From Ken Swauger:
"Hi Carl,
Erasing is done with the same high frequency signal that is used to bias the recording process. In your deck this is 150kHz, certainly requiring a close contact between the erase head and the tape. Also, certain "high bias" tapes can be more difficult to completely erase. If I had to venture a guess I would say that there is slightly less tape-to-head tension in one direction than in the other, causing the reduction in erasing efficiency.
To be honest, it is probably best to continue bulk erasing just to insure completely quiet tapes. As a test you could try using a 7" reel but set the tension for 10.5" to see if there's better erasure.
Cheers, Ken"
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