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    ST70 hum when PAM1 is connected to the takeoff socket

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    antsap

    Posts : 15
    Join date : 2019-03-10

    ST70 hum when PAM1 is connected to the takeoff socket - Page 2 Empty Re: ST70 hum when PAM1 is connected to the takeoff socket

    Post by antsap on Fri Nov 29, 2019 10:52 am

    I finally solved the problem and I can't believe it. Both of the 7199 tubes were microphonic and the vibrations of the chassis shaking them was the ultimate problem.

    I spent hours looking for every other cause, and even swapped out the choke for a new one.
    I re-flowed all the ground solder joints and ... ug hours.

    In desperation I ordered a NOS 6u8 and a 6u8 -> 7199 adaptor, and the amp is dead silent with it. I tried both channels with the new tube and each is silent with it, and hums like crazy with one of the old tubes.

    This should be 60hz hum no? I still can't explain why I'm getting 120hz from the microphonic tubes, but this was definitely the problem and fix.

    Note that the PAM's being plugged in must tax the power supply more, and cause slightly more aggressive power supply vibration shakes and that is why the hum got louder with them plugged in. The tip off here was a) trying everything else first, and then b) feeling the chassis vibrate more with a PAM plugged in.

    I'm both thrilled and mad at the same time Smile.
    peterh
    peterh

    Posts : 1228
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST70 hum when PAM1 is connected to the takeoff socket - Page 2 Empty Re: ST70 hum when PAM1 is connected to the takeoff socket

    Post by peterh on Fri Nov 29, 2019 12:29 pm

    antsap wrote:I finally solved the problem and I can't believe it. Both of the 7199 tubes were microphonic and the vibrations of the chassis shaking them was the ultimate problem.

    I spent hours looking for every other cause, and even swapped out the choke for a new one.
    I re-flowed all the ground solder joints and ... ug hours.

    In desperation I ordered a NOS 6u8 and a 6u8 -> 7199 adaptor, and the amp is dead silent with it. I tried both channels with the new tube and each is silent with it, and hums like crazy with one of the old tubes.

    This should be 60hz hum no? I still can't explain why I'm getting 120hz from the microphonic tubes, but this was definitely the problem and fix.

    Note that the PAM's being plugged in must tax the power supply more, and cause slightly more aggressive power supply vibration shakes and that is why the hum got louder with them plugged in. The tip off here was a) trying everything else first, and then b) feeling the chassis vibrate more with a PAM plugged in.

    I'm both thrilled and mad at the same time :).

    This was a long shot ! Thanks for sharing this exotic fault !
    avatar
    antsap

    Posts : 15
    Join date : 2019-03-10

    ST70 hum when PAM1 is connected to the takeoff socket - Page 2 Empty Re: ST70 hum when PAM1 is connected to the takeoff socket

    Post by antsap on Sat Nov 30, 2019 12:02 pm

    peterh wrote:
    antsap wrote:I finally solved the problem and I can't believe it. Both of the 7199 tubes were microphonic and the vibrations of the chassis shaking them was the ultimate problem.

    I spent hours looking for every other cause, and even swapped out the choke for a new one.
    I re-flowed all the ground solder joints and ... ug hours.

    In desperation I ordered a NOS 6u8 and a 6u8 -> 7199 adaptor, and the amp is dead silent with it. I tried both channels with the new tube and each is silent with it, and hums like crazy with one of the old tubes.

    This should be 60hz hum no? I still can't explain why I'm getting 120hz from the microphonic tubes, but this was definitely the problem and fix.

    Note that the PAM's being plugged in must tax the power supply more, and cause slightly more aggressive power supply vibration shakes and that is why the hum got louder with them plugged in. The tip off here was a) trying everything else first, and then b) feeling the chassis vibrate more with a PAM plugged in.

    I'm both thrilled and mad at the same time Smile.

    This was a long shot ! Thanks for sharing this exotic fault !

    My pleasure. I hope this helps some other bloke along their way with a similar problem.

    Thanks to everyone that made suggestions, offered their advice, and followed along until the end.
    avatar
    rjpjnk

    Posts : 209
    Join date : 2018-07-18

    ST70 hum when PAM1 is connected to the takeoff socket - Page 2 Empty Re: ST70 hum when PAM1 is connected to the takeoff socket

    Post by rjpjnk on Sat Nov 30, 2019 7:45 pm

    Wow, what an unusual problem! So happy to hear you solved it. Thanks very much for following up.

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