Originally Posted by Beneficial
I'm calibrating my MCI/SONY JH110C 2 track deck and trying to make sure I'm setting the bias right. I'm using Ampex 406 tape. The MCI manual says to overbias at 10Khz +2db for Scotch 206 tape. I've read Scotch 206 would use the same bias as Ampex 406... anyone know if that's true?
Also... I initially set the bias by turning the bias screw clockwise until the VU meter peaked... then continuing to turn it until the needle dropped an addition 2 VU positions... however I'm thinking that's different than 2db... am I right? When I overbiased by 2VU it seemed like I was still losing a little high end compared to other positions.
I hooked the tape deck up to my RME fireface and ran some spectral analysis software as I messed with the bias. If I recall correctly I remember seeing the third harmonic distortion dropping to it's lowest point at about 3/4VU overbias. Is it always best to go with what the manufacturer recommends.. or would the ideal bias setting really be the point at which the third harmonic distortion is minimal? (unless of course you thought a different setting sounded better)... this is at 15ips by the way.
My JH-110 manuals don't list the over-bias settings for Ampex 406.
I can't recall if it is the same as Scotch 206 or not.
To over-bias by 2 db you would turn the pot clockwise until the VU passed odb and fell to -2 db. Over-bias is when the bias is cranked above the 0 db point.
You are correct that you might have better distortion figures at one bias setting opposed to another. Proper bias is a trade off between freq. response and distortion.
I have always biased decks at the manufacturer's recommended levels.
I do have a nice laminated card that shows the calibration data for all of the Scotch/3M formulations.
It doesn't have the bias and operating levels for Scotch 206 though.
Another factor to consider is head gap.
The bias is different if the head gap is different.
The card says that Ampex AG-440s, 1000s, ATR100s and Otari 7800, 5050B(F) and MKll-1 decks all have .50 mil heads.
It says that ALL other deck models have .25 mil head gaps, so I guess "all other models" would include MCI decks.
Still, the MCI recommended 10K over bias settings are closer to the .50 mil head gap settings.
So, I'd follow the manual and go with 2 db over over bias at 10K.
That will probably produce the flattest response and the best IM distortion levels.
It has been a while, but I have actually plotted the freq. response on a few decks while playing with optimal bias settings.
I did this on an Ampex AG-440 and a n ATR-102 and the manufacturers recommended settings produced the best over-all freq. response/ IM distortion situations.
You can always experiment, too!