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    Transformer Hum

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    Belvedere2

    Posts : 3
    Join date : 2013-04-30

    Transformer Hum

    Post by Belvedere2 on Tue Apr 30, 2013 8:39 pm

    Hello new to the forum and of course I have a question. I have a pair of Dynaco MK3s. One of them is dead quiet. The other one always made some transformer noise. Today I turned them on and the one that hummed made a loud humming noise through the speaker that just continued to get louder. I shut it off. Unfortunately I have no idea what to check. I can set bias on it but thats about it. Complete novice here. Thanks for the help.

    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: Transformer Hum

    Post by kaner on Tue Apr 30, 2013 10:00 pm

    Your hum is probably not from the transformer if it's coming through the speaker. Could be a bad quad cap or problems with the filament windings or any number of issues. It would help to know if it's 60Hz or 120Hz. If you have an iPhone you can use a guitar tuning app to figure out which one.

    The guys on this forum really know that amp and will be happy to help you. Next thing you know you will be building your own, swapping out caps, and learning more about vacuum tube amps than you can imagine. I started with an VTA st70 and have now built about 10 amps.

    Good luck and enjoy the ride!

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: Transformer Hum

    Post by j beede on Tue Apr 30, 2013 10:07 pm

    When people talk about transformer hum they usually are referring to mechanical hum due to loose windings or loose core laminations. From your description it sounds like you may be hearing 60Hz ac getting past your power supply filtering. The filter caps in a stock MkIII are contained in the silver "quad cap" mounted on your chassis. This is a common failure in the MkIII. Your options would be to replace the quad cap (they are available) or replace the quad cap with more conventional/modern capacitors that can be fitted under the chassis.

    Belvedere2

    Posts : 3
    Join date : 2013-04-30

    Re: Transformer Hum

    Post by Belvedere2 on Tue Apr 30, 2013 10:49 pm

    j beede wrote:When people talk about transformer hum they usually are referring to mechanical hum due to loose windings or loose core laminations. From your description it sounds like you may be hearing 60Hz ac getting past your power supply filtering. The filter caps in a stock MkIII are contained in the silver "quad cap" mounted on your chassis. This is a common failure in the MkIII. Your options would be to replace the quad cap (they are available) or replace the quad cap with more conventional/modern capacitors that can be fitted under the chassis.
    I believe it has been replaced. From pictures I've seen the silver tall cap is now a smaller blue cap.

    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: Transformer Hum

    Post by kaner on Tue Apr 30, 2013 11:07 pm

    I had a new quad cap fail on an amp. I tried all sorts of fixes, convinced that my new quad cap couldn't be the problem. I changed tubes, re-wired the power supply (several times), ran new filaments, and much more. Finally I rigged up temporary filter capacitors (modern ones as j beede suggests) and the hum was gone.

    Not sure if that's your problem but its's worth checking. 60 Hz hum sounds like a low buzz, if that helps. A new quad cap will run about $40 to $50.

    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: Transformer Hum

    Post by kaner on Tue Apr 30, 2013 11:13 pm

    By the way, if you decide to try a quad cap replacement, it's pretty simple BUT they store A LOT OF ELECTRICITY! You need to learn about how to safely discharge them.

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Transformer Hum

    Post by Bob Latino on Wed May 01, 2013 7:29 am

    The most common cause of a steady hum in older Dynaco amps is a quad cap that has gone bad. Original Mark III's ran a B+ of 475 VDC or so on the 115 - 117 VAC line voltages of the 1950's and 1960's. If used on modern 120 VAC line voltages, the B+ is now probably closer to 500 VDC. If you have a "blue" quad cap then someone did replace the cap - BUT - Look on the side of this cap for its voltage rating. I would be willing to bet that it is either a 450 or 500 VDC rated cap. This is too low a voltage rating for this amp. You need at minimum a 525 volt rated cap or better yet > a 550 volt rated cap.

    The best replacement cap to use is the 550 volt rated 80, 40, 30, 20 that Dynakitparts sells. Link below ...

    Dynakitparts 550 volt rated quad cap

    Bob

    Belvedere2

    Posts : 3
    Join date : 2013-04-30

    Re: Transformer Hum

    Post by Belvedere2 on Wed May 01, 2013 9:07 pm

    Bob Latino wrote:The most common cause of a steady hum in older Dynaco amps is a quad cap that has gone bad. Original Mark III's ran a B+ of 475 VDC or so on the 115 - 117 VAC line voltages of the 1950's and 1960's. If used on modern 120 VAC line voltages, the B+ is now probably closer to 500 VDC. If you have a "blue" quad cap then someone did replace the cap - BUT - Look on the side of this cap for its voltage rating. I would be willing to bet that it is either a 450 or 500 VDC rated cap. This is too low a voltage rating for this amp. You need at minimum a 525 volt rated cap or better yet > a 550 volt rated cap.

    The best replacement cap to use is the 550 volt rated 80, 40, 30, 20 that Dynakitparts sells. Link below ...



