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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I did tack on extra 50uF in parallel with 30uF - no difference in hum...
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Originally posted by g1 View PostSingle 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.
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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.
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Originally posted by Enzo View PostPower supply ripple is 120Hz, so filter caps won't help 60Hz hum.
But should I expect this amp to be totally silent?
Power supply in this amp is the rectifier tube....
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Originally posted by Enzo View PostPower supply ripple is 120Hz, so filter caps won't help 60Hz hum.
60Hz ripple paired with SE output requires good filtering.
I would try tacking the 50µ cap in parallel with the screen node filter.Last edited by Helmholtz; 06-25-2022, 11:03 PM.
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Power supply ripple is 120Hz, so filter caps won't help 60Hz hum.
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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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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.
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Originally posted by mtlbasslad View PostThe 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?
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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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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Originally posted by The Dude View PostFYI: 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.
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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.
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Originally posted by glebert View PostDoes 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.
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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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