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Old 3rd December 2012, 07:31 PM   #11
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I had exactly the same problem on my last project, using a similar Hammond PT, with diode rectifier. I tried all sorts of capacitor/resistor suggestions, different diodes, etc, to try to kill the noise. Couldn't get rid of the noise. I clearly identified the source of the noise as the filament supply (receiving noise from the B+ winding) coupling noise into the grids.

My solution was to use a small filament transformer to supply my preamp tubes. Using the noisy filament supply on the push-pull power adds no noise.
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Old 6th December 2012, 12:44 AM   #12
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Better, but still not perfect:
Click the image to open in full size.
Not dead quiet yet, but the original problem is gone. Need to double check all my joints and see about a little shielding on the the power supply side.
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Old 6th December 2012, 01:09 AM   #13
JMFahey is offline JMFahey  Argentina
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Hey !! That looks GOOD !!
Congratulations.
Now you have a real chassis which ypu cam later bolt into a head or combo cabinet.
Yes, you will still have a little noise, everybody does by the way, but now you are fighting dogs, not Godzilla.
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Old 9th December 2012, 10:27 PM   #14
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Perfect now:
Click the image to open in full size.
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Old 9th December 2012, 11:01 PM   #15
JMFahey is offline JMFahey  Argentina
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Wow !! Now that's something !!!
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Old 24th July 2014, 08:43 PM   #16
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Are your filament center tape to ground?
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Old 30th July 2014, 02:43 AM   #17
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Quote:
Originally Posted by Ibere Sabatini View Post
Are your filament center tape to ground?
Make sure that the transformer's B supply winding's center tap is grounded AND that the transformer's filament supply winding's center tap is grounded. If so at least you got that as designed!

Put small ceramic disc or mica capacitors across the diodes to eliminate any diode switching noise at its source.

Put some small-value poly caps across the main filter cap (the only one connected directly to the diodes) to do a better job of filtering out any diode switching noise.

Keep in mind that a single-ended amp is unforgiving of power noise.

A common fix easily tried is to make a 10:1 voltage divider between B+ and ground and reference the filament to that instead of to ground. Put a 1meg resistor from the filament center tap to the B+ (right after the diodes) and a 100K resistor from the filament center tap to ground. Oh, and bypass the 100K with a small cap to ground out any high frequency noise on the filament supply. This really works! It made a huge difference with my Sound City amp.

I would also mention that my Alembic preamp had a lot more stages of filter cap & resistor, filter cap and resistor, filter cap and resistor...
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Old 30th July 2014, 02:57 AM   #18
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And as Indiana Joe asked, is it 60-hz noise or 120-hz noise?

If you inject a very very low-level input with the amp's gain way up (but input so weak it's still not loud) if you pull the power plug the music will (hopefully) fade away slowly as the amp continues to run on the caps and filaments take a few seconds to cool. BUT: did the hum disappear as soon as you pulled the plug?

Did you try putting the heaters on a battery with one side grounded or lifted to some additional + with respect to ground (one side conneted to that voltage divider I described)?

Since you're using shielded wire to the input jack, sometimes it helps to use an insulated jack that doesn't connect to chassis ground, and let the shielded cable supply the ground to your guitar. Other times the grounded jack isn't a problem.

If you decide to put the transformers in cans, get some "loaded" heat-conductive potting epoxy and fill the can with the trans inside, instead of making any cooling holes. Wrap the trans in mu-metal?

Does the hum disappear if you turn the volume off?

Good luck.
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Old 30th July 2014, 02:58 AM   #19
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BTW these other guys know a lot more than I do.
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Old 30th July 2014, 03:04 AM   #20
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You have the speaker jack is grounded to the chassis too. Sometimes speaker current thru the chassis can make a better high-current path than the wire, but induce currents in the chassis that can couple to other things like the input jack. So, you might want to try using an insulated speaker jack.
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