Need help with tape bias...

[AlexHerd]

Hi all. I'm starting to experiment with tape and I need some help re: setting bias. Please be patient with me, I'm new.

When I get to bias I'm stuck. I understand how to set bias, just not where. Lets say that I want to I want to bias my tape at +6/185 nWb/m. As I understand it, thats (pretty much) the same as saying +3/250, correct? So using 911 as the example, could one person say to me that like to bias 911 at +6/185 and another +3/250 and mean the same thing?

So lets say I bias that tape to +6. If someone told me they wanted to align to +3/250 am I still going to over bias by 6?

Am I making any sense? haha. Thanks!

[Matt C.]

i could be wrong but it sounds like you are mixing up your terms here. Bias is different than Operating Level. +6/185 is an expression of the operating level you are calibrating for. I can never remember the exact numbers, but i think yes, +6/185 is the same as saying +3/250.

i believe the bias level is somewhat dependent on the operating level (given the same reel of tape, the optimum bias level will change depending on what operating level you're using), but they aren't the same thing.

[MikeRivers]

Matt is almost correct Those numbers are correctly called reference fluxivity, but operating level is the common term.

As far as the + goes, that's calculated in dB referenced to 185 nanowebers/meter, the Ampex standard level. So +6 is 6 dB above 185, or 370 nW/m. (6 dB is a ratio of 2:1). Since 3 dB is a ratio of 1.41:1, +3 over 250 would be 352.5 - close enough for rock'n'roll.

[AlexHerd]

thank you guys! I think my problem is just one of terminology. I'm getting what you're saying. I'm going to keep learning and trying!

[drumsound]

Setting overbias is related to tape formula and recording speed. I'm sure the RMGI site probably has the specks on their site.

You first have first find the bias peak by turning the bias adjustment counterclockwise until the meter starts to fall instead of climb, then go clockwise the and adjust the amount to the speck.

[doremi]

I calibrated my tape recorder for SM911 too! and over biased by 3 db and 1.5 db for 7.5 ips.

I know that the over bias value varies not only by speed but also by the type of machine, because it is also affected by the head gap.

Actually my operating level is higher at 510 Nanowebmeter and this is what my calibration tape is, then it is near +9 over Ampex/185 nwb/m.

I wonder if it is more optimal to set up the bias level with a THD analyzer for the lowest distortion ? What do you think ?

[jumpskins]

long time reader, first time poster. hi folks!

i run a fostex m80 - with RMG LPR-35, and what i can find of old stock quantegy 457 online.
i bought the deck second hand, and understand that its factory biased for 457. ive had no problems with it yet, aside having to get the rec/play head relapped as it'd worn to the edges. it now runs fine.

anyway, im making this post as ive never understood how bias actually affects the recorded signal. there is a pot for each channel on the m80 situated at the bottom of the deck for bias adjustment, but ive never dared touch it as i don't want to mess with it before i know what i'm doing.

id like to understand how to find the correct bias for each type of tape i may use, and the effect it has on the sound... ive been home recording for about 7 years to portastudios/DAWs, but this is the first time i've ever owned a r2r multitrack.

i suppose i don't understand the procedure of calibrating the bias. i might also add that i can't afford an MRL tape at this moment in time.

many thanks. sorry if i dont make much sense.