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    ST70 hum when PAM1 is connected to the takeoff socket

    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 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 : 1352
    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 : 253
    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.
    MechEngVic
    MechEngVic

    Posts : 58
    Join date : 2019-01-16

    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 MechEngVic on Mon Dec 16, 2019 11:54 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.

    Those darn pentode/triode driver tubes! The 6gh8a's are subject to the same kinds of problems! You'd think a driver tube would outlast the power tubes, but I've had to swap out more microphonic 6gh8a's than I care to admit. And the ones that last a long time don't sound good...

    I think it has to do with the fact that a pentode (2 plates, 3 screens) is crammed into HALF of a tiny driver tube.

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