Preamp tubes going microphonic in AD140TC

[bclaire]

My amp seems to be eating preamp tubes. The first tube in the preamp of both channels went microphonic on me (at the same time which is weird...) I replaced them both and everything was fine for three months but now the channel two tube went microphonic again. I replaced it and everything's back to normal but is it just bad tubes or is anyone else experiencing this? I run the amp pretty hot BTW and it sounds fabulous. Thanks Billy