Dac acts like a microphone

>>"I bought them on Ebay. Why do they sound good now?. If it is a bad microphonic tube will it show itself again?"<<
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That is just a chance you will have to take.
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>>"When I powered the unit up there was a strange sound (as the sound of an AC unit running)"<<
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That was noise, could be caused from grid emissions, or a slight intermittent short.

Some times when an old tube lays around for many years strange things can happen when put back into service. If after you get a few more hours on the tube and it remains quiet you may luck out and have a good tube. If it does it again I would pull it and replace it. Shorted tubes can cause damage.

As for the Herbie's HAL-O tubes dampers I use them in my Sonic Frontiers Preamp, and ARC VT50 Power amp. Imo they are great.
In my preamp I have them installed near the top of main body of the tubes.

Yeah, all tubes are microphonic, so no tapping. What I consider a bad microphonic tube is one that rings constantly with the faintest vibration in the room.

I solved microphonic tubes by using Herbie hal-o tube dampeners. Problem never came back so I recommend them.

All tubes are microphonic. I find that is best not to tap on equipment especially while it is playing.

Jea48,

I bought them on Ebay. Why do they sound good now?. If it is a bad microphonic tube will it show itself again?

Gary

Sounds like a bad microphonic tube. If by chance you just bought them from a dealer send him an email. Most dealers will replace microphonic tubes. If they are a matched pair he should replace both. If you bought them on Ebay, well....