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    1channel fading

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    grizzinca

    Posts : 40
    Join date : 2012-01-01

    1channel fading

    Post by grizzinca on Sun Jun 29, 2014 6:09 pm

    Hi everyone. Been running my rebuilt st70 for about 2 years now. Recently I've noticed one channel (right) fading after an hour or so of use. I suspect a capacitor but would like to know what other areas I should look at. I've already swapped output tubes, checked my preamp and used other interconnects. Would appreciate any ideas. Thanks

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: 1channel fading

    Post by Bob Latino on Sun Jun 29, 2014 7:45 pm

    Hi,

    Is this a STOCK or stock type driver board ?

    Have you tried swapping DRIVER tubes ? Also - try cleaning the right channel driver tubes socket(s) with an electrical contact cleaner. If it is a tube, a slow fade like that is more common a driver tube problem than an output tube problem IF it is a tube problem.

    Bob

    grizzinca

    Posts : 40
    Join date : 2012-01-01

    Re: 1channel fading

    Post by grizzinca on Sun Jun 29, 2014 11:06 pm

    Thanks for the reply Bob. Haven't swapped the driver tubes, but have cleaned ALL tube sockets. I'll try this and see what happens. Thanks again. And the driver board is not stock, but VTA upgrade.

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: 1channel fading

    Post by Bob Latino on Mon Jun 30, 2014 8:06 am

    grizzinca wrote:Thanks for the reply Bob. Haven't swapped the driver tubes, but have cleaned ALL tube sockets. I'll try this and see what happens. Thanks again. And the driver board is not stock, but  VTA upgrade.

    If it is a VTA driver board, then either the center or the right driver tube could be the cause of the right channel fading. What you should do is buy ONE new 12AU7 (or 12AT7 if you have an older board) and substitute the tube into the center position and see if that helps. If that doesn't do it then try the new tube in the right position. Swapping the tubes you have in there is probably NOT the best way. If swapping in a new driver tube at the center and/or right positions doesn't help, then you have to look at the other components on the right side of the board.

    Bob

    grizzinca

    Posts : 40
    Join date : 2012-01-01

    Re: 1channel fading

    Post by grizzinca on Mon Jun 30, 2014 8:24 am

    Thanks Bob.

    sKiZo

    Posts : 1312
    Join date : 2013-04-01
    Location : Michigan USA

    Re: 1channel fading

    Post by sKiZo on Mon Jun 30, 2014 2:06 pm

    Why stop at one?  sunny 



    Swapping tubes can really change the entire personality of the amp, and you can get some real bargains in the 12AU7 family. RCA clear tops are very highly regarded and extremely affordable. IMHO, you can pay a whole lot more for a lot less.

    Any working tube should be good for testing, but rule of thumb, it's good to eventually match the side drivers for the best results. Center can be a different brand, and even a different tube type, depending on your tastes. I'd pick up at least one complete spare set for "sonic emergencies" ... Don't forget to check with Bob to see what he's stocking right now.

    Luck with it, and be sure to post back with your results ...

    grizzinca

    Posts : 40
    Join date : 2012-01-01

    Re: 1channel fading

    Post by grizzinca on Thu Jul 03, 2014 8:45 pm

    Hi again. Got new tubes and swapped as instructed. Amp played nice for about 20 minutes then right channel quit. If I run the volume up the right channel comes back, then fades again. I'll put it to rest till I can get some expert advise as to where to look next. Thanks for helping.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: 1channel fading

    Post by tubes4hifi on Sun Jul 06, 2014 8:32 pm

    sounds like a bad socket or a bad solder connection somewhere on the bad channel, easy fix with a solder iron!

    grizzinca

    Posts : 40
    Join date : 2012-01-01

    Re: 1channel fading

    Post by grizzinca on Sun Jul 06, 2014 8:58 pm

    I'll take a look. Thanks

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