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    ST-70 Rehab project

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    ejc

    Posts : 32
    Join date : 2010-10-16
    Age : 66
    Location : Connecticut

    ST-70 Rehab project

    Post by ejc on Wed Nov 23, 2016 6:28 pm

    I had toyed with the idea of picking up a tube amp and a year ago I pulled the trigger on a Harmon Kardon HK 500A Integrated amp.
    Turned out to be a lovely piece of equipment. So far I've only replaced the filter caps but plan on updating the rest of the caps.
    In the meantime, I'm hooked. The amp sounds simply wonderful, which is why I haven't rushed to update anything else.

    Moving forward a year.

    My son was using my old HK870 power amp and it is slowly dying. I found an ST-70 in pretty good shape and it will be his for the holidays.
    The EL-34s (may be the originals) appear weak and will be replaced.
    I am aware of the power supply issue and it will be taken care of.

    I wondering what to do with the 7199 tubes. Should I use them until they die or replace them?
    What seems to be the best tube to replace them with?

    GP49

    Posts : 771
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST-70 Rehab project

    Post by GP49 on Wed Nov 23, 2016 6:48 pm

    Use the 7199s until they die, which may not ever happen. Many Dynaco Stereo 70s are still running with their original 7199s.

    Agreement seems to be that the best substitute is the 6GH8, but you will either have to rewire the tube socket (the pinout differs) or purchase plug-in adapters. Since Dynaco used the 6AN8 on the Mark II with EL34 output tubes, I consider that an eminently successful substitute, too, but it also requires rewiring and there aren't any adapters on the market.

    ejc

    Posts : 32
    Join date : 2010-10-16
    Age : 66
    Location : Connecticut

    Re: ST-70 Rehab project

    Post by ejc on Wed Nov 23, 2016 7:11 pm

    GP49 wrote:Use the 7199s until they die, which may not ever happen.  Many Dynaco Stereo 70s are still running with their original 7199s.

    Agreement seems to be that the best substitute is the 6GH8, but you will either have to rewire the tube socket (the pinout differs) or purchase plug-in adapters.  Since Dynaco used the 6AN8 on the Mark II with EL34 output tubes, I consider that an eminently successful substitute, too, but it also requires rewiring and there aren't any adapters on the market.

    Thanks.
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    Peter W.

    Posts : 356
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: ST-70 Rehab project

    Post by Peter W. on Sun Nov 27, 2016 11:50 am

    GP49 wrote:Use the 7199s until they die, which may not ever happen.  Many Dynaco Stereo 70s are still running with their original 7199s.

    Agreement seems to be that the best substitute is the 6GH8, but you will either have to rewire the tube socket (the pinout differs) or purchase plug-in adapters.  Since Dynaco used the 6AN8 on the Mark II with EL34 output tubes, I consider that an eminently successful substitute, too, but it also requires rewiring and there aren't any adapters on the market.


    Seconding this motion on using the 7199s until they die. At present, I have four amps that use them, and in over 40 years, I have never seen a bad one that was not physically damaged.

    Also agree on the substitution options - and were you to go in that direction, you might consider replacing the entire board.
    avatar
    peterh

    Posts : 797
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST-70 Rehab project

    Post by peterh on Sun Nov 27, 2016 2:41 pm

    Peter W. wrote:
    GP49 wrote:Use the 7199s until they die, which may not ever happen.  Many Dynaco Stereo 70s are still running with their original 7199s.

    Agreement seems to be that the best substitute is the 6GH8, but you will either have to rewire the tube socket (the pinout differs) or purchase plug-in adapters.  Since Dynaco used the 6AN8 on the Mark II with EL34 output tubes, I consider that an eminently successful substitute, too, but it also requires rewiring and there aren't any adapters on the market.


    Seconding this motion on using the 7199s until they die. At present, I have four amps that use them, and in over 40 years, I have never seen a bad one that was not physically damaged.

    Also agree on the substitution options - and were you to go in that direction, you might consider replacing the entire board.

    I have actually replaced one 7199 ( in a sca-35 ) due to hum leaking from filaments to
    cathode. AT that time ( 5 years ago ?) there was no problems obtaining 7199, i still
    have a few around.

    If replacement is needed and no 7199 is in range 6GH8 or 6U8 ( ecf82 ) can be
    used , adapters can be used to "repin" the socket to avoid soldering the board.
    avatar
    Pillo69

    Posts : 132
    Join date : 2012-04-11
    Location : Granada (España)

    Re: ST-70 Rehab project

    Post by Pillo69 on Mon Nov 28, 2016 5:16 am

    GP49 wrote:Use the 7199s until they die, which may not ever happen.  Many Dynaco Stereo 70s are still running with their original 7199s.

    Agreement seems to be that the best substitute is the 6GH8, but you will either have to rewire the tube socket (the pinout differs) or purchase plug-in adapters.  Since Dynaco used the 6AN8 on the Mark II with EL34 output tubes, I consider that an eminently successful substitute, too, but it also requires rewiring and there aren't any adapters on the market.


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