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In the Forum: Analog Playback
In the Thread: 7788 tube mic pre design
Post Subject: 7788 RingingPosted by Lucian on: 1/13/2008

While the question was addressed to Romy, may I be allowed to barge in :-)?...

A question before anything: do you feed the 7788 DC or AC on the filament?

Many tubes are perfectly able to oscillate mechanically on 1KHz--20KHz range, specially when they are very high gain (see your CCS load...). This self- or microphony- induced oscillation seems to be excited further by the 60Hz heater power (electromotor induction into the cathode). DC on the filament tends to minimize/eliminated this electromechanically induced oscillation.

Obviously, against microphony there is the normal array of measures that one can try: soft "lantern" shock absorbers on a separate tube plate and mesh shielding, but all this can only do so much... Microphony seems to be one unavoidable evil with tubes...


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