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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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mijohn
Bob Latino
MechEngVic
7 posters

    Very low bias and hum on Dynakit ST-70

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    mijohn

    Posts : 97
    Join date : 2013-06-19

    Very low bias and hum on Dynakit ST-70 - Page 2 Empty Re: Very low bias and hum on Dynakit ST-70

    Post by mijohn 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.
    lagwil
    lagwil

    Posts : 18
    Join date : 2019-05-29

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    Post by lagwil 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 : 1589
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Very low bias and hum on Dynakit ST-70 - Page 2 Empty Re: Very low bias and hum on Dynakit ST-70

    Post by peterh 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

    Posts : 18
    Join date : 2019-05-29

    Very low bias and hum on Dynakit ST-70 - Page 2 Empty Re: Very low bias and hum on Dynakit ST-70

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

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

    Very low bias and hum on Dynakit ST-70 - Page 2 Empty Re: Very low bias and hum on Dynakit ST-70

    Post by Guest Thu Dec 05, 2019 7:23 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 8:13 pm; edited 1 time in total
    peterh
    peterh

    Posts : 1589
    Join date : 2012-12-25
    Location : gothenburg, sweden

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    Post by peterh 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!
    Rick Green
    Rick Green

    Posts : 4
    Join date : 2017-01-09
    Age : 57
    Location : Stuart Florida

    Very low bias and hum on Dynakit ST-70 - Page 2 Empty Re: Very low bias and hum on Dynakit ST-70

    Post by Rick Green 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

    Posts : 18
    Join date : 2019-05-29

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    Post by lagwil 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

    Posts : 18
    Join date : 2019-05-29

    Very low bias and hum on Dynakit ST-70 - Page 2 Empty *** SOLVED ***

    Post by lagwil 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.

    DC92 likes this post

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    DC92

    Posts : 21
    Join date : 2015-05-28

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    Post by DC92 Wed Oct 06, 2021 1:04 pm

    So, as usual, Bob Latino's original diagnosis of a bad QuadCap was correct! So did the replacement QuadCap fix the Bias issue, too?

    lagwil likes this post

    lagwil
    lagwil

    Posts : 18
    Join date : 2019-05-29

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    Post by lagwil Wed Oct 06, 2021 1:46 pm

    DC92 wrote:So, as usual, Bob Latino's original diagnosis of a bad QuadCap was correct! So did the replacement QuadCap fix the Bias issue, too?

    Yes it did.

    /Wilhelm

    DC92 likes this post

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    DC92

    Posts : 21
    Join date : 2015-05-28

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    Post by DC92 Wed Oct 06, 2021 1:58 pm

    I was curious because I found your post looking for a solution to my low bias set readings, but without the hum. It seems my issue was caused by fading output tubes. Anyway, rock on!

    lagwil likes this post


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