    Bob
    Thanks Bob. Yep it's a 500. I live in Mass. so I'll end up bringing these to you. I could replace parts but I think they need to be looked at. At least for my peace of mind. I'll give you a call. Thanks!

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: Transformer Hum

    Post by j beede on Thu May 02, 2013 9:11 pm

    FYI: I built these 800V filter cap boards for about $15 each. They fit under the Mk III chassis with no modifications. I have been "quad cap" and hum free for about two years. I left the old quad caps in place to preserve the stock look of the Mk III. They are not connected.
    ...j





    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Transformer Hum

    Post by PeterCapo on Mon Apr 28, 2014 4:40 pm

    j,

    Sorry if this is a repeat question, but can you please say exactly what kind of paint was used for the transformer end bells? Perhaps a brand and part number? They look really good.

    Thanks.
    Peter

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Transformer Hum

    Post by sKiZo on Mon Apr 28, 2014 6:53 pm

    Belvedere2 wrote: Yep it's a 500. I live in Mass. so I'll end up bringing these to you. I could replace parts but I think they need to be looked at. At least for my peace of mind. I'll give you a call. Thanks!

    Will there be cookies?
    I was told there'd be cookies!!

    If it was closer, I'd probably been knocking on his door every other day on my build.  geek 

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: Transformer Hum

    Post by tubes4hifi on Tue Apr 29, 2014 7:41 pm

    as everyone else has said, it's most likely the quad cap. However, it could also be the bias voltage capacitors, those are the two 50uF capacitors on the terminal strip,
    they can be replaced with 47-100uF 100v capacitors.

    deepee99

    Posts : 1336
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Transformer Hum

    Post by deepee99 on Thu May 01, 2014 2:58 pm

    sKiZo wrote:
    Belvedere2 wrote: Yep it's a 500. I live in Mass. so I'll end up bringing these to you. I could replace parts but I think they need to be looked at. At least for my peace of mind. I'll give you a call. Thanks!

    Will there be cookies?
    I was told there'd be cookies!!

    If it was closer, I'd probably been knocking on his door every other day on my build.  geek 

    Why else do you think he lives in the boon-docks? We'd all be pounding on his door if it weren't for the air-fare. Bob, move to Idaho. Promise we'll be best friends.

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Transformer Hum

    Post by Bob Latino on Thu May 01, 2014 5:29 pm

    Not to hijack the OP thread about transformer hum but > I wish I could move out of "Taxachusetts" .. The people in nearby New Hampshire call us "Ma$$holes" for living here because we have a sales tax and they have NO sales tax in NH. The truth of the matter is that both my wife and I still have elderly mothers living here in Mass. who need constant attention because of failing health ..

    As Roy and other's have said > Hum from the speakers is usually a problem with quad cap or any other associated high voltage caps. If the "hum" comes as a vibrational hum directly from the power transformer itself then you should try tightening (or loosening) the nuts that hold the transformer to the chassis.

    Bob

    Guest
    Guest

    Re: Transformer Hum

    Post by Guest on Thu May 01, 2014 6:46 pm

    deepee99 wrote:
    sKiZo wrote:
    Belvedere2 wrote: Yep it's a 500. I live in Mass. so I'll end up bringing these to you. I could replace parts but I think they need to be looked at. At least for my peace of mind. I'll give you a call. Thanks!

    Will there be cookies?
    I was told there'd be cookies!!

    If it was closer, I'd probably been knocking on his door every other day on my build.  geek 

    Why else do you think he lives in the boon-docks? We'd all be pounding on his door if it weren't for the air-fare. Bob, move to Idaho. Promise we'll be best friends.

    Idaho shmidaho  Razz 
    Montana...Big Sky country...we just went for a walk on our property...it is drop dead gorgeous out here, about 68, a few puffy clouds, slight breeze......paradise!  sunny 

    deepee99

    Posts : 1336
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Transformer Hum

    Post by deepee99 on Thu May 01, 2014 9:40 pm

    MontanaWay wrote:
    deepee99 wrote:
    sKiZo wrote:
    Belvedere2 wrote: Yep it's a 500. I live in Mass. so I'll end up bringing these to you. I could replace parts but I think they need to be looked at. At least for my peace of mind. I'll give you a call. Thanks!

    Will there be cookies?
    I was told there'd be cookies!!

    If it was closer, I'd probably been knocking on his door every other day on my build.  geek 

    Why else do you think he lives in the boon-docks? We'd all be pounding on his door if it weren't for the air-fare. Bob, move to Idaho. Promise we'll be best friends.

    Idaho shmidaho  Razz 
    Montana...Big Sky country...we just went for a walk on our property...it is drop dead gorgeous out here, about 68, a few puffy clouds, slight breeze......paradise!  sunny 

    Yeah, just keep Robert Redford over there, OK? And Streisand and all those other sheep-lovers. We don't need 'em here. We've got house-pets bigger than their egos.

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