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Old 11th September 2012, 10:25 AM   #101
6J7 is offline 6J7
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I tried a new power supply setup (disco): 50uF > 100R > 50uF > choke > 200uF

- output caps (0.68uF, too much) are now in aluminium enclosure

- 50 Hz hum seems to be gone

- welcome to 100Hz hum

Although, it's not loud (full gain, step down transformers not yet installed) and hum is now "clean", no bzzzzzz anymore.

Not the most important thing at this time but sound is 100% boring, I'll try with bypassed cathode resistor again.

About 135V on plate, 20K load and 3.3V with led bias, not a good operating point. But I expected a fat mellow sound, not at all ! thin.. (mid-band gone ?)

I am thinking to not use anymore these big capacitors and go for electrolytics (short wires).

And two die-cast Hammond enclosure, one for the PS, the other for the voltage amp.
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Old 13th September 2012, 12:44 PM   #102
6J7 is offline 6J7
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hum problems are now almost solved.

But there is a ground loop (?) when adding Sowter 8920 at the input. Screen (black) has no effect, connected or not.

When connecting the input ground (orange) to starground, it's ok.

Did I miss something ? an isolating input transformer has no interest if input ground is connected to secondary ground, right ?

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