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    ST-120 hum

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    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    ST-120 hum

    Post by baddog1946 on Tue Jul 27, 2010 10:23 am

    Hi Bob:
    I have a ST-120 you built for me last spring that has started to hum. When the amp warms up after about a minute it has a steady but low hum. I replaced all the power tubes (Gold Lion T-88's)and it does the same thing with the hum. I have not replaced the 12AX's but they do not appear to be microphonic when lightly tapped and appear to light up normally.About two moths ago I had a KT-88 die on me and it looked like the tube was frosted inside. I replaced the pair and it weas fine until this hum began. (I'm trying to give you as much info to go on as possible here).

    There have been the occasional sharp cracking sounds before this began when I was playing music.
    It also blew a couple of fuses just before the hum began but it then began to operate normally except for the hum I now have.
    My question is what do I check first and how do I logically proceed to isolate and repair this hum.
    Can you give me a list of things that might be the problem. Should I be using the amp at all until this is resolved?
    Hope you can help.
    Baddog1946



    Bob Latino
    Admin

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

    Re: ST-120 hum

    Post by Bob Latino on Tue Jul 27, 2010 10:37 am

    baddog1946 wrote:Hi Bob:
    I have a ST-120 you built for me last spring that has started to hum. When the amp warms up after about a minute it has a steady but low hum. I replaced all the power tubes (Gold Lion T-88's)and it does the same thing with the hum. I have not replaced the 12AX's but they do not appear to be microphonic when lightly tapped and appear to light up normally.About two moths ago I had a KT-88 die on me and it looked like the tube was frosted inside. I replaced the pair and it weas fine until this hum began. (I'm trying to give you as much info to go on as possible here).

    There have been the occasional sharp cracking sounds before this began when I was playing music.
    It also blew a couple of fuses just before the hum began but it then began to operate normally except for the hum I now have.
    My question is what do I check first and how do I logically proceed to isolate and repair this hum.
    Can you give me a list of things that might be the problem. Should I be using the amp at all until this is resolved?
    Hope you can help.
    Baddog1946



    Hi Baddog,

    Is the hum the same output level on both channels?

    Try changing out the CENTER 12AT7 and see if the hum goes away. The center 12AT7, which is the initial voltage amplifier for BOTH channels, can cause hum sometimes as the tube ages. If changing out the center 12AT7 does not get rid of the hum then maybe there is an issue with the high voltage B+ storage - possibly a bad quad cap ?

    Bob

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: ST-120 hum

    Post by baddog1946 on Wed Jul 28, 2010 11:48 pm

    Thanks again Bob.
    Although the center tube was not microphonic it was bad. I swapped out the Philips JAN 12AT7 for a Mullard 4024 and the hum was gone.
    I just completed a set of DIY Decware NFX's (a very challenging DIY design)and they are awesome with the 120!
    I am using a couple of Scott Endler stepped attenuators instead of my foreplay preamp now and this thing never gets turned up over 20-30% of its power. Even with these very sensitive speakers 99dB it is now dead silent again.

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