itm/383917040636

    Pavel

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