I have an F2-B with the blue case and the silver knobs, old style 2 prong cord. I bought it from a recording studio going out of business. The paper schematic glued to the inside top says Alembic F-2B Stereo Preamp and Starting with Serial Number 143 so I presume the prior units had some slightly different circuit design.
Channel One works fine; Channel Two will work fine typically at first, but then after some period of time that varies from 2 minutes to 20 minutes will get very distorted and it seems like it loses lower frequencies.
I've swapped and replaced tubes, but the problem doesn't follow, so it can't be the tubes and the power supply is common to both, so that's not it. It also seems the time delay is a bit misleading (not a true heating issue, more just an intermittent).
Given the reduced low frequencies, I've wondered if the bass tone pot might be intermittent or something in that specific section going south.
Has anybody experienced anything like this? Thoughts, guidance, direction, suggestions would be much appreciated.
John
