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    Mark II Preamp Wiring Change Question

    AA6U
    AA6U

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    Post by AA6U on Tue Jan 21, 2020 12:59 am

    I have a question about the Mark II modifications provided in the manual allowing it be used with an external preamp that needs a power supply like the Heathkit WA-P2 (plugs into the octal socket in the side of the Mark II).

    One of the modification steps is to cut the jumper wire on the terminal strip between terminal 2 (center tap of 6AN8/EL34 filament winding) and terminal 3 (chassis ground). However, the modification instructions never tells you to connect terminal 2 to anything else thus leaving it floating above ground.

    If you do not use a preamp needing power like the WA-P2, should this jumper be reinstalled? I am refurbishing two Mark IIs and one has the preamp mod and one does not. I will never use them with a preamp like the WA-P2, so should I reinstall the jumper, or does it matter?
    WLT
    WLT

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    Post by WLT on Wed Jan 22, 2020 11:12 am

    The MK II manual states some vintage preamps require the amp to not have the amp heater circuit grounded. The later MK III manual and schematic show the amplifier to be grounded thru a .02 uF cap. I have a pair of MK IIs that I have solidly grounded and a pair of MK IIIs with the cap to ground. Both work and sound fine with modern preamps and direct connected to streaming audio devices. I would make the solid connection first and then check for hum or other noise related issues.

    Others may have a different view so lets hope you get more responses.
    AA6U
    AA6U

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    Post by AA6U on Wed Jan 22, 2020 12:25 pm

    Thanks. That's what I thought. I also posted this on Audiokarma and someone said the same thing.
    tubes4hifi
    tubes4hifi
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    Post by tubes4hifi on Mon Jan 27, 2020 3:01 pm

    the front octal socket and it's original connections on any and all Dynaco tube amps are not needed for anything, unless you are powering a 50s era Dynaco preamp.
    AA6U
    AA6U

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    Post by AA6U on Mon Jan 27, 2020 6:51 pm

    Not planning on powering any 50s era Dynaco preamps, so I am going to follow the general scheme of the ST-70 and run the filament center tap to ground through a .47 cap.

    I'm still waiting for the rest of the parts to finish the rebuild. When I'm done, I'll report back on how it works.
    WLT
    WLT

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    Post by WLT on Mon Jan 27, 2020 9:52 pm

    The ST 70, MK III and MK VI schematics (that I have) all show a .02 uF cap not a .47 uF.. The MK II and MK IV show as solidly grounded.
    AA6U
    AA6U

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    Post by AA6U on Tue Jan 28, 2020 4:41 am

    Some of the locals I've talked to said that anything between a .02 uF and a solid ground connection will work. I used the .47 uF because it is between the .02 uF on the ST-70 and the solid ground connection as it was originally wired, and I had two .47 uFs handy (I have two units I'm refurbishing).

    I haven't finished the refurbishment yet, and I think I may just go with the solid ground connection because I can't go wrong with that.
    AA6U
    AA6U

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    Post by AA6U on Sun Feb 02, 2020 8:21 pm

    I finished the Mark II rebuilds and ended up wiring the center tap directly to ground. They sound great. No hum or noise.

    I did find that most of the resistors were way out of spec. Anyone restoring one of these needs to check all of the resistors. Better yet, just replace them all. There aren't very many.
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Sun Feb 02, 2020 9:16 pm

    AA6U wrote:

    I did find that most of the resistors were way out of spec.

    Dynaco used a lot of (if not all) carbon composition resistors in those amps. After many years most of the carbon composition resistors will stray from their original value. Most of the time the value of a carbon composition resistors will RISE as time goes by. I would replace the board and install metal film, metal oxide or carbon film resistors. Either of these three types of resistors will hold their value for many years.

    Bob
    AA6U
    AA6U

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    Post by AA6U on Mon Feb 03, 2020 12:20 am

    Surprisingly, some of the resistors seem to have been of better quality than others and held their value very closely. However, most were very cheap carbon comps and I replaced those.

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