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Some octal tubes are very sensitive to hum

6J5s or 6SN7s have a very high grid impedance, meaning that the cathode-grid structure will sometimes act like a powerful hum "antenna".
First of all you need to screen the input tubes (should be a basic rule on phono stages). My standard trick is buy some nice perforated metal pencil holders at a stationery store. Cut a hole in the pencil holder bottom to clear the tube, either screw the pencil holder to the chassis or attach a wire from "screen" to chassis ground.
Do not use russian 6SN7s on phono stages, their heaters are not hum-proof (coiled).
You may want to increase the grid-stopper value to 1000 ohms, make sure they are non-inductive: Allen-Bradley, Riken or Kiwame carbon resistors make excellent grid-stoppers, IMO. If you hear a loss of "air", decrease to 470 ohms.

If you hear 120 Hz buzz instead of hum (60 Hz), this means that your choke input PS is not clean enough (too much ripple).
First, try moving the large cap to the first pi filter, the small cap to the second filter. The choke input PS will output less ripple and the tubes will be fed from a relatively small, fast cap. Anything above 20 uF sounds too slow IMS.
Second, try increasing the capacitance of the first LC filter, to 100-300 uF. As long as you use an oil cap for the final filter, the large lytic will not do much damage.
Better still, use Unilytics. I actually prefer a mix of Unilytics and oil caps to avoid a "slow" sound (the downside of large PIO caps IMO).

Measure AC Volts at the speaker terminals with the preamp at a fixed volume setting. Make notes and compare several values of capacitance.

The best cap in a choke input PS is the smallest cap that will sound reasonably quiet. It is a balancing act: big caps, no ripple, lousy sound; small caps, lots of ripple, fast and musical sound.
It's up to you to find "the zone" in your system (within its particular gain structure).
I hope this helps.
Carlos


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  • Some octal tubes are very sensitive to hum - Carlos 10/26/0421:20:27 10/26/04 (0)


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