Effects Of Overbiasing

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SteveM

SteveM

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I'm wondering how the bias effects the overall playback/record on tape. On the 38 you're supposed to turn the bias screw clockwise until the meter drops about 5db below the peak meter reading. So, what happens if you let if drop a few db less or a few db more? I'm trying to see what other options there are for different sounds. The eq calibration on the 38 is only one screw so I don't understand how you can change the eq for a different sound . Any ideas?
 
too much bias = more bass less highs and less hiss; not enough bias = more highs, less bass and more hiss.

http://en.wikipedia.org/wiki/Tape_bias

I would try to bias based on what you hear vs what the manual says. I took a whole day to try several different levels and attemted to get the repro to sound identical to the input on one of my decks.
 
Interesting Falken, thanks for that information. I'm going to try experimenting with that a bit. Even though it adds more hiss is it sometimes still a practice to underbias?
 
The bias adjustment should be used to minimize distortion, not adjust frequency response. Since most users don't have adequate methods of measuring distortion, the tape machines' manufacturers usually just tell you to peak the output and then keep increasing the bias until the output drops by so many dB, to get you to approximately the best place distortionwise.
 
yes but what kind of distortion are you referring to?
 
Wil816 said:
The bias adjustment should be used to minimize distortion, not adjust frequency response. Since most users don't have adequate methods of measuring distortion, the tape machines' manufacturers usually just tell you to peak the output and then keep increasing the bias until the output drops by so many dB, to get you to approximately the best place distortionwise.
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So, how do you adjust eq with only one eq adjustment screw?
 
Most machines only have one record eq adjustment (if that) which only affects the high frequencies.

It's only there to correct high frequency record level errors, when the machine is lined up. It's not some sort of user effect like on a mixer. That's why it's not on the front panel.

cheers Tim.
 
Yeah, I guess that makes sense. I understand you're usually after a flat response but I guess I'm just seeing if theres a different dynamic response than I'm getting.
 
ive read

Ive read about Joe Meek altering the bias of his machines for certain "effects", but they never really say what he was doing or why. Maybe it was just maintenance that in the retelling of history has now become stranger than what the reality was.
 
According to the 38 manual, tweaking bias and rec/rep eq you can fine tune to get a flatter or different response. So, you can play with the bias settings to alter the dynamic effect or suit your taste.
 
Tim Gillett said:
Most machines only have one record eq adjustment (if that) which only affects the high frequencies.

It's only there to correct high frequency record level errors, when the machine is lined up. It's not some sort of user effect like on a mixer. That's why it's not on the front panel.

cheers Tim.
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If I understand correctly [with everything else being aligned], the high EQ is there to compensate [to a point] for wear on the heads?
 
Hi Daniel,
I think the main effect of record head wear (loss of metal at the head face) is a tendency to become overbiased. You would normally adjust for that by redoing the recommended overbias adjustment re that tape.

Record eq is normally adjusted after bias and record level have been done, and to get a flat playback response. I think it is more to compensate for different tapes.

I wouldnt be using the record hi eq to compensate for a worn record head as that should be taken care of by the bias adjustment.

If by worn you mean a head that can no longer reliably maintain good tape contact then bias adjustment cant fix that. It needs relap or replace. This is a separate issue from loss of metal, leading to overbias.

Always check the bias is correct as gradual head wear and tape type can have a big effect on the bias requirement. Then adjust hi eq last to get a good flat response.

My 2 cents.

Tim
 
Ok, cool, thank you very much Tim. :)

I meant just typical head wear where the surface progressively gets flatter and flatter. I actually didn't know that more wear means a rebias.
 
It doesnt always need rebiasing but often it does. Depends on the bias oscillator. As the record head wears down it changes its electrical characteristics, sort of like changing from a 40watt bulb into a 60 watt. So to bring the 60 watt bulb back to producing only 40 watts of output you have to decrease the voltage a bit.

The problem with the head wearing flat is that it spreads the tape to head pressure too thin, and the tape no longer sits as tight against the gap. It's a recipe for dropouts and weak highs. Relapping can restore the curve so there's more focussed tape pressure right on the head gap again, at the top of the curve.

Cheers Tim
 
Thus you relap it, as you say, or struggle continually to re-align / re-calibrate the machine, as the head wear gets worse. Got it, thanks again.:)
 
SteveM said:
According to the 38 manual, tweaking bias and rec/rep eq you can fine tune to get a flatter or different response. So, you can play with the bias settings to alter the dynamic effect or suit your taste.
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Absolutely true. I bias my track 1 for bass/kick drum sounds. You can tailor any channel for the material provided you remember you did this!

There is a whole world beyond standard bias if you want to really make your machine sing.
 
how far do you overbias for kick/bass, MCI2424?
 
FALKEN said:
how far do you overbias for kick/bass, MCI2424?
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I run a 40Hz sine running at 15ips and adjust bias for best sound. I am not really sure of the exact numbers as they are meaningless to me. The "adjust by ear" method is just that.

I do use GP9 and the overbias will definately be higher than +4db. I don't do this on my E-16 because there is no playback head (2 -head) and it is a monumental pain-in-the-ass to set back. The 2" is easy.
 
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