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    Very low bias and hum on Dynakit ST-70

    peterh
    peterh

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    Post by peterh on Wed Dec 04, 2019 6:11 pm

    lagwil wrote:Nope but I do have a 5U4GB and exactly the same resultat with that rectifier. Alao swaped all EL-34 Tubes. Same...
    Time for a scope ( with > 500V probes) to look for that actual rectifier of the B+.  
    The midpoint red-yel should be grounded, the red wires connected with the
    gz34 at pin 4 and 6 should be examined for symmetrical voltages. At pin 8 of gz34
    look for the 435V and the pulsating peaks from both sides of the rectifier.

    The humming transformer indicated unbalance from DC magnetizing , as if one side
    of the rectifier don't work.
    lagwil
    lagwil

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    Post by lagwil on Wed Dec 04, 2019 6:17 pm

    mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

    PA521 is what I have.

    /Wilhelm


    Last edited by lagwil on Wed Dec 04, 2019 6:41 pm; edited 1 time in total
    peterh
    peterh

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    Post by peterh on Wed Dec 04, 2019 6:28 pm

    lagwil wrote:
    mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

    PA521 is what I got.

    /Wilhelm

    Du kan höra med http://www.lh-musik.se/ för reparation
    lagwil
    lagwil

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    Post by lagwil on Wed Dec 04, 2019 6:44 pm

    peterh wrote:
    lagwil wrote:
    mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

    PA521 is what I got.

    /Wilhelm

    Du kan höra med http://www.lh-musik.se/  för reparation
    Tack för tipset! De skriver iofs att ”vi tar inte emot HiFi”

    /Wille
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    mijohn

    Posts : 82
    Join date : 2013-06-19

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    Post by mijohn on Wed Dec 04, 2019 7:40 pm

    lagwil wrote:
    mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

    PA521 is what I have.

    /Wilhelm
    That's good, you have the right transformer. I made that comment because you wrote that the amp was stamped 220V.
    PeterCapo
    PeterCapo

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    Join date : 2008-12-05

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    Post by PeterCapo on Wed Dec 04, 2019 11:24 pm

    lagwil wrote: ... I use a slow 1,5A fuse since it is a 220V amp.

    /Wilhelm

    Thank you, I did not realize that.
    lagwil
    lagwil

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    Post by lagwil on Thu Dec 05, 2019 10:14 am

    peterh wrote:
    lagwil wrote:
    mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

    PA521 is what I got.

    /Wilhelm

    Du kan höra med http://www.lh-musik.se/  för reparation

    Peter, är det du som är Mr Tubular Well?
    peterh
    peterh

    Posts : 1251
    Join date : 2012-12-25
    Location : gothenburg, sweden

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    Post by peterh on Thu Dec 05, 2019 12:04 pm

    lagwil wrote:
    peterh wrote:
    lagwil wrote:
    mijohn wrote:Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

    PA521 is what I got.

    /Wilhelm

    Du kan höra med http://www.lh-musik.se/  för reparation

    Peter, är det du som är Mr Tubular Well?

    Yes
    lagwil
    lagwil

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    Post by lagwil on Thu Dec 05, 2019 5:29 pm

    @peterh I just ordered a new GZ34 from you!
    PeterCapo
    PeterCapo

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    Post by PeterCapo on Thu Dec 05, 2019 7:23 pm

    Cool beans. Would be fun to come over there for a visit and buy some tubes!
    peterh
    peterh

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    Join date : 2012-12-25
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    Post by peterh on Fri Dec 06, 2019 4:41 am

    lagwil wrote:@peterh I just ordered a new GZ34 from you!
    Thanks for that. It's in the post together with santa claus million parcels!
    deepee99
    deepee99

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    Location : Wallace, Idaho

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    Post by deepee99 on Sun Dec 15, 2019 11:28 pm

    Maybe a loose (or too tight) transformer bolt?
    Rick Green
    Rick Green

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    Join date : 2017-01-09
    Age : 55
    Location : Stuart Florida

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    Post by Rick Green on Mon Dec 16, 2019 11:31 am

    Hello , I am curious if you have resolved your hum problem? If so what was it? I suspect it is coming from the main transformer mechanically. It would be nice to see where the amp is located some pictures of what it is sitting on in its normal environment. Can you measure the loudness of the hum or is it just perceptible? Is the hum 50 Hz or 100Hz?
    lagwil
    lagwil

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    Join date : 2019-05-29

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    Post by lagwil on Tue Dec 17, 2019 4:47 am

    Rick Green wrote:Hello ,  I am curious if you have resolved your hum problem? If so what was it? I suspect it is coming from the main transformer mechanically. It would be nice to see where the amp is located some pictures of what it is sitting on in its normal environment. Can you measure the loudness of the hum or is it just perceptible? Is the hum 50 Hz or 100Hz?

    Hi, no the hum is not solved. I tried two different GZ34 rectifiers and both gives only 0.44 V bias. Something is definitely wrong. I will try to get some professional help to solve it.
    lagwil
    lagwil

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    Post by lagwil on Sun Feb 02, 2020 12:48 pm

    At last I found the problem with the help of a professional Dynaco repairman! The new quadcap was broken.

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