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Supreme (Garnet) Amp

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  • mtlbasslad
    replied
    Well, the customer thinks hum level is acceptable, even if I don't. He's taken it home.
    Thanks once again for the help everyone.

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  • mtlbasslad
    replied
    I did tack on extra 50uF in parallel with 30uF - no difference in hum...

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  • mtlbasslad
    replied
    Originally posted by g1 View Post
    Single ended power amp will always have more hum than a push-pull arrangement.
    You can't increase the value of the cap at the rectifier tube, but you can increase the screen node cap like Helmholtz mentioned. It's the 30uF shown on the schematic. Just tack the extra one you have in parallel for a temporary test.

    Click image for larger version  Name:	fetch?id=961873&d=1654647103.jpg Views:	3 Size:	177.3 KB ID:	963027
    The extra one I have is 50uF - in parallel with the 30uF to test? Or substitute the 50uF for the 30uF?

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  • g1
    replied
    Single ended power amp will always have more hum than a push-pull arrangement.
    You can't increase the value of the cap at the rectifier tube, but you can increase the screen node cap like Helmholtz mentioned. It's the 30uF shown on the schematic. Just tack the extra one you have in parallel for a temporary test.

    Click image for larger version

Name:	fetch?id=961873&d=1654647103.jpg
Views:	52
Size:	177.3 KB
ID:	963027

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  • mtlbasslad
    replied
    Originally posted by Enzo View Post
    Power supply ripple is 120Hz, so filter caps won't help 60Hz hum.
    So where do I look? And when I said 60hz hum, that is by (old) ear - not by any serious techy analysis... could have been 120hz
    But should I expect this amp to be totally silent?
    Power supply in this amp is the rectifier tube....

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  • Helmholtz
    replied
    Originally posted by Enzo View Post
    Power supply ripple is 120Hz, so filter caps won't help 60Hz hum.
    Amp uses half-wave rectification, so 60Hz ripple here.

    60Hz ripple paired with SE output requires good filtering.
    I would try tacking the 50 cap in parallel with the screen node filter.
    Last edited by Helmholtz; 06-25-2022, 11:03 PM.

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  • Enzo
    replied
    Power supply ripple is 120Hz, so filter caps won't help 60Hz hum.

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  • mtlbasslad
    replied
    Put everything together & fired it up - no magic smoke, but still a bit of 60hz buzz. Would adding another 50uF cap (in stock) help that? Or is this little amp prone to some hum?

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  • mtlbasslad
    replied
    Originally posted by Helmholtz View Post

    Wire the shield to safety ground/earth, typically amp chassis.

    All transformer secondary voltages sag with load. So no-load voltage being higher by around 10% than with rated load is normal.
    Thanks for that Helmholtz - I'll wire that shield to ground now, last step before I plug it in. Hoping for no 'magic smoke'. And that hum is gone - crossing my fingers.

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  • Helmholtz
    replied
    Originally posted by mtlbasslad View Post
    The secondary has a 3rd wire called 'static shield' - what do I do with that?
    Also the ISO trans secondary is 140V with a 125V input - with no load is that normal?
    Wire the shield to safety ground/earth, typically amp chassis.

    All transformer secondary voltages sag with load. So no-load voltage being higher by around 10% than with rated load is normal.

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  • mtlbasslad
    replied
    Got the parts & installed the ISO trans. The secondary has a 3rd wire called 'static shield' - what do I do with that?
    Also the ISO trans secondary is 140V with a 125V input - with no load is that normal?
    Now trying to figure out if the connections to the old can cap were in the right place before I install the 3 new caps...

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  • mtlbasslad
    replied
    Originally posted by The Dude View Post
    FYI: You don't need a 3 cap can. You could save money by using separate caps and even more money if you use radials. I would use radial caps and terminal strips. There's usually plenty of room in those older amps.
    I took your advice & actually managed to find terminal strips, rubber grommets, iso trans at D-K. Couldn't find radial caps though? Went with axial. Last time when I really needed axial, I had to take radial, go figure.

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  • mtlbasslad
    replied
    Originally posted by g1 View Post

    Use the 68, it has the highest VA rating, so most capable. The $ difference is negligible so may as well use the best one.
    I managed to survive the DigiKey website with your help about the 68. Parts on the way.

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  • mtlbasslad
    replied
    Originally posted by glebert View Post
    Does the hum go up and down with the volume? Also, is it 60 or 120 cycle hum? If necessary download an audio spectrum app for your phone.
    No, hum is a constant - 60hz & 120hz. No apps on my phone, only thing around here dumber than my phone is me...

    Leave a comment:


  • Enzo
    replied
    Oh, I thought your readings were going to be voltages. 865uf is only about 15% high for 750, not a problem. 750 may be a problem for the rectifier, but the cap itself can easily be that far off. In those days 20% was close tolerance.

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