Obviously that would be an issue due to the fact you can record hotter on the 996. re-calibrate your repro/sync/and record head to get the hotter sound, but still to come out at Ovu. This is an easy conversion. The object is to get the minimum noise, lowest distortion, and flat frequency response. You'll need a reference tape (probably recorded at +3), if you're gonna set your level to +9, then you can do that conversion. The reference tape has tones (I would use 1k and 10k) recorded on it at +3vu. Do your repro gain and sync gain at the 1k (which if you are aligning to +9 with the +3 reference, you'd have to have your needles at -6) After you've accomplished this, use the 10k to record your hi speed or low speed eq, depending on if you are recording at 15 or 30 ips., also to the -6. Then you worry about the bias. You have toknow your tape speed, bias frequency (10k), record head gap, and the type of tape you will be using to make this conversion. Example: after you determine all of that, take of the reference tape and load a blank or old tape whatever, and hit record, increase your bias (clockwise) until it peaks, then it will start to go back down, if you decided you need to be 1.5dB below peak with your conversion, keep moving the pot clockwise, never go backwards, until it comes down 1.5dB from your peak. (All this time sending a 1k tone out of your oscillator to tape.) At this point, you have achieved perfect bias.
Here, let me help you move that slab of cement that NTDunn dropped on your foot...
the answer is "yes".
Ampex 456 is a "+3" tape, and 3M 996 is a "+9" tape.
At 15ips, the typical overbias on 456 is about 2-3db (again, depends on head gap, tape width, might vary for you), while the typical overbias for 3M 996 is almost twice that (again, depending on head gap and tape width).
You can definitely get away with just tossing on the new reels, but THD will be compromised. Better to rebias. Overbiasing, in case anyone was wondering, is applying more bias voltage (level), which will move you away from achieving the optimal sensitivity and output level from the tape, but will lower the THD by a fair amount. Most folks figure the tradeoff to be worth it! Sacrifice a little level to achieve better fidelity.
Also, "NTDunn" seems to be suggesting that you bias using a 1k tone applied to the record inputs. Most everyone else uses a 10k tone. (some fostex machines are hard to bias properly using 10k, though).
Also, don't set your record levels until you've re-biased. The order is:
1) set playback level
2) set playback HF level
3) set bias
4) set record level
5) set record HF level/LF level