You can not run a tube amp without a cab (load). You need a dummy load to do this. Google it or look on ebay perhaps for dummy loads. If you attempt this without one you'll fry your output transformer. You'll also probably need a DI box or interface before running a tube head into a soundcard.
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I act like I want to, not like I should!
If you have a tube head,,warped is correct. you'll need a weber mass for the speaker load..then get the behinger gi100 D.I. box and run that to your soundcard.....
there ya go....silent recording.
I use a engl preamp connected directly to my presonus sound card...jam at 3am no problems.
good luck.
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