Originally Posted by Mike H
Yes, I have gone through the manual (a very detailed manual, indeed). I did not mention bias, azimuth, etc. because these have already been set by ATR Services, and I have checked them for my own education.
I think I have sorted out my confusion, at least partially. I measured voltages of a sine wave out of Pro Tools and found that the voltage of 1.23 volts RMS occurs at about +8 dB on my Dorrough meters. This reminds me that I followed Bob Katz's advice (see the book Mastering Audio) when setting everything up and actually set my analog meters so 0 VU = -4dBu. So, when I send a signal out of the tape deck which reads 0 on the deck meters, it reads +8 on my other analog meters. So, both are corresponding to +4 dBu = -12 dBFS. (I now need to decide if I do anything about this 8dB "offset", which would mean recalibrating my deck meter or all my other analog meters, or leave everything as-is.)
I still have the same question, though:
If I run peaks above -12 dBFS (+4 dBu) to the tape, it seems to me I will be hitting the 456 tape harder than +6. For example, if I let peaks go to -6 dBFS, it seems to me I will be hitting the tape at +6 + 6 = +12. Am I wrong on this?
If you've transported the machine, you should redo the alignment [all levels and bias]. If you are using a different tape than the one they used to set up your machine, you absolutely need to do a complete alignment to that particular brand of tape.
As far as your level question, it all depends on the reference level you use to set up the machine. If you are keen to keep your dig reference at -20 dbFS, then you might want top consider aligning the machine for +3/185 or even 0/185. Part of the fun of the tape machine is that the reference level you choose will affect the tape saturation characteristics of a given input level.
I wouldn't get caught up in the numbers as much as experimenting with different reference levels to find which one sounds the best to you. Plus, it'll be great practice aligning the machine!