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    ST-70 Chassis Hum

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    breading

    Posts : 23
    Join date : 2010-01-08

    ST-70 Chassis Hum

    Post by breading on Sat Feb 13, 2010 10:02 pm

    I have developed a chassis hum and cannot isolate its cause. The hum is definitely coming from the amp itself, not the speakers.

    My question is, when this type of problem arises is it generally tube related, or is it in the transformers?

    This problem began after I replaced the quad cap with a new 30/20/20/20 from Dynakitparts.

    Thanks for any input.

    Bruce

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 Chassis Hum

    Post by Bob Latino on Sun Feb 14, 2010 7:56 am

    breading wrote:I have developed a chassis hum and cannot isolate its cause. The hum is definitely coming from the amp itself, not the speakers.

    My question is, when this type of problem arises is it generally tube related, or is it in the transformers?

    This problem began after I replaced the quad cap with a new 30/20/20/20 from Dynakitparts.

    Thanks for any input.

    Bruce

    Hi Bruce,

    A few things to check ..

    1. First try replacing the rectifier tube with a new or a "known to be good" rectifier tube. See if the hum/vibration continues or stops .. If it continues ..

    2. Try tightening or loosening the nuts on the power transformer (with the amp OFF) then turn the amp back on and see if your "hum" is gone or is reduced. If it is still there then probably the power transformer should be replaced.

    You didn't say which Dynaco tube amp you had but if it is an ST-70 the stock PA-060 power transformer was notoriously undersized to run that amp. After 40-50 years of use many do develop hum, buzz, vibration issues. The monoblock Dynaco amps - Mark II, Mark III and Mark IV can have issues also (hum, vibration, noisy etc.) but to a lesser degree than the ST-70 since they only supply power to one channel and have had "an easier life" over the years.

    Kevin at Dynakitparts carrys replacement power transformers for all the Dynaco tube amps. They are all upgraded versions of the original power transformers. Check them out at the link below ...

    Dynakitparts replacement transformers

    Bob

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: ST-70 Chassis Hum

    Post by Bugs on Mon Feb 15, 2010 2:45 am

    Make sure you have your quad cap oriented correctly. I had the same problem after replacing mine and it turned out I used the wrong quad cap directions. There are different directions for different caps. I blew the cap out after a couple of months. Big mistake on my part.

    Best

